Securities - Additional Information (Detail) $ in Thousands		Jun. 30, 2018 USD ($) Investment	Dec. 31, 2017 USD ($) Investment
Schedule of Investments [Line Items]
Federal Home Loan Bank stock, at cost | $	[1]	$ 653,075	$ 603,819
Investment securities designated as having a continuous loss position for twelve months or more, unrealized losses | $		$ 9,800	$ 10,100
Investment securities designated as having a continuous loss position for twelve months or more, percentage below collective amortized cost		7.60%	8.50%
Investment securities designated as having a continuous loss position for twelve months or more, amortized cost | $		$ 128,500	$ 118,200
Mortgage-Related Securities
Schedule of Investments [Line Items]
Number of investment securities designated as having a continuous loss position for twelve months or more		6	6
Capital trust notes
Schedule of Investments [Line Items]
Number of investment securities designated as having a continuous loss position for twelve months or more		5	5
Municipal bonds
Schedule of Investments [Line Items]
Number of investment securities designated as having a continuous loss position for twelve months or more		3	3
Mutual funds
Schedule of Investments [Line Items]
Number of investment securities designated as having a continuous loss position for twelve months or more		1	1

[1] Carrying value and estimated fair value are at cost.