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Securities - Additional Information (Detail)
$ in Thousands
 6 Months Ended
Jun. 30, 2017
USD ($)
Investment
Dec. 31, 2016
USD ($)
Investment
Schedule of Investments [Line Items]    
Federal Home Loan Bank stock, at cost [1] $ 589,067 $ 590,934
Amortized cost from sales of held to maturity securities 521,000 3,721,368 [2]
Proceeds from sales of held to maturity securities 547,925  
Gross realized gains from sales of held to maturity securities 26,900  
Securities transferred from held to maturity to available for sale 3,040,305  
Unrealized gain on securities transferred from held to maturity to available for sale 82,800  
Investment securities designated as having a continuous loss position for twelve months or more, unrealized losses $ 11,300 $ 14,100
Investment securities designated as having a continuous loss position for twelve months or more, percentage below collective amortized cost 25.90% 32.20%
Investment securities designated as having a continuous loss position for twelve months or more, amortized cost $ 43,700 $ 43,700
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
[1] Carrying value and estimated fair value are at cost.
[2] Held-to-maturity securities are reported at a carrying amount equal to amortized cost less the non-credit portion of OTTI recorded in AOCL. At December 31, 2016, the non-credit portion of OTTI recorded in AOCL was $8.6 million (before taxes).