Condensed Consolidated Statements of Shareholders' Equity - USD ($) shares in Millions, $ in Millions	Total	Common Stock	Retained Earnings	Accumulated Other Comprehensive Loss
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adoption of new ASC			$ 29.7
Adoption of new ASC | Accounting Standards Update 2014-09	$ 29.7
Shares outstanding, beginning (in shares) at Dec. 31, 2017		46.2
Shareholders' equity, beginning at Dec. 31, 2017	640.3	$ 7.3	634.3	$ (1.3)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	182.0		182.0
Issuance of common stock (in shares)		0.1
Issuance of common stock	0.0	$ 0.0
Repurchase of common stock (in shares)		(5.7)
Repurchase of common stock	(501.5)	$ (13.5)	(488.0)
Restricted Stock, Shares Issued Net of Shares for Tax Withholdings		(0.1)
Stock Issued During Period, Value, Restricted Stock Award, Forfeitures	(12.9)	$ 0.0	(12.9)
Issuance of restricted stock awards, net of forfeitures (in shares)		0.1
Issuance of restricted stock awards, net of forfeitures	0.0	$ 0.0
Stock-based compensation	6.2	$ 6.2
Other	0.4			0.4
Shares outstanding, ending (in shares) at Mar. 31, 2018		40.6
Shareholders' equity, ending at Mar. 31, 2018	344.2	$ 0.0	345.1	(0.9)
Shares outstanding, beginning (in shares) at Dec. 31, 2017		46.2
Shareholders' equity, beginning at Dec. 31, 2017	640.3	$ 7.3	634.3	(1.3)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	285.1
Issuance of restricted stock awards, net of forfeitures	12.9
Shares outstanding, ending (in shares) at Jun. 30, 2018		40.7
Shareholders' equity, ending at Jun. 30, 2018	453.2	$ 6.0	448.1	(0.9)
Shares outstanding, beginning (in shares) at Mar. 31, 2018		40.6
Shareholders' equity, beginning at Mar. 31, 2018	344.2	$ 0.0	345.1	(0.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	103.1		103.1
Issuance of common stock (in shares)		0.1
Issuance of common stock		$ 0.0
Repurchase of common stock (in shares)		0.0
Repurchase of common stock	(0.3)	$ (0.3)
Stock-based compensation	6.4	6.4
Other	(0.2)	$ (0.1)	(0.1)
Shares outstanding, ending (in shares) at Jun. 30, 2018		40.7
Shareholders' equity, ending at Jun. 30, 2018	453.2	$ 6.0	448.1	(0.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock, no par value; 150.0 shares authorized; 40.0 shares issued and outstanding at June 30, 2019 and 40.4 shares at December 31, 2018	0.0
Adoption of new ASC			(0.3)
Adoption of new ASC | Accounting Standards Update 2016-02	(0.3)
Shares outstanding, beginning (in shares) at Dec. 31, 2018		40.4
Shareholders' equity, beginning at Dec. 31, 2018	473.3	$ 0.0	474.2	(0.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	11.6		11.6
Issuance of common stock (in shares)		0.1
Issuance of common stock	0.0	$ 0.0
Repurchase of common stock (in shares)		(0.3)
Repurchase of common stock	(25.0)	$ (4.7)	(20.3)
Restricted Stock, Shares Issued Net of Shares for Tax Withholdings		(0.1)
Stock Issued During Period, Value, Restricted Stock Award, Forfeitures	(7.6)	$ 0.0	(7.6)
Issuance of restricted stock awards, net of forfeitures (in shares)		0.1
Issuance of restricted stock awards, net of forfeitures	0.0	$ 0.0
Stock-based compensation	4.7	$ 4.7
Other	0.2		0.2
Shares outstanding, ending (in shares) at Mar. 31, 2019		40.2
Shareholders' equity, ending at Mar. 31, 2019	456.9	$ 0.0	457.8	(0.9)
Shares outstanding, beginning (in shares) at Dec. 31, 2018		40.4
Shareholders' equity, beginning at Dec. 31, 2018	473.3	$ 0.0	474.2	(0.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	118.7
Issuance of restricted stock awards, net of forfeitures	7.6
Shares outstanding, ending (in shares) at Jun. 30, 2019		40.0
Shareholders' equity, ending at Jun. 30, 2019	553.3	$ 2.9	551.3	(0.9)
Shares outstanding, beginning (in shares) at Mar. 31, 2019		40.2
Shareholders' equity, beginning at Mar. 31, 2019	456.9	$ 0.0	457.8	(0.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	107.1		107.1
Repurchase of common stock (in shares)		(0.2)
Repurchase of common stock	(18.0)	$ (4.4)	(13.6)
Stock-based compensation	7.4	7.4
Other	(0.1)	$ (0.1)
Shares outstanding, ending (in shares) at Jun. 30, 2019		40.0
Shareholders' equity, ending at Jun. 30, 2019	553.3	$ 2.9	$ 551.3	$ (0.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock, no par value; 150.0 shares authorized; 40.0 shares issued and outstanding at June 30, 2019 and 40.4 shares at December 31, 2018	$ 2.9