Stock Compensation - Effects on Cash Flows (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Proceeds from common stock transactions	[1]	$ 373	$ 483	$ 472
Tax benefit realized from stock compensation		179	341	255
Reduction to deferred tax asset		(43)	(91)	(105)
Excess tax benefit for stock compensation		$ 136	$ 250	$ 150

[1] Net of taxes paid for employee shares withheld of $60 million, $83 million and $70 million in 2018, 2017 and 2016, respectively.