Income Tax Matters, Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income tax provision	$ 8.2	$ 16.1	$ 21.9	$ 36.8
Effective tax rate (percent)	27.30%	44.70%	24.30%	37.80%
Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Amount			$ (2.0)	$ (1.7)
Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Percent			(2.30%)	(1.70%)
Effective tax rate reconciliation, increase (decrease) to the valuation allowance for certain state net operating losses	$ (0.4)		$ (0.9)	$ (0.5)
Effective tax rate reconciliation, increase (decrease) to the valuation allowance for certain state net operating losses (percent)	(1.30%)		(1.00%)	(0.60%)
Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount		$ 2.5		$ 2.5
Effective Income Tax Rate Reconciliation,Other Reconciling Items, Percent		7.10%		2.60%
Effective Income Tax Rate Reconciliation, Unrecognized Tax Benefit Amount				$ (0.2)
Effective Income Tax Rate Reconciliation Unrecognized Tax Benefit Percent				(0.20%)
Effective Income Tax Rate Reconciliation Sequestration of AMT Credits Amount	$ 0.2		$ 0.6
Effective Income Tax Rate Reconciliation Sequestration of AMT Credits Percent	0.70%		0.60%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amount	$ 0.6		$ 1.7
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Percent	1.90%		1.90%
Effective Income Tax Rate Reconciliation Advance Pricing Agreement, Amount	$ 0.3		$ 0.3
Effective Income Tax Rate Reconciliation Advance Pricing Agreement, Percent	1.00%		0.30%
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