Income Taxes (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Sep. 30, 2017 USD ($) item	Sep. 30, 2016 USD ($)	Dec. 31, 2017
Effective income tax rate			37.00%	32.00%
Estimated loss contingency accrual expense			$ 210.0
Impairment charges	$ 3.1		$ 49.6
Number of investments held at cost for which impairment charge recorded | item			2
Deferred income tax asset recorded on non-deductible impairment expenses	18.3		$ 18.3
Effective income tax rate impacted for valuation allowance			3.00%
Uncertain tax positions	0.5		$ 0.5	$ 0.5
Positions of uncertain tax, if realized, would impact the effective tax rate	0.3		0.3	0.3
Accrued interest expense related to uncertain tax positions	0.0		$ 0.0	$ 0.0
Forecast
Effective income tax rate impacted for valuation allowance					2.00%
U.S. Department of Justice
Estimated loss contingency accrual expense		$ 210.0
Tax benefit related to accrual	$ 57.0	$ 0.0