Income Tax Matters, Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income tax provision	$ 7.8	$ 2.2	$ 13.7	$ 20.7
Effective tax rate (percent)	27.50%	31.70%	22.80%	33.80%
Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Amount	$ (0.3)	$ (0.1)	$ (2.0)	$ (1.7)
Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Percent	(0.90%)	(1.50%)	(3.40%)	(2.70%)
Effective tax rate reconciliation, increase (decrease) to the valuation allowance for certain state net operating losses	$ 0.3	$ (0.1)	$ (0.5)	$ (0.5)
Effective tax rate reconciliation, increase (decrease) to the valuation allowance for certain state net operating losses (percent)	1.20%	(1.40%)	(0.80%)	(0.90%)
Effective Income Tax Rate Reconciliation, Unrecognized Tax Benefit Amount		$ (0.2)		$ (0.2)
Effective Income Tax Rate Reconciliation Unrecognized Tax Benefit Percent		(3.00%)		(0.30%)
Effective Income Tax Rate Reconciliation Sequestration of AMT Credits Amount	$ 0.2		$ 0.4
Effective Income Tax Rate Reconciliation Sequestration of AMT Credits Percent	0.70%		0.60%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amount	$ 0.6		$ 1.2
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Percent	2.10%		2.00%
Gross unrecognized tax benefits	$ 1.5		$ 1.5		$ 1.5
Gross unrecognized tax benefits that would impact effective tax rate	$ 0.4		$ 0.4		$ 0.4