Stock Compensation - Effects on Cash Flows (Details) - USD ($) $ in Millions		3 Months Ended				12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Proceeds from common stock transactions	[1]					$ 472	$ 396	$ 554
Tax benefit realized from stock compensation						255	171	218
Reduction to deferred income tax asset						(105)	(81)	(108)
Excess tax benefit for stock compensation		$ 17	$ 50	$ 40	$ 43	$ 150	$ 90	$ 110

[1] Net of taxes paid for employee shares withheld of $70 million in 2016, $46 million in 2015 and $62 million in 2014.