Consolidated Statements of Shareholders' Equity - USD ($) shares in Millions, $ in Millions	Total	Common Stock	Retained Earnings	Accumulated Other Comprehensive Loss	Noncontrolling Interest
Shares outstanding, beginning (in shares) at Dec. 31, 2016		49.5
Shareholders' equity, beginning at Dec. 31, 2016	$ 685.0	$ 116.5	$ 569.7	$ (1.2)	$ 0.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	140.5		140.5
Issuance of common stock (in shares)		0.1
Issuance of common stock	2.1	$ 2.1
Repurchase of common stock (in shares)		(3.4)
Repurchase of common stock	(180.9)	$ (138.4)	(42.5)
Taxes paid related to net share settlement of stock awards (in shares)		(0.2)
Taxes paid related to net share settlement of stock awards	(10.0)		(10.0)
Issuance of restricted stock awards, net of forfeitures (in shares)		0.2
Stock-based compensation	27.1	$ 27.1
Cash dividends	(23.4)		(23.4)
Foreign currency translation, net of tax	(0.1)			(0.1)
Change in pension benefits, net of tax	0.0
Shares outstanding, ending (in shares) at Dec. 31, 2017		46.2
Shareholders' equity, ending at Dec. 31, 2017	640.3	$ 7.3	634.3	(1.3)	0.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adoption of ASC	29.7		29.7
Net income	352.8		352.8
Issuance of common stock (in shares)		0.3
Issuance of common stock	1.5	$ 1.5
Repurchase of common stock (in shares)		(6.1)
Repurchase of common stock	(533.9)	$ (29.9)	(504.0)
Taxes paid related to net share settlement of stock awards (in shares)		(0.1)
Taxes paid related to net share settlement of stock awards	(15.6)		(15.6)
Issuance of restricted stock awards, net of forfeitures (in shares)		0.1
Stock-based compensation	21.1	$ 21.1
Cash dividends	(23.0)		(23.0)
Foreign currency translation, net of tax	0.6			0.6
Change in pension benefits, net of tax	(0.2)			(0.2)
Shares outstanding, ending (in shares) at Dec. 31, 2018		40.4
Shareholders' equity, ending at Dec. 31, 2018	473.3	$ 0.0	474.2	(0.9)	0.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adoption of ASC	(0.3)		(0.3)
Net income	137.2		137.5		(0.3)
Contributions from noncontrolling interest	3.0				3.0
Issuance of common stock (in shares)		0.2
Issuance of common stock	1.9	$ 1.9
Repurchase of common stock (in shares)		(0.9)
Repurchase of common stock	(93.0)	$ (25.7)	(67.3)
Taxes paid related to net share settlement of stock awards (in shares)		(0.1)
Taxes paid related to net share settlement of stock awards	(11.5)		(11.5)
Issuance of restricted stock awards, net of forfeitures (in shares)		0.1
Stock-based compensation	23.8	$ 23.8
Cash dividends	(23.4)		(23.4)
Foreign currency translation, net of tax	0.0
Change in pension benefits, net of tax	0.0
Shares outstanding, ending (in shares) at Dec. 31, 2019		39.7
Shareholders' equity, ending at Dec. 31, 2019	$ 511.0	$ 0.0	$ 509.2	$ (0.9)	$ 2.7