Share-Based Compensation - Summary of Non-qualified Option Activity (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Weighted- Average Exercise Price
Income tax benefit	$ 11	$ 9	$ 11
Non-qualified Options
Non-qualified Options
Shares, Outstanding at beginning of year	154,551	204,434	415,960
Shares Granted	0	0	0
Shares, Exercised	(132,905)	(47,393)	(209,976)
Shares, Forfeited	0	0	0
Shares, Expired	(5,941)	(2,490)	(1,550)
Shares, Outstanding at end of year	15,705	154,551	204,434
Shares, Exercisable at end of period	15,705	154,551	204,434
Weighted- Average Exercise Price
Outstanding at beginning of year (in USD per share)	$ 35.96	$ 36.66	$ 40.19
Granted (in USD per share)	0.00	0.00	0.00
Exercised (in USD per share)	36.86	38.60	43.53
Forfeited (in USD per share)	0.00	0.00	0.00
Expired (in USD per share)	41.25	43.35	51.62
Outstanding at end of year (in USD per share)	26.34	35.96	36.66
Exercisable at end of year (in USD per share)	$ 26.34	$ 35.96	$ 36.66
Aggregate intrinsic value, Outstanding	$ 1
Aggregate intrinsic value, Exercisable	$ 1
Weighted-average remaining contractual term, Outstanding	8 months 12 days
Weighted-average remaining contractual term, Exercisable	8 months 12 days
Intrinsic value for options exercised	$ 8	$ 2	$ 10
Options vested in period	0	0	0
Cash received from the exercise of options	$ 5	$ 2
Income tax benefit	$ 2	$ 1