Condensed 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, 2017	$ 31.9	$ 289.7	$ (24.9)	$ 380.5	$ 677.2
Balance (in shares) at Dec. 31, 2017	42.5
Increase (Decrease) in Stockholders' Equity
Adoption of new lease accounting standard			(6.0)	6.0
Net income				14.2	14.2
Other comprehensive income (loss), net of tax			0.2		0.2
Share-based compensation		2.7			2.7
Shares issued, net of shares withheld for employee taxes	$ 0.1	(4.3)			(4.2)
Shares issued, net of shares withheld for employee taxes (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)	392.0	681.4
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)	380.5	677.2
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)	448.5	742.7
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)	392.0	681.4
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, net of shares withheld for employee taxes		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)	406.9	698.5
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, net of shares withheld for employee taxes		(0.2)			(0.2)
Balance at the end of the period at Sep. 30, 2018	$ 32.0	293.7	(31.5)	448.5	742.7
Balance (in shares) at Sep. 30, 2018	42.7
Balance at the beginning of the period at Dec. 31, 2018	$ 32.0	297.8	(34.5)	460.0	755.3
Balance (in shares) at Dec. 31, 2018	42.7
Increase (Decrease) in Stockholders' Equity
Adoption of new lease accounting standard				4.4	4.4
Net income				12.5	12.5
Other comprehensive income (loss), net of tax			(0.2)		(0.2)
Share-based compensation		3.2			3.2
Shares issued, net of shares withheld for employee taxes	$ 0.1	(3.2)			(3.1)
Shares issued, net of shares withheld for employee taxes (in shares)	0.1
Dividends				(9.1)	(9.1)
Balance at the end of the period at Mar. 31, 2019	$ 32.1	297.8	(34.7)	467.8	763.0
Balance (in shares) at Mar. 31, 2019	42.8
Balance at the beginning of the period at Dec. 31, 2018	$ 32.0	297.8	(34.5)	460.0	755.3
Balance (in shares) at Dec. 31, 2018	42.7
Increase (Decrease) in Stockholders' Equity
Net income					67.1
Other comprehensive income (loss), net of tax					(1.2)
Balance at the end of the period at Sep. 30, 2019	$ 32.2	303.2	(35.7)	498.0	797.7
Balance (in shares) at Sep. 30, 2019	42.9
Balance at the beginning of the period at Mar. 31, 2019	$ 32.1	297.8	(34.7)	467.8	763.0
Balance (in shares) at Mar. 31, 2019	42.8
Increase (Decrease) in Stockholders' Equity
Net income				18.4	18.4
Other comprehensive income (loss), net of tax			(0.5)		(0.5)
Share-based compensation		3.0			3.0
Shares issued, net of shares withheld for employee taxes		(0.1)			(0.1)
Shares issued, net of shares withheld for employee taxes (in shares)	0.1
Dividends				(18.6)	(18.6)
Balance at the end of the period at Jun. 30, 2019	$ 32.1	300.7	(35.2)	467.6	765.2
Balance (in shares) at Jun. 30, 2019	42.9
Increase (Decrease) in Stockholders' Equity
Adoption of new lease accounting standard				(5.6)	(5.6)
Net income				36.2	36.2
Other comprehensive income (loss), net of tax			(0.5)		(0.5)
Share-based compensation		2.5			2.5
Shares issued, net of shares withheld for employee taxes	$ 0.1				0.1
Dividends				(0.2)	(0.2)
Balance at the end of the period at Sep. 30, 2019	$ 32.2	$ 303.2	$ (35.7)	$ 498.0	$ 797.7
Balance (in shares) at Sep. 30, 2019	42.9