Accumulated Other Comprehensive Income (Loss) - Reclassifications out of AOCI (Detail) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Debt securities gains (losses), net	$ 4,921			$ 424			$ 10,606	$ 1,486
Income tax (expense) benefit	(12,154)			(13,254)			(24,854)	(40,717)
Interest income (expense)	145,573			153,096			435,015	455,374
Salaries and employee benefits	(28)			(20)			(82)	(61)
Net income	77,944	$ 51,705	$ 22,640	69,781	$ 62,964	$ 56,276	152,289	189,021
Amount Reclassified from AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	4,985			(659)			10,951	(370)
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	4,921			424			10,606	1,486
Income tax (expense) benefit	(1,084)			(89)			(2,347)	(339)
Net income	3,837			335			8,259	1,147
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			392			0	627
Interest income (expense)	0			(1,384)			0	(2,405)
Net income	0			(992)			0	(1,778)
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	(381)			(8)			(897)	(105)
Interest income (expense)	1,550			21			3,651	412
Net income	1,169			13			2,754	307
Amount Reclassified from AOCI | Defined Benefit Pension Plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax (expense) benefit	7			5			20	15
Salaries and employee benefits	(28)			(20)			(82)	(61)
Net income	$ (21)			$ (15)			$ (62)	$ (46)