Condensed Consolidated Statements of Shareholders' Equity - USD ($) shares in Millions, $ in Millions	Total	Common Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adoption of new ASC | Accounting Standards Update 2014-09	$ 29.7		$ 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
Repurchase of common stock (in shares)		(5.8)
Repurchase of common stock	(514.4)	$ (13.5)	(500.9)
Issuance of restricted stock awards, net of forfeitures (in shares)		0.1
Issuance of restricted stock awards, net of forfeitures	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]
Adoption of new ASC | Accounting Standards Update 2016-02	(0.3)		(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
Repurchase of common stock (in shares)		(0.3)
Repurchase of common stock	(32.6)	$ (4.7)	(27.9)
Issuance of restricted stock awards, net of forfeitures (in shares)		0.1
Issuance of restricted stock awards, net of forfeitures	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)