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]
Shares, Outstanding		46.2
Stockholders' Equity Attributable to Parent	$ 640.3	$ 7.3	$ 634.3	$ (1.3)
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
Stock Issued During Period, Value, Restricted Stock Award, 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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, Outstanding		46.2
Stockholders' Equity Attributable to Parent	640.3	$ 7.3	634.3	(1.3)
Net income	182.0		182.0
Issuance of common stock (in shares)		0.1
Issuance of common stock	0.0	$ 0.0
Stock-based compensation	6.2	$ 6.2
Other	0.4			0.4
Repurchase of common stock (in shares)		5.7
Repurchase aggregate cost	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
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	341.4
Shares outstanding, ending (in shares) at Sep. 30, 2018		40.7
Shareholders' equity, ending at Sep. 30, 2018	514.7	$ 11.5	504.1	(0.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, Outstanding		46.2
Stockholders' Equity Attributable to Parent	640.3	$ 7.3	634.3	(1.3)
Net income	341.4
Adoption of new ASC			29.7
Adoption of new ASC | Accounting Standards Update 2014-09	29.7
Shares, Outstanding		40.6
Stockholders' Equity Attributable to Parent	344.2	$ 0.0	345.1	(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	$ 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]
Shares, Outstanding		40.6
Stockholders' Equity Attributable to Parent	344.2	$ 0.0	345.1	(0.9)
Net income	103.1		103.1
Issuance of common stock (in shares)		0.1
Issuance of common stock	0.0	$ 0.0
Stock-based compensation	6.4	6.4
Other	(0.2)	$ (0.1)	(0.1)
Repurchase of common stock (in shares)		0.0
Repurchase aggregate cost	0.3	$ 0.3
Shares, Outstanding		40.7
Stockholders' Equity Attributable to Parent	453.2	$ 6.0	448.1	(0.9)
Net income	56.3		56.3
Issuance of common stock (in shares)		0.0
Issuance of common stock	1.5	$ 1.5
Stock-based compensation	3.9	3.9
Other	(0.2)	$ 0.1	(0.3)
Shares outstanding, ending (in shares) at Sep. 30, 2018		40.7
Shareholders' equity, ending at Sep. 30, 2018	514.7	$ 11.5	504.1	(0.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, Outstanding		40.7
Stockholders' Equity Attributable to Parent	453.2	$ 6.0	448.1	(0.9)
Net income	56.3		56.3
Issuance of common stock (in shares)		0.0
Issuance of common stock	1.5	$ 1.5
Stock-based compensation	3.9	3.9
Other	(0.2)	$ 0.1	(0.3)
Shares, Outstanding		40.7
Stockholders' Equity Attributable to Parent	514.7	$ 11.5	504.1	(0.9)
Common stock, no par value; 150.0 shares authorized; 39.9 shares issued and outstanding at September 30, 2019 and 40.4 shares at December 31, 2018	0.0
Shares, Outstanding		40.4
Stockholders' Equity Attributable to Parent	473.3	$ 0.0	474.2	(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	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
Stock Issued During Period, Value, Restricted Stock Award, 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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, Outstanding		40.4
Stockholders' Equity Attributable to Parent	473.3	$ 0.0	474.2	(0.9)
Net income	11.6		11.6
Issuance of common stock (in shares)		0.1
Issuance of common stock	0.0	$ 0.0
Stock-based compensation	4.7	$ 4.7
Other	0.2		0.2
Repurchase of common stock (in shares)		0.3
Repurchase aggregate cost	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
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	133.5
Shares outstanding, ending (in shares) at Sep. 30, 2019		39.9
Shareholders' equity, ending at Sep. 30, 2019	550.4	$ 0.0	551.3	(0.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, Outstanding		40.4
Stockholders' Equity Attributable to Parent	550.4	$ 0.0	474.2	(0.9)
Net income	133.5
Adoption of new ASC			(0.3)
Adoption of new ASC | Accounting Standards Update 2016-02	(0.3)
Shares, Outstanding		40.2
Stockholders' Equity Attributable to Parent	456.9	$ 0.0	457.8	(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]
Shares, Outstanding		40.2
Stockholders' Equity Attributable to Parent	456.9	$ 0.0	457.8	(0.9)
Net income	107.1		107.1
Stock-based compensation	7.4	7.4
Other	(0.1)	$ (0.1)
Repurchase of common stock (in shares)		0.2
Repurchase aggregate cost	18.0	$ 4.4	13.6
Shares, Outstanding		40.0
Stockholders' Equity Attributable to Parent	553.3	$ 2.9	551.3	(0.9)
Net income	14.8		14.8
Issuance of common stock (in shares)		0.1
Issuance of common stock	1.9	$ 1.9
Repurchase of common stock (in shares)		(0.2)
Repurchase of common stock	(25.0)	$ (10.4)	(14.6)
Restricted Stock, Shares Issued Net of Shares for Tax Withholdings		0.0
Stock Issued During Period, Value, Restricted Stock Award, Forfeitures	(0.1)	$ 0.0	(0.1)
Stock-based compensation		5.5
Other	0.0	$ 0.1	(0.1)
Shares outstanding, ending (in shares) at Sep. 30, 2019		39.9
Shareholders' equity, ending at Sep. 30, 2019	550.4	$ 0.0	551.3	(0.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, Outstanding		40.0
Stockholders' Equity Attributable to Parent	550.4	$ 2.9	551.3	(0.9)
Net income	14.8		14.8
Issuance of common stock (in shares)		0.1
Issuance of common stock	1.9	$ 1.9
Stock-based compensation		5.5
Other	0.0	$ 0.1	(0.1)
Repurchase of common stock (in shares)		0.2
Repurchase aggregate cost	25.0	$ 10.4	14.6
Restricted Stock, Shares Issued Net of Shares for Tax Withholdings		0.0
Stock Issued During Period, Value, Restricted Stock Award, Forfeitures	0.1	$ 0.0	0.1
Common stock, no par value; 150.0 shares authorized; 39.9 shares issued and outstanding at September 30, 2019 and 40.4 shares at December 31, 2018	0.0
Shares, Outstanding		39.9
Stockholders' Equity Attributable to Parent	$ 550.4	$ 0.0	$ 551.3	$ (0.9)