Stock Compensation Plans - Options (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Options Outstanding Activity
Options outstanding, beginning of period (in shares)	6,597,098
Options granted (in shares)	0
Options exercised (in shares)	(684,374)
Options expired (in shares)	(72,205)
Options cancelled (in shares)	(260,067)
Options outstanding, end of period (in shares)	5,580,452	6,597,098
Weighted Average Exercise Price
Options outstanding, beginning of period (in dollars per share)	$ 19.91
Options granted (in dollars per share)	0.00
Options exercised (in dollars per share)	14.43
Options expired (in dollars per share)	23.46
Options cancelled (in dollars per share)	28.57
Options outstanding, end of period (in dollars per share)	$ 20.14	$ 19.91
Other Options Information
Options vested and expected to vest (in shares)	5,485,525
Options vested and expected to vest, weighted average exercise price (in dollars per share)	$ 19.93
Options exercisable (in shares)	4,717,476
Options exercisable, weighted average exercise price (in dollars per share)	$ 17.93
Options available for grant (in shares)	16,211,852
Options outstanding, weighted average remaining contractual term	4 years 6 months
Options vested and expected to vest, weighted average remaining contractual term	4 years 6 months
Options exercisable, weighted average remaining contractual term	4 years
Options exercised, aggregate intrinsic value	$ 9	$ 18	$ 14
Options outstanding, aggregate intrinsic value	24
Options vested and expected to vest, aggregate intrinsic value	24
Options exercisable, aggregate intrinsic value	$ 24