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Reserves for Losses and Loss Adjustment Expenses - Narrative (Details) - USD ($)
$ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Insurance [Abstract]      
Reserve releases $ (80.2) $ (74.9) $ (105.4)
Amount of reserves ceded     $ 125.5