Securities - Additional Information (Detail) $ in Thousands		12 Months Ended
		Dec. 31, 2017 USD ($) Investment	Dec. 31, 2016 USD ($) Investment		Dec. 31, 2015 USD ($)
Schedule of Investments [Line Items]
Federal Home Loan Bank stock, at cost	[1]	$ 603,819	$ 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	1,297		$ 44,104
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		$ 10,100	$ 14,100
Investment securities designated as having a continuous loss position for twelve months or more, percentage below collective amortized cost		8.50%	32.20%
Investment securities designated as having a continuous loss position for twelve months or more, amortized cost		$ 43,700	$ 118,200
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

[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).