In IDLE the following code silently works:
>>> del __builtins__
>>> min
<built-in function min>

In the standard interpreter, it produces an error:
>>> del __builtins__
>>> min
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'min' is not defined

Note that saying __builtins__ = 7 fails in idle as well.

If you type del __builtins__;min an error is raise in both IDLE and the standard interpreter.

__builtins__ is a private implementation detail in CPython. There is no guarantees made about whether it exists or not. E.g. it doesn't exist in Jython.

steve@orac:~/python$ jython
Jython 2.5.1+ (Release_2_5_1, Aug 4 2010, 07:18:19)
[OpenJDK Server VM (Sun Microsystems Inc.)] on java1.6.0_31
Type "help", "copyright", "credits" or "license" for more information.
>>> __builtins__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name '__builtins__' is not defined

You should use __builtin__ in Python 2 and builtins in Python 3. Anything you do to __builtins__ with an S is implementation-dependent.

I don't think it is a bug that CPython behaves differently regarding __builtins__ depending on whether IDLE is running or not.

We try to have IDLE work the same as Python (and CPython, where relevant), except where differences are intentional or unavoidable. I am trying to eliminate some unintentional avoidable differences in other issues. However, this one is unavoidable given IDLE's basic design. Also, Steven is correct; see
https://docs.python.org/3/library/builtins.html#module-builtins

IDLE executes user code with "exec(code, self.locals)" (run.py, l.351 in 3.5).
https://docs.python.org/3/library/functions.html#exec
says "If the globals dictionary does not contain a value for the key __builtins__, a reference to the dictionary of the built-in module builtins is inserted under that key."

(Doc issus: From the builtins doc and the Jython example, this seems implementation dependent. Steven, does
  >>> d = {}; exec('', d); d.keys()
  dict_keys(['__builtins__'])
have this result in Jython?)

In the IDLE shell, each statement is exec'ed separately. With two statements, __builtins__ is added back before 'min' , while with one statement, it is not.

Editor: Run Module execs the entire file at once. I expected print(min) to fail either way, but it works either way. I verified that globals().keys() lost '__builtins__', so I don't know how __builtins__.min is found.

I left this open to consider adding a line to
https://docs.python.org/3/library/idle.html#idle-console-differences

Steven: "You should use __builtin__ in Python 2 and builtins in Python 3." I presume this is for import statements.

ppperry: Titles should fit in the box, so they fit in search listing results.

I am thinking of something like "Since Python inserts '__builtins__' into the exec global namespace when not present and IDLE uses exec, '__builtins__' is defined at the start of each statement or file even when it otherwise would not be."

Terry J. Reedy added the comment:

Steven: "You should use __builtin__ in Python 2 and builtins in
Python 3." I presume this is for import statements.

My understanding is that __builtins__ is intended to be for the private
use of the CPython interpreter only. It may not be available in other
Pythons, or in the future, and code that needs access to the built-ins
should treat it as a regular module and import it via __builtin__ in
Python 2 and builtins in Python 3.

In other words, unless you're the CPython interpreter, don't touch
__builtins__.

I don't know whether IDLE is considered sufficiently closely integrated
to CPython that it is allowed to rely on __builtins__, but for an
ordinary module (even one in the stdlib) I wouldn't touch it at all.

New changeset 862761e4376e by Terry Jan Reedy in branch '2.7':
Issue bpo-25564: Mention exec and __builtins__ in IDLE-console difference section.
https://hg.python.org/cpython/rev/862761e4376e

New changeset 641852513b8e by Terry Jan Reedy in branch '3.5':
Issue bpo-25564: Mention exec and __builtins__ in IDLE-console difference section.
https://hg.python.org/cpython/rev/641852513b8e

I added what I think is an improved version of what I posted.

For 2.7 only, a change in the generated html outside of the text proper from using — and » to using &bpo-8212; and &#187' resulted in the extraneous characters being shown. The displayed text was preceded by '—»»»» ' and followed by the closers. Adding the missing guard 'if self.show' to the charref function prevents this.

What I meant is that one cannot use __builtin__ or builtins until one has done the import, which is why people tend not to bother and instead do things like ppperry was doing.

To the extent that tkinter is limited to CPython, so is IDLE. Still, the only reference to __builtins__ is in
F:\Python\dev\36\lib\idlelib\autocomplete.py: 188: (fetch completions)
namespace.update(main.__builtins__.__dict__)

I don't know why not just use __builtins__.__dict__, but this follows
import __main__
namespace = __main__.__dict__.copy()

To be cross-platform, you are saying that this should be on 3.x
import builtins; namespace.update(builtins.__dict__)