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, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Beginning Balance		6,723,611
Options exercised		(1,522,952)
Options forfeited		0
Options expired		(1,173,007)
Ending Balance		4,027,652	6,723,611
Weighted Average Exercise Price (in usd per share)		$ 43.95	$ 47.69
Options Exercised, Weighted Average Exercise Price (in usd per share)		36.54
Options Forfeited, Weighted Average Exercise Price (in usd per share)		0.00
Options Expired, Weighted Average Exercise Price (in usd per share)		$ 74.99
Shares Outstanding, Weighted Average Remaining Contractual Life		2 years 10 months 24 days	2 years 7 months 24 days
Exercisable at December 31, 2017	[1]	4,027,652
Shares Exercisable, Weighted Average Exercise Price (in usd per share)	[1]	$ 43.95
Shares Exercisable, Weighted Average Remaining Contractual Life	[1]	2 years 10 months 24 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value		$ 14	$ 7
Shares Exercisable, Aggregate Intrinsic Value	[1]	$ 14

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