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Reserves for Losses and Adjustment Expenses - Additional Information (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Insurance [Abstract]      
Provision for losses and LAE for claims incurred $ 105.4 $ 129.3 $ 156.5
Additional loss reserves assumed   5.7  
Liability for unpaid claims and claims adjustment expense amount ceded $ (125.5) $ (85.8) $ 0.0