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Junior Subordinated Debentures - Schedule of Maybrooke Junior Subordinated Debentures (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Principal $ 143.8 $ 143.8
Junior Subordinated Debentures, Maybrooke [Member]    
Debt Instrument [Line Items]    
Issue Date Sep. 15, 2007  
Maturity Sep. 15, 2037  
Rate Structure 3M LIBOR + 3.15%  
Interest Rate 4.74%  
Principal $ 91.8  
Carrying Value $ 83.9