Income Taxes (Unrecognized Tax Benefits) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in reserve for uncertain tax positions, excluding related accrued interest and penalties
Unrecognized tax benefits, beginning of year	$ 384	$ 446	$ 300
Additions for current year tax positions	15	49	27
Additions for prior year tax positions	18	67	204
Reductions for prior year tax positions	(34)	(26)	(60)
Cash settlements	(2)	(149)	(19)
Statute of limitations lapses	(9)	(3)	(6)
Reclassification to deferred income tax liability	(64)
Unrecognized tax benefits, end of year	$ 308	$ 384	$ 446