Income Taxes - Effective Tax Reconciliation (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
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									$ 14.9	$ 16.0	$ 25.3
Permanent differences									0.8	0.2	0.3
State taxes, net of federal benefit									4.5	6.3	2.5
Effect of foreign tax rates									0.6	0.6	0.0
Effect of 2017 Tax Act									0.0	(1.5)	(1.0)
Noncontrolling interest									(0.6)	(0.7)	(1.1)
Current year adjustment to deferred taxes									0.8	0.4	1.6
Recognition of tax credits									(1.8)	(2.7)	(1.5)
Other									0.0	0.0	1.1
Income tax expense before change in valuation allowance									19.2	18.6	27.2
Change in valuation allowance									0.2	1.0	0.5
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
Effective tax rate									27.30%	25.80%	38.40%
Income tax expense related to increase in valuation allowance											$ 1.2