Consolidated Statements of Changes in Equity - USD ($) $ in Millions	Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Treasury Stock	APIC	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	AOCI
Beginning balance (in shares) at Dec. 31, 2018			46,767,164
Beginning balance at Dec. 31, 2018	$ 807.3		$ 0.5	$ (940.3)	$ 539.0	$ 1,208.1		$ 0.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	5.3					5.3
Loss on interest rate hedge, net of tax	0.0
Purchase of treasury stock	(13.3)			(13.3)
Issuance of treasury stock	0.0			5.6	(5.6)
Amounts related to share-based compensation	(4.1)				(4.1)
Share-based compensation expense	2.6				2.6
Ending balance (in shares) at Mar. 31, 2019			46,767,164
Ending balance at Mar. 31, 2019	797.8		$ 0.5	(948.0)	531.9	1,213.4		0.0
Beginning balance (in shares) at Dec. 31, 2018			46,767,164
Beginning balance at Dec. 31, 2018	807.3		$ 0.5	(940.3)	539.0	1,208.1		0.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	38.0
Ending balance (in shares) at Jun. 30, 2019			46,767,164
Ending balance at Jun. 30, 2019	816.3		$ 0.5	(964.7)	534.4	1,246.1		0.0
Beginning balance (in shares) at Mar. 31, 2019			46,767,164
Beginning balance at Mar. 31, 2019	797.8		$ 0.5	(948.0)	531.9	1,213.4		0.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	32.7					32.7
Loss on interest rate hedge, net of tax	0.0
Purchase of treasury stock	(16.8)			(16.8)
Issuance of treasury stock	0.0			0.1	(0.1)
Amounts related to share-based compensation	(0.2)				(0.2)
Share-based compensation expense	2.8				2.8
Ending balance (in shares) at Jun. 30, 2019			46,767,164
Ending balance at Jun. 30, 2019	816.3		$ 0.5	(964.7)	534.4	1,246.1		0.0
Beginning balance (in shares) at Dec. 31, 2019			46,767,164
Beginning balance at Dec. 31, 2019	803.0	$ 1.1	$ 0.5	(1,099.8)	538.7	1,362.9	$ 1.1	0.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	89.3					89.3
Loss on interest rate hedge, net of tax	(2.7)							(2.7)
Purchase of treasury stock	(140.6)			(140.6)
Issuance of treasury stock	(0.5)			5.2	(5.7)
Amounts related to share-based compensation	(5.6)				(5.6)
Share-based compensation expense	2.8				2.8
Ending balance (in shares) at Mar. 31, 2020			46,767,164
Ending balance at Mar. 31, 2020	746.8		$ 0.5	(1,235.2)	530.2	1,453.3		(2.0)
Beginning balance (in shares) at Dec. 31, 2019			46,767,164
Beginning balance at Dec. 31, 2019	803.0	$ 1.1	$ 0.5	(1,099.8)	538.7	1,362.9	$ 1.1	0.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	258.2
Ending balance (in shares) at Jun. 30, 2020			46,767,164
Ending balance at Jun. 30, 2020	918.0		$ 0.5	(1,235.2)	533.0	1,622.2		(2.5)
Beginning balance (in shares) at Mar. 31, 2020			46,767,164
Beginning balance at Mar. 31, 2020	746.8		$ 0.5	(1,235.2)	530.2	1,453.3		(2.0)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	168.9					168.9		(0.5)
Loss on interest rate hedge, net of tax	(0.5)							(0.5)
Purchase of treasury stock	0.0			0.0
Issuance of treasury stock				0.0
Amounts related to share-based compensation	0.0				0.0
Share-based compensation expense	2.8				2.8
Ending balance (in shares) at Jun. 30, 2020			46,767,164
Ending balance at Jun. 30, 2020	$ 918.0		$ 0.5	$ (1,235.2)	$ 533.0	$ 1,622.2		$ (2.5)