Income Taxes - Effective Tax Reconciliation (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
Reconciliation of income tax provision (benefit) to the amount computed by multiplying book income/(loss) before income taxes by federal income tax rate
Tax at statutory rate									$ 25.3	$ 14.6	$ 8.8
Permanent differences									0.3	0.8	1.4
State taxes, net of federal benefit									2.5	1.3	0.3
Effect of foreign tax rates									0.0	0.0	(0.1)
Effect of 2017 Tax Act									(1.0)	0.0	0.0
Minority interest									(1.1)	(1.0)	(1.0)
Current year adjustment to deferred taxes									1.6	1.3	1.5
Recognition of tax credits									(1.5)	(1.4)	(1.2)
Other									1.1	0.4	0.6
Income tax expense before change in valuation allowance									27.2	16.0	10.3
Change in valuation allowance									0.5	(0.2)	(5.5)
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
Effective tax rate									38.40%	37.80%	19.10%
Income tax expense related to increase in valuation allowance									$ 1.2