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SCHEDULE IV - Reinsurance Reinsurance (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]      
Gross Amount $ 26,425.7 $ 23,111.2 $ 20,454.1
Ceded to Other Companies 695.8 637.2 555.0
Property, Liability and Casualty Insurance Product Line      
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]      
Gross Amount 26,425.7 23,111.2 20,454.1
Ceded to Other Companies 695.8 637.2 555.0
Assumed From Other Companies 0.0 0.0 0.0
Net Amount $ 25,729.9 $ 22,474.0 $ 19,899.1
Percentage of Amount Assumed to Net 0.00% 0.00% 0.00%