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Reserves for Losses and Loss Adjustment Expenses - Narrative (Details) - USD ($)
 3 Months Ended 12 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Dec. 31, 2015
Insurance [Abstract]        
Reserve releases $ (26,200,000) $ (21,600,000) $ (129,300,000)  
Liability for Claims and Claims Adjustment Expense [Line Items]        
Loss portfolio transfers (disposed) $ 0 5,700,000.0 (80,100,000)  
Liability For Unpaid Claims And Claims Adjustment Expense Amount Disposed of or Ceded     $ 85,800,000 $ 0
AgriLogic        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Loss portfolio transfers (disposed)   $ 5,700,000