Stock-Based Compensation Plans - Summary of Stock Option Activity (Details) - Employee Stock Option [Member] - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
December 31, 2016		12,531,591
Options exercised		(3,093,156)
Options forfeited		0
Options expired		(2,714,824)
December 31, 2017		6,723,611	12,531,591
Weighted Average Exercise Price (in usd per share)		$ 47.69	$ 46.23
Options Exercised, Weighted Average Exercise Price (in usd per share)		34.69
Options Forfeited, Weighted Average Exercise Price (in usd per share)		0.00
Options Expired, Weighted Average Exercise Price (in usd per share)		$ 55.78
Shares Outstanding, Weighted Average Remaining Contractual Life		2 years 7 months 23 days	3 years 6 months
Exercisable at December 31, 2017	[1]	6,723,611
Shares Exercisable, Weighted Average Exercise Price (in usd per share)	[1]	$ 47.69
Shares Exercisable, Weighted Average Remaining Contractual Life	[1]	2 years 7 months 23 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value	[1]	$ 7	$ 13
Shares Exercisable, Aggregate Intrinsic Value		$ 7

[1] Includes stock options issued to retirement eligible employees.