Stock-based Compensation - Stock Options Activity (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Number of options
Outstanding, beginning of period (in shares)	627,550	898,732	1,528,590
Exercised (in shares)	(523,400)	(267,182)	(617,358)
Forfeited (in shares)		(4,000)	(12,500)
Outstanding, end of period (in shares)	104,150	627,550	898,732
Weighted average exercise price
Outstanding at beginning of period, Weighted average exercise price (in dollars per share)	$ 36.41	$ 36.55	$ 36.09
Exercised, Weighted average exercise price (in dollars per share)	36.00	36.84	35.37
Forfeited, Weighted average exercise price (in dollars per share)		39.71	38.36
Outstanding at end of period, Weighted average exercise price (in dollars per share)	$ 38.46	$ 36.41	$ 36.55
Outstanding, Weighted average remaining life	1 year 3 months 18 days	2 years 2 months 12 days
Exercised, Weighted average remaining life	1 year 1 month 6 days
Outstanding, Aggregate intrinsic value (in dollars)	$ 3	$ 11