You sometimes even need docs now if you're flying international with one parent and not the other--like a notarized letter from the non-traveling parent that you have permission to take the child to another country.
Yes, notarized consent for international flights if both parents aren't traveling with the child.
However, 3 times in as many years, my wife has not been asked to show the letter when traveling alone with our kids...SQ, CX and UA.