Accumulated Other Comprehensive Income (Loss) - Reclassifications out of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Debt securities gains (losses), net	$ 5,174	$ (103)
Income tax (expense) benefit	(2,939)	(13,104)
Interest income (expense)	143,771	147,048
Salaries and employee benefits	(27)	(20)
Net income	22,640	56,276
Amount Reclassified from AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	4,305	(155)
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	5,174	(103)
Income tax (expense) benefit	(1,174)	24
Net income	4,000	(79)
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	106
Interest income (expense)	0	(457)
Net income	0	(351)
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	(106)	(95)
Interest income (expense)	431	385
Net income	325	290
Amount Reclassified from AOCI | Defined Benefit Pension Plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax (expense) benefit	7	5
Salaries and employee benefits	(27)	(20)
Net income	$ (20)	$ (15)