Income Tax Matters, Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income tax provision	$ 2.2	$ 15.7	$ 20.7	$ 30.8
Effective tax rate (percent)	31.70%	37.70%	33.80%	37.10%
Effective tax rate reconciliation, decrease due to recognition of excess tax benefits from stock-based compensation	$ (0.1)		$ (1.7)
Effective tax rate reconciliation, decrease due to recognition of excess tax benefits from stock-based compensation (percent)	(1.50%)		(2.70%)
Effective tax rate reconciliation, decrease to the valuation allowance for certain state net operating losses	$ (0.1)		$ (0.5)
Effective tax rate reconciliation, decrease to the valuation allowance for certain state net operating losses (percent)	(1.40%)		(0.90%)
Effective tax rate reconciliation, decrease related to unrecognized tax benefits , including interest and penalties	$ (0.2)		$ (0.2)
Effective tax rate reconciliation, decrease related to unrecognized tax benefits , including interest and penalties (percent)	(3.00%)		(0.30%)
Gross unrecognized tax benefits	$ 1.5		$ 1.5		$ 1.8
Gross unrecognized tax benefits that would impact effective tax rate	$ 0.4		$ 0.4		$ 0.7