Income Taxes - Effective Income Tax Rate 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, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Taxes [Line Items]
Federal income tax provision at the statutory rate										$ 16.2	$ 12.6	$ 9.9
Increase (decrease) resulting from:
State income taxes, net of federal benefit										2.7	2.6	1.6
Reduction in federal statutory rate									$ 0.0	0.0		(14.6)
Decrease in unrecognized tax benefits (inclusive of related interest and penalty)										0.0	0.0	(0.3)
Effect of foreign operations										0.1	0.5	0.1
Excess tax benefits from stock-based award payments										(0.1)	(0.2)	1.5
Change in valuation allowance										1.7	0.0	0.0
Tax credits										(1.1)	(0.7)	(0.4)
Adjustments of prior years’ estimates										(0.7)	(0.5)	(0.4)
Other, net										0.7	(0.3)	0.5
Income tax provision (benefit)	$ 4.6	$ 8.1	$ 6.5	$ 0.5	$ 3.1	$ 7.4	$ 4.4	$ (0.5)		$ 19.7	$ 14.4	$ (5.1)
Federal income tax provision at the statutory rate										21.00%	21.00%	35.00%
Increase (decrease) resulting from:
State income taxes, net of federal benefit										3.50%	4.30%	5.70%
Reduction in federal statutory rate										0.00%	0.00%	(51.60%)
Decrease in unrecognized tax benefits (inclusive of related interest and penalty)										0.00%	0.00%	(1.00%)
Effect of foreign operations										0.10%	0.80%	0.40%
Excess tax benefits from stock-based award payments										0.10%	0.30%	(5.30%)
Change in valuation allowance										2.20%	0.00%	0.00%
Tax credits										(1.40%)	(1.20%)	(1.40%)
Adjustments of prior years’ estimates										(0.90%)	(0.80%)	(1.40%)
Other, net										0.90%	(0.40%)	1.60%
Effective tax rate										25.50%	24.00%	(18.00%)
Net income tax benefit									$ 14.6
Accounting Standards Update 2016-09
Increase (decrease) resulting from:
Excess tax benefits from stock-based award payments										$ 0.1		$ (1.5)