Consolidated Statements of Changes in Equity - USD ($) $ in Millions	Total	Common Stock	Treasury Stock	APIC	Retained Earnings	AOCI
Balance at Dec. 31, 2016	$ 697.0	$ 0.5	$ (608.0)	$ 555.3	$ 749.2	$ 0.0
Balance (in shares) at Dec. 31, 2016		46,767,164
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	245.3				245.3
Purchase of treasury stock	(206.0)		(206.0)
Issuance of common stock	0.0	$ 0.0
Issuance of treasury stock	0.1		7.5	(7.4)
Amounts related to share-based compensation	(5.6)			(5.6)
Share-based compensation expense	7.6			7.6
Balance at Dec. 31, 2017	738.4	$ 0.5	(806.5)	549.9	994.5	0.0
Balance (in shares) at Dec. 31, 2017		46,767,164
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	213.6				213.6
Purchase of treasury stock	(144.4)		(144.4)
Issuance of common stock	0.0	$ 0.0
Issuance of treasury stock			10.6	(10.6)
Amounts related to share-based compensation	(9.4)			(9.4)
Share-based compensation expense	9.1			9.1
Balance at Dec. 31, 2018	$ 807.3	$ 0.5	(940.3)	539.0	1,208.1	0.0
Balance (in shares) at Dec. 31, 2018	46,767,164	46,767,164
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 154.8				154.8
Gain on interest rate hedge, net of tax	0.7					0.7
Purchase of treasury stock	(165.8)		(165.8)
Issuance of common stock	0.0	$ 0.0
Issuance of treasury stock			6.3	(6.3)
Amounts related to share-based compensation	(4.5)			(4.5)
Share-based compensation expense	10.5			10.5
Balance at Dec. 31, 2019	$ 803.0	$ 0.5	$ (1,099.8)	$ 538.7	$ 1,362.9	$ 0.7
Balance (in shares) at Dec. 31, 2019	46,767,164	46,767,164