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 Cumulative Effect, Period of Adoption, Adjustment SSAT	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Retained Earnings	Cumulative Effect, Period of Adoption, Adjustment SSAT	Cumulative Effect, Period of Adoption, Adjustment	Total
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						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)		$ 4.4	467.8		$ 4.4	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)	$ (5.6)		498.0	$ (5.6)		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)		$ 4.4	467.8		$ 4.4	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
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)	$ (5.6)		498.0	$ (5.6)		797.7
Balance (in shares) at Sep. 30, 2019	42.9
Balance at the beginning of the period at Dec. 31, 2019	$ 32.2	306.2	(36.9)			504.2			805.7
Balance (in shares) at Dec. 31, 2019	42.9
Increase (Decrease) in Stockholders' Equity
Net income						3.8			3.8
Other comprehensive income (loss), net of tax			(0.6)						(0.6)
Share-based compensation		3.1							3.1
Shares issued, net of shares withheld for employee taxes	$ 0.1	(4.6)							(4.5)
Shares issued, net of shares withheld for employee taxes (in shares)	0.1
Equity interest in SSAT						2.2			2.2
Dividends						(9.5)			(9.5)
Balance at the end of the period at Mar. 31, 2020	$ 32.3	304.7	(37.5)			500.7			800.2
Balance (in shares) at Mar. 31, 2020	43.0
Balance at the beginning of the period at Dec. 31, 2019	$ 32.2	306.2	(36.9)			504.2			805.7
Balance (in shares) at Dec. 31, 2019	42.9
Increase (Decrease) in Stockholders' Equity
Net income									107.5
Other comprehensive income (loss), net of tax									(1.0)
Balance at the end of the period at Sep. 30, 2020	$ 32.3	312.6	(37.9)			584.8			891.8
Balance (in shares) at Sep. 30, 2020	43.1
Balance at the beginning of the period at Mar. 31, 2020	$ 32.3	304.7	(37.5)			500.7			800.2
Balance (in shares) at Mar. 31, 2020	43.0
Increase (Decrease) in Stockholders' Equity
Net income						32.8			32.8
Other comprehensive income (loss), net of tax			0.1						0.1
Share-based compensation		3.0							3.0
Shares issued, net of shares withheld for employee taxes		(1.0)							(1.0)
Shares issued, net of shares withheld for employee taxes (in shares)	0.1
Dividends						(19.6)			(19.6)
Balance at the end of the period at Jun. 30, 2020	$ 32.3	306.7	(37.4)			513.9			815.5
Balance (in shares) at Jun. 30, 2020	43.1
Increase (Decrease) in Stockholders' Equity
Net income						70.9			70.9
Other comprehensive income (loss), net of tax			(0.5)						(0.5)
Share-based compensation		6.0							6.0
Shares issued, net of shares withheld for employee taxes		(0.1)							(0.1)
Balance at the end of the period at Sep. 30, 2020	$ 32.3	$ 312.6	$ (37.9)			$ 584.8			$ 891.8
Balance (in shares) at Sep. 30, 2020	43.1