Income Taxes - Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Taxes [Line Items]
Federal income tax provision at the statutory rate									$ 17.5	$ 16.2	$ 12.6
Increase (decrease) resulting from:
State income taxes, net of federal benefit									1.1	1.9	2.9
Foreign tax rate differential									0.3	0.1	0.0
U.S. branch taxes									2.5	(0.1)	0.5
Excess tax benefits from stock-based award payments									0.2	0.1	0.2
Change in valuation allowance									(1.1)	1.7	0.0
Other tax credits									(0.9)	(1.2)	(1.2)
Other, net									0.5	1.0	(0.6)
Income tax provision	$ 3.4	$ 9.2	$ 5.7	$ 1.8	$ 4.6	$ 8.1	$ 6.5	$ 0.5	$ 20.1	$ 19.7	$ 14.4
Federal income tax provision at the statutory rate									21.00%	21.00%	21.00%
Increase (decrease) resulting from:
State income taxes, net of federal benefit									1.30%	2.50%	4.80%
Foreign tax rate differential									0.40%	0.10%	0.00%
U.S. branch taxes									3.00%	(0.10%)	0.80%
Excess tax benefits from stock-based award payments									0.20%	0.10%	0.30%
Change in valuation allowance									(1.30%)	2.20%	0.00%
Other tax credits									(1.10%)	(1.60%)	(2.00%)
Other, net									0.60%	1.30%	(0.90%)
Effective tax rate									24.10%	25.50%	24.00%
Cumulative Effect, Period of Adoption, Adjustment
Increase (decrease) resulting from:
Excess tax benefits from stock-based award payments									$ 0.2