Accumulated Other Comprehensive Income (Loss) - Reclassifications out of AOCI (Detail) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Debt securities gains (losses), net	$ 511		$ 1,165		$ 5,685	$ 1,062
Income tax (expense) benefit	(9,761)		(14,359)		(12,700)	(27,463)
Interest income (expense)	145,671		155,230		289,442	302,278
Salaries and employee benefits	(27)		(21)		(54)	(41)
Net income	51,705	$ 22,640	62,964	$ 56,276	74,345	119,240
Amount Reclassified from AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	1,661		445		5,966	290
Amount Reclassified from AOCI | Unrealized Gains and Losses on Available- for-Sale Debt Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Debt securities gains (losses), net	511		1,165		5,685	1,062
Income tax (expense) benefit	(89)		(273)		(1,263)	(249)
Net income	422		892		4,422	813
Amount Reclassified from AOCI | Unrealized Gains and Losses on Held-to- Maturity Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax (expense) benefit	0		129		0	235
Interest income (expense)	0		(564)		0	(1,021)
Net income	0		(435)		0	(786)
Amount Reclassified from AOCI | Gains and Losses on Cash Flow Hedges | Interest rate contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax (expense) benefit	(410)		(2)		(516)	(97)
Interest income (expense)	1,670		6		2,101	391
Net income	1,260		4		1,585	294
Amount Reclassified from AOCI | Defined Benefit Pension Plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax (expense) benefit	6		5		13	10
Salaries and employee benefits	(27)		(21)		(54)	(41)
Net income	$ (21)		$ (16)		$ (41)	$ (31)