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Securities - Additional Information (Detail)
$ in Thousands
Mar. 31, 2018
USD ($)
Investment
Dec. 31, 2017
USD ($)
Investment
Schedule of Investments [Line Items]    
Federal Home Loan Bank stock, at cost [1] $ 622,989 $ 603,819
Investment securities designated as having a continuous loss position for twelve months or more, unrealized losses $ 9,200 $ 10,100
Investment securities designated as having a continuous loss position for twelve months or more, percentage below collective amortized cost 7.80% 8.50%
Investment securities designated as having a continuous loss position for twelve months or more, amortized cost $ 118,200 $ 117,900
Capital trust notes    
Schedule of Investments [Line Items]    
Number of investment securities designated as having a continuous loss position for twelve months or more | Investment 5 5
Mortgage-Related Securities    
Schedule of Investments [Line Items]    
Number of investment securities designated as having a continuous loss position for twelve months or more | Investment 6 6
[1] Carrying value and estimated fair value are at cost.