Income Taxes - Income Tax Components (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Components of income before taxes
United States									$ 69.7	$ 74.9	$ 70.0
Foreign									1.4	1.1	2.2
Earnings before income taxes	$ 14.3	$ 20.7	$ 22.1	$ 14.0	$ 12.5	$ 20.1	$ 22.2	$ 21.2	71.1	76.0	72.2
Current provision
U.S. federal									9.6	9.9	21.5
Foreign									0.9	1.0	1.0
State									4.7	7.4	3.3
Total current									15.2	18.3	25.8
Deferred provision
U.S. federal									2.9	1.3	2.6
Foreign									(0.1)	(0.3)	0.6
State									1.4	0.3	(1.3)
Total deferred									4.2	1.3	1.9
Income tax (benefit) expense	$ 4.8	$ 5.7	$ 5.8	$ 3.1	$ 2.7	$ 5.6	$ 6.0	$ 5.3	$ 19.4	$ 19.6	$ 27.7