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Securities - Amortized Cost and Estimated Fair Value (Detail) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Aug. 31, 2016
Schedule of Available-for-sale Securities [Line Items]      
Total, Face Amount   $ 522,391  
Total, Amortized Cost   514,599  
Total, Gross Unrealized Gain   292  
Total, Gross Unrealized Loss   (37,463)  
Total, Carrying Value/Estimated Fair Value   477,428  
Commercial Mortgage Backed Securities [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Fair Value Option, Amortized Cost     $ 86,676
Held to Maturity, Face Amount   146,530  
Held to Maturity, Amortized Cost   146,352  
Held to Maturity, Gross Unrealized Gain   0  
Held to Maturity, Gross Unrealized Loss   0  
Held to Maturity, Carrying Value/Estimated Fair Value   146,352  
Commercial Mortgage Backed Securities [Member] | Estimated Fair Value [Member] | Available-for-sale Securities [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Fair Value Option, Face Amount $ 222,522 375,861  
Fair Value Option, Amortized Cost 217,242 368,247  
Fair Value Option, Gross Unrealized Gain 247 292  
Fair Value Option, Gross Unrealized Loss (25,587) (37,463)  
Fair Value Option, Carrying Value/Estimated Fair Value $ 191,902 $ 331,076