Stock-Based Compensation - Exercise Price Range (Details) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options outstanding (in shares)	520	652	648	650
Weighted-Average Remaining Contractual Term (in years)	4 years 3 months	4 years 9 months 10 days	4 years 10 months 6 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 42.56	$ 39.27	$ 30.50	$ 25.56
Exercisable at the end of the year (in shares)	381	363	493
Exercisable at the end of the year (in dollars per share)	$ 36.39	$ 30.21	$ 25.03
$18.25 to $21.95
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices, lower range limit (in dollars per share)	18.25
Range of exercise prices, upper range limit (in dollars per share)	$ 21.95
Options outstanding (in shares)	155
Weighted-Average Remaining Contractual Term (in years)	3 years 9 months 14 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 19.92
Exercisable at the end of the year (in shares)	155
Exercisable at the end of the year (in dollars per share)	$ 19.92
26.48 to 27.74
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices, lower range limit (in dollars per share)	26.48
Range of exercise prices, upper range limit (in dollars per share)	$ 27.74
Options outstanding (in shares)	17
Weighted-Average Remaining Contractual Term (in years)	3 months 18 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 27.02
Exercisable at the end of the year (in shares)	17
Exercisable at the end of the year (in dollars per share)	$ 27.02
35.28
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices, lower range limit (in dollars per share)	35.28
Range of exercise prices, upper range limit (in dollars per share)	$ 35.28
Options outstanding (in shares)	85
Weighted-Average Remaining Contractual Term (in years)	3 years 1 month 13 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 35.28
Exercisable at the end of the year (in shares)	85
Exercisable at the end of the year (in dollars per share)	$ 35.28
51.38 to 65.54
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices, lower range limit (in dollars per share)	51.38
Range of exercise prices, upper range limit (in dollars per share)	$ 65.54
Options outstanding (in shares)	263
Weighted-Average Remaining Contractual Term (in years)	5 years 1 month 20 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 59.31
Exercisable at the end of the year (in shares)	124
Exercisable at the end of the year (in dollars per share)	$ 59.10