Stock-Based Compensation Plans - Activity for Stock Options Outstanding (Details) - Stock Options - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Shares Under Option
Stock options outstanding, beginning of period (in shares)	4	9	10
Granted (in shares)	0	0	0
Exercised (in shares)	(4)	(5)	(1)
Cancelled/forfeited (in shares)	0	0	0
Stock options outstanding, end of period (in shares)	0	4	9
Weighted Average Exercise Price
Stock options outstanding, beginning of period, weighted average exercise price (in dollars per share)	$ 43.74	$ 48.37	$ 48.63
Granted, weighted average exercise price (in dollars per share)	0.00	0.00	0.00
Exercises, weighted average exercise price (in dollars per share)	43.74	52.58	49.95
Canceled/forfeited, weighted average exercise price (in dollars per share)	0.00	0.00	0.00
Stock options outstanding, end of period, weighted average exercise price (in dollars per share)	$ 0.00	$ 43.74	$ 48.37