Consolidated Statements of Stockholders' Equity - USD ($) shares in Millions, $ in Millions	Common Stock	Additional Paid In Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Total
Balance at the beginning of the period at Dec. 31, 2014	$ 32.4	$ 274.9	$ (53.3)	$ 109.8	$ 363.8
Balance (in shares) at Dec. 31, 2014	43.2
Increase (Decrease) in Stockholders' Equity
Net income				103.0	103.0
Other comprehensive income (loss), net of tax			6.4		6.4
Tax benefit from stock-based compensation and share withholding		2.6			2.6
Share-based compensation		12.2			12.2
Shares issued	$ 0.3	(1.0)			(0.7)
Shares issued (in shares)	0.4
Shares repurchased	$ (0.1)	(0.8)		(5.0)	(5.9)
Shares repurchased (in shares)	(0.1)
Dividends ($0.78, $0.74, and $0.70 per share) for the years 2017, 2016 and 2015 respectively				(30.8)	(30.8)
Balance at the end of the period at Dec. 31, 2015	$ 32.6	287.9	(46.9)	177.0	450.6
Balance (in shares) at Dec. 31, 2015	43.5
Increase (Decrease) in Stockholders' Equity
Net income				81.4	81.4
Other comprehensive income (loss), net of tax			23.3		23.3
Tax benefit from stock-based compensation and share withholding		2.2			2.2
Share-based compensation		11.2			11.2
Shares issued	$ 0.2	(4.9)			(4.7)
Shares issued (in shares)	0.3
Shares repurchased	$ (0.7)	(6.6)		(29.6)	(36.9)
Shares repurchased (in shares)	(0.9)
Dividends ($0.78, $0.74, and $0.70 per share) for the years 2017, 2016 and 2015 respectively				(32.2)	(32.2)
Balance at the end of the period at Dec. 31, 2016	$ 32.1	289.8	(23.6)	196.6	$ 494.9
Balance (in shares) at Dec. 31, 2016	42.9				42.9
Increase (Decrease) in Stockholders' Equity
Net income				232.0	$ 232.0
Other comprehensive income (loss), net of tax			(1.3)		(1.3)
Share-based compensation		11.1			11.1
Shares issued	$ 0.3	(5.7)			(5.4)
Shares issued (in shares)	0.3
Shares repurchased	$ (0.5)	(5.5)		(13.3)	(19.3)
Shares repurchased (in shares)	(0.7)
Dividends ($0.78, $0.74, and $0.70 per share) for the years 2017, 2016 and 2015 respectively				(33.8)	(33.8)
Balance at the end of the period at Dec. 31, 2017	$ 31.9	$ 289.7	$ (24.9)	$ 381.5	$ 678.2
Balance (in shares) at Dec. 31, 2017	42.5				42.5