Securities - Additional Information (Detail) $ in Thousands		3 Months Ended	9 Months Ended
		Sep. 30, 2019 USD ($) Investment	Sep. 30, 2019 USD ($) Investment	Dec. 31, 2018 USD ($) Investment
Schedule of Investments [Line Items]
Federal Home Loan Bank stock, at cost | $	[1]	$ 606,371	$ 606,371	$ 644,590
Investment securities designated as having a continuous loss position for twelve months or more, unrealized losses | $		$ 8,400	$ 8,400	$ 22,500
Investment securities designated as having a continuous loss position for twelve months or more, percentage below collective amortized cost		2.80%	2.80%	2.90%
Investment securities designated as having a continuous loss position for twelve months or more, amortized cost | $		$ 299,400	$ 299,400	$ 782,400
Net unrealised gains losses on equity securities recognised | $		$ 275,000	$ 2,300
Mortgage-related Securities
Schedule of Investments [Line Items]
Number of investment securities designated as having a continuous loss position for twelve months or more		3	3	9
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	5
Municipal bonds
Schedule of Investments [Line Items]
Number of investment securities designated as having a continuous loss position for twelve months or more		1	1	3
Mutual Fund
Schedule of Investments [Line Items]
Number of investment securities designated as having a continuous loss position for twelve months or more		1	1	1
US Government agencies securities
Schedule of Investments [Line Items]
Number of investment securities designated as having a continuous loss position for twelve months or more		6	6	9
Collateralized mortgage obligations
Schedule of Investments [Line Items]
Number of investment securities designated as having a continuous loss position for twelve months or more		2	2	7

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