Stock Compensation Plans - Options (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Options Outstanding Activity
Options outstanding, beginning of period (in shares)	7,173,048
Options granted (in shares)	722,440
Options exercised (in shares)	(1,142,039)
Options expired (in shares)	(107,080)
Options cancelled (in shares)	(49,271)
Options outstanding, end of period (in shares)	6,597,098	7,173,048
Weighted Average Exercise Price
Options outstanding, beginning of period (in dollars per share)	$ 18.21
Options granted (in dollars per share)	35.16
Options exercised (in dollars per share)	18.13
Options expired (in dollars per share)	26.14
Options cancelled (in dollars per share)	23.72
Options outstanding, end of period (in dollars per share)	$ 19.91	$ 18.21
Other Options Information
Options vested and expected to vest (in shares)	6,411,897
Options vested and expected to vest, weighted average exercise price (in dollars per share)	$ 19.59
Options exercisable (in shares)	4,913,448
Options exercisable, weighted average exercise price (in dollars per share)	$ 16.01
Options available for grant (in shares)	17,764,947
Options outstanding, weighted average remaining contractual term	5 years 3 months 18 days
Options vested and expected to vest, weighted average remaining contractual term	5 years 2 months 12 days
Options exercisable, weighted average remaining contractual term	4 years 3 months 18 days
Options exercised, aggregate intrinsic value	$ 18	$ 14	$ 40
Options outstanding, aggregate intrinsic value	84
Options vested and expected to vest, aggregate intrinsic value	85
Options exercisable, aggregate intrinsic value	$ 80