Income Taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income Tax Examination [Line Items]
Statutory rate	21.00%	35.00%	21.00%	35.00%	35.00%
Effect of tax cuts and jobs act - Deferred Federal Income Tax Expense (Benefit)	$ 0.2		$ (0.3)
Additions for tax positions of prior years			1.6	$ 0.1
Unrecognized tax benefits including interest and penalties that would affect the Company's effective tax rate if recognized	10.4	$ 5.3	10.4	5.3
Total accrued interest and penalties related to uncertain tax positions	4.3		4.3		$ 4.1
Increase (decrease) in income tax expense related to interest and penalties on unrecognized tax benefits	0.0	$ 0.1	(0.2)	$ (5.3)
Settlement with taxing authority and lapse in statute of limitations
Income Tax Examination [Line Items]
Reasonably possible decrease in unrecognized federal and state tax benefits within twelve months	$ (5.2)		$ (5.2)