Stock Compensation - Effects on Cash Flows (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Proceeds from common stock transactions	[1]	$ 483	$ 472	$ 396
Tax benefit realized from stock compensation		341	255	171
Reduction to deferred tax asset		(91)	(105)	(81)
Excess tax benefit for stock compensation		$ 250	$ 150	$ 90

[1] Net of taxes paid for employee shares withheld of $83 million in 2017, $70 million in 2016 and $46 million in 2015.