Income Taxes - Income Taxes (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Examination [Line Items]
Goodwill, impairment loss	$ 358.2
Non-deductibility of goodwill impairment		29.50%	0.00%	0.00%
Tax cuts and jobs act of 2017, change in tax rate, income tax expense (benefit)		18.20%
Effective tax rate, decrease from change in state tax rates, percent		1.90%
Valuation allowance, noncurrent		$ 0.0	$ 1.1
Deferred tax assets, net		1.1
Operating loss carryforwards		0.6	10.7
Unrecognized tax benefits		5.9	3.9	$ 3.9	$ 3.4
Unrecognized tax benefit changes in next 12 months		2.9
Unrecognized tax benefits, interest on income taxes accrued		1.1	1.0
Unrecognized tax benefits, income tax penalties accrued		0.1	$ 0.1
Change in accrued interest related to increase in unrecognized tax benefits and penalties		0.1
Domestic Tax Authority [Member] | Internal Revenue Service (IRS) [Member]
Income Tax Examination [Line Items]
Income tax examination, increase (decrease) in deferred tax asset		$ 30.4