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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
Valuation Allowance [Line Items]          
Tax expense (benefit) $ (1.2) $ 2.3 $ 2.7 $ 3.8  
Federal income tax at statutory rate (as a percent)     21.00%   35.00%
Unrecognized tax benefits 15.8   $ 15.8   $ 19.5
2012 and 2013 Tax Years          
Valuation Allowance [Line Items]          
Discrete tax benefit $ 4.8