Income Taxes - Income Tax Components (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of income before taxes
United States									$ 70.0	$ 38.9	$ 21.7
Foreign									2.2	2.9	3.4
Earnings before income taxes	$ 14.9	$ 19.3	$ 28.0	$ 10.0	$ 15.2	$ 12.8	$ 12.3	$ 1.5	72.2	41.8	25.1
Current provision
U.S. federal									21.5	13.9	11.5
Foreign									1.0	1.4	1.2
State									3.3	2.6	1.8
Total current									25.8	17.9	14.5
Deferred provision
U.S. federal									2.6	(2.5)	(4.9)
Foreign									0.6	(0.4)	0.1
State									(1.3)	0.8	(4.9)
Total deferred									1.9	(2.1)	(9.7)
Income tax (benefit) expense	$ 6.4	$ 7.3	$ 10.7	$ 3.3	$ 4.9	$ 5.1	$ 4.9	$ 0.9	$ 27.7	$ 15.8	$ 4.8