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				231.0	231.0
Other comprehensive income (loss), net of tax			(1.3)		(1.3)
Share-based compensation		11.1			11.1
Shares issued, net of shares withheld for employee taxes	$ 0.3	(5.7)			(5.4)
Shares issued, net of shares withheld for employee taxes (in shares)	0.3
Shares repurchased	$ (0.5)	(5.5)		(13.3)	(19.3)
Shares repurchased (in shares)	(0.7)
Dividends				(33.8)	(33.8)
Balance at the end 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				109.0	109.0
Other comprehensive income (loss), net of tax			(9.6)	6.0	(3.6)
Share-based compensation		12.1			12.1
Shares issued, net of shares withheld for employee taxes	$ 0.1	(4.0)			(3.9)
Shares issued, net of shares withheld for employee taxes (in shares)	0.2
Shares repurchased				(0.1)	(0.1)
Dividends				(35.4)	(35.4)
Balance at the end 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				42.7
Increase (Decrease) in Stockholders' Equity
Net income				82.7	$ 82.7
Adoption of new lease accounting standard (see Note 9) | ASU 2016-02 New Lease Accounting Standard				4.4	4.4
Other comprehensive income (loss), net of tax			(2.4)		(2.4)
Share-based compensation		11.3			11.3
Shares issued, net of shares withheld for employee taxes	$ 0.2	(2.9)			(2.7)
Shares issued, net of shares withheld for employee taxes (in shares)	0.2
Dividends				(37.3)	(37.3)
SSAT's adoption of new lease accounting standard (see Note 4) | ASU 2016-02 New Lease Accounting Standard				(5.6)	(5.6)
Balance at the end 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				42.9