Consolidated Statements of Stockholders' Equity - USD ($) shares in Millions, $ in Millions	Common Stock	Additional Paid In Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Total
Balance at the beginning of the period at Dec. 31, 2016	$ 32.1	$ 289.8	$ (23.6)	$ 196.6	$ 494.9
Balance (in shares) at Dec. 31, 2016	42.9
Increase (Decrease) in Stockholders' Equity
Net income				7.0	7.0
Other comprehensive income (loss), net of tax			0.7		0.7
Share-based compensation		2.6			2.6
Shares issued	$ 0.2	(6.9)			(6.7)
Shares issued (in shares)	0.3
Shares repurchased		(0.3)		(0.5)	(0.8)
Dividends				(8.3)	(8.3)
Balance at the end of the period at Mar. 31, 2017	$ 32.3	285.2	(22.9)	194.8	489.4
Balance (in shares) at Mar. 31, 2017	43.2
Balance at the beginning of the period at Dec. 31, 2016	$ 32.1	289.8	(23.6)	196.6	494.9
Balance (in shares) at Dec. 31, 2016	42.9
Increase (Decrease) in Stockholders' Equity
Net income					65.1
Other comprehensive income (loss), net of tax					1.2
Balance at the end of the period at Sep. 30, 2017	$ 31.8	285.8	(22.4)	223.4	518.6
Balance (in shares) at Sep. 30, 2017	42.5
Balance at the beginning of the period at Mar. 31, 2017	$ 32.3	285.2	(22.9)	194.8	489.4
Balance (in shares) at Mar. 31, 2017	43.2
Increase (Decrease) in Stockholders' Equity
Net income				24.0	24.0
Other comprehensive income (loss), net of tax			1.0		1.0
Share-based compensation		2.4		(0.1)	2.3
Shares issued		(0.1)			(0.1)
Shares repurchased		(0.1)		(0.3)	(0.4)
Dividends				(17.0)	(17.0)
Balance at the end of the period at Jun. 30, 2017	$ 32.3	287.4	(21.9)	201.4	499.2
Balance (in shares) at Jun. 30, 2017	43.2
Increase (Decrease) in Stockholders' Equity
Net income				34.1	34.1
Other comprehensive income (loss), net of tax			(0.5)		(0.5)
Share-based compensation		2.9			2.9
Shares repurchased	$ (0.5)	(4.5)		(12.1)	(17.1)
Shares repurchased (in shares)	(0.7)
Balance at the end of the period at Sep. 30, 2017	$ 31.8	285.8	(22.4)	223.4	518.6
Balance (in shares) at Sep. 30, 2017	42.5
Balance at the beginning of the period at Dec. 31, 2017	$ 31.9	289.7	(24.9)	381.5	678.2
Balance (in shares) at Dec. 31, 2017	42.5
Increase (Decrease) in Stockholders' Equity
Net income				14.2	14.2
Other comprehensive income (loss), net of tax			0.2		0.2
Reclassification resulting from adoption of new accounting pronouncements (see Note 2)			(6.0)	6.0
Share-based compensation		2.7			2.7
Shares issued	$ 0.1	(4.3)			(4.2)
Shares issued (in shares)	0.2
Dividends				(8.7)	(8.7)
Balance at the end of the period at Mar. 31, 2018	$ 32.0	288.1	(30.7)	393.0	682.4
Balance (in shares) at Mar. 31, 2018	42.7
Balance at the beginning of the period at Dec. 31, 2017	$ 31.9	289.7	(24.9)	381.5	678.2
Balance (in shares) at Dec. 31, 2017	42.5
Increase (Decrease) in Stockholders' Equity
Net income					88.4
Other comprehensive income (loss), net of tax					(0.6)
Balance at the end of the period at Sep. 30, 2018	$ 32.0	293.7	(31.5)	449.5	743.7
Balance (in shares) at Sep. 30, 2018	42.7
Balance at the beginning of the period at Mar. 31, 2018	$ 32.0	288.1	(30.7)	393.0	682.4
Balance (in shares) at Mar. 31, 2018	42.7
Increase (Decrease) in Stockholders' Equity
Net income				32.6	32.6
Other comprehensive income (loss), net of tax			(0.9)		(0.9)
Share-based compensation		2.8			2.8
Shares issued		0.3			0.3
Dividends				(17.7)	(17.7)
Balance at the end of the period at Jun. 30, 2018	$ 32.0	291.2	(31.6)	407.9	699.5
Balance (in shares) at Jun. 30, 2018	42.7
Increase (Decrease) in Stockholders' Equity
Net income				41.6	41.6
Other comprehensive income (loss), net of tax			0.1		0.1
Share-based compensation		2.7			2.7
Shares issued		(0.2)			(0.2)
Balance at the end of the period at Sep. 30, 2018	$ 32.0	$ 293.7	$ (31.5)	$ 449.5	$ 743.7
Balance (in shares) at Sep. 30, 2018	42.7