CONSOLIDATED STATEMENTS OF SHAREHOLDERS’ EQUITY - USD ($) $ in Millions	Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Additional Paid-in Capital	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Treasury Stock	Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss) Cumulative Effect, Period of Adoption, Adjustment
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201802Member
Balance (in shares) at Dec. 31, 2018			41,065,069				21,719,339
Balance at Dec. 31, 2018	$ 473.2	$ 0.0	$ 0.4	$ 572.9	$ 922.7	$ 0.2	$ (1,023.4)	$ 0.6	$ (0.2)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	40.9				40.9
Change in fair value of cash flow swaps, net of reclassification adjustment	(1.3)							(1.3)
Comprehensive income	39.6				40.9			(1.3)
Share-based compensation	3.9			3.9
Issuance of common stock, net of forfeitures in connection with share-based payment arrangements (in shares)			238,078
Issuance of common stock, net of forfeitures in connection with share-based payment arrangements	0.0
Repurchase of common stock associated with net share settlements of employee share-based awards (in shares)							66,912
Repurchase of common stock associated with net share settlements of employee share-based awards	(4.7)						$ (4.7)
Share repurchases (in shares)							108,978
Share repurchases	(7.4)						$ (7.4)
Retirement of previously repurchased common stock (in shares)			(108,978)				(108,978)
Retirement of previously repurchased common stock	0.0			(1.3)	(6.1)		$ 7.4
Balance (in shares) at Mar. 31, 2019			41,194,169				21,786,251
Balance at Mar. 31, 2019	504.6		$ 0.4	575.5	957.7		$ (1,028.1)	(0.9)
Balance (in shares) at Dec. 31, 2018			41,065,069				21,719,339
Balance at Dec. 31, 2018	473.2	$ 0.0	$ 0.4	572.9	922.7	$ 0.2	$ (1,023.4)	0.6	$ (0.2)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	140.8
Balance (in shares) at Sep. 30, 2019			41,096,408				21,791,707
Balance at Sep. 30, 2019	600.0		$ 0.4	580.8	1,050.9		$ (1,028.6)	(3.5)
Balance (in shares) at Mar. 31, 2019			41,194,169				21,786,251
Balance at Mar. 31, 2019	504.6		$ 0.4	575.5	957.7		$ (1,028.1)	(0.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	54.9				54.9
Change in fair value of cash flow swaps, net of reclassification adjustment	(1.9)							(1.9)
Comprehensive income	53.0				54.9			(1.9)
Share-based compensation	2.9			2.9
Issuance of common stock, net of forfeitures in connection with share-based payment arrangements (in shares)			(3,656)
Issuance of common stock, net of forfeitures in connection with share-based payment arrangements	0.0
Repurchase of common stock associated with net share settlements of employee share-based awards (in shares)							3,328
Repurchase of common stock associated with net share settlements of employee share-based awards	(0.3)						$ (0.3)
Share repurchases (in shares)							50,436
Share repurchases	(3.9)						$ (3.9)
Retirement of previously repurchased common stock (in shares)			(50,436)				(50,436)
Retirement of previously repurchased common stock	0.0			(0.6)	(3.3)		$ 3.9
Balance (in shares) at Jun. 30, 2019			41,140,077				21,789,579
Balance at Jun. 30, 2019	556.3		$ 0.4	577.8	1,009.3		$ (1,028.4)	(2.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	45.0				45.0
Comprehensive income	(0.7)							(0.7)
Share-based compensation	3.6			3.6
Issuance of common stock, net of forfeitures in connection with share-based payment arrangements (in shares)			(704)
Issuance of common stock, net of forfeitures in connection with share-based payment arrangements	0.0
Repurchase of common stock associated with net share settlements of employee share-based awards (in shares)							2,128
Repurchase of common stock associated with net share settlements of employee share-based awards	(0.2)						$ (0.2)
Share repurchases (in shares)							42,965
Share repurchases	(4.0)						$ (4.0)
Retirement of previously repurchased common stock (in shares)			(42,965)				(42,965)
Retirement of previously repurchased common stock	0.0			(0.6)	(3.4)		$ 4.0
Balance (in shares) at Sep. 30, 2019			41,096,408				21,791,707
Balance at Sep. 30, 2019	600.0		$ 0.4	580.8	1,050.9		$ (1,028.6)	(3.5)
Balance (in shares) at Dec. 31, 2019			41,072,080				21,791,707
Balance at Dec. 31, 2019	646.3		$ 0.4	582.9	1,094.5		$ (1,028.6)	(2.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	19.5				19.5
Change in fair value of cash flow swaps, net of reclassification adjustment	(3.4)							(3.4)
Comprehensive income	16.1				19.5			(3.4)
Share-based compensation	3.8			3.8
Issuance of common stock, net of forfeitures in connection with share-based payment arrangements (in shares)			68,577
Issuance of common stock, net of forfeitures in connection with share-based payment arrangements	(0.3)			(0.3)
Repurchase of common stock associated with net share settlements of employee share-based awards (in shares)							53,915
Repurchase of common stock associated with net share settlements of employee share-based awards	(5.0)						$ (5.0)
Balance (in shares) at Mar. 31, 2020			41,140,657				21,845,622
Balance at Mar. 31, 2020	660.9		$ 0.4	586.4	1,114.0		$ (1,033.6)	(6.3)
Balance (in shares) at Dec. 31, 2019			41,072,080				21,791,707
Balance at Dec. 31, 2019	646.3		$ 0.4	582.9	1,094.5		$ (1,028.6)	(2.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	165.3
Balance (in shares) at Sep. 30, 2020			41,133,862				21,848,314
Balance at Sep. 30, 2020	811.9		$ 0.4	592.1	1,259.8		$ (1,033.7)	(6.7)
Balance (in shares) at Mar. 31, 2020			41,140,657				21,845,622
Balance at Mar. 31, 2020	660.9		$ 0.4	586.4	1,114.0		$ (1,033.6)	(6.3)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	49.6				49.6
Change in fair value of cash flow swaps, net of reclassification adjustment	(0.4)							(0.4)
Comprehensive income	49.2				49.6			(0.4)
Share-based compensation	3.1			3.1
Forfeitures in connection with share-based payment arrangements (in shares)			(2,916)
Forfeitures in connection with share-based payment arrangements	0.0
Repurchase of common stock associated with net share settlements of employee share-based awards (in shares)							2,552
Repurchase of common stock associated with net share settlements of employee share-based awards	(0.1)						$ (0.1)
Balance (in shares) at Jun. 30, 2020			41,137,741				21,848,174
Balance at Jun. 30, 2020	713.1		$ 0.4	589.5	1,163.6		$ (1,033.7)	(6.7)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	96.2				96.2
Comprehensive income	96.2				96.2			0.0
Share-based compensation	2.8			2.8
Issuance of common stock, net of forfeitures in connection with share-based payment arrangements	(0.2)			(0.2)
Forfeitures in connection with share-based payment arrangements (in shares)			(3,879)
Repurchase of common stock associated with net share settlements of employee share-based awards (in shares)							140
Repurchase of common stock associated with net share settlements of employee share-based awards	0.0
Balance (in shares) at Sep. 30, 2020			41,133,862				21,848,314
Balance at Sep. 30, 2020	$ 811.9		$ 0.4	$ 592.1	$ 1,259.8		$ (1,033.7)	$ (6.7)