Other Comprehensive Income - Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Unrealized gains (losses) on securities available for sale:
Change in fair value of securities available for sale, Pre Tax Amount	$ (94,979)	$ 33,802
Less: reclassification adjustments for gains included in net income, Pre-Tax Amount	1,164	122
Net change in fair value of securities available for sale, Pre Tax Amount	(96,143)	33,680
Unrealized gains (losses) on cash flow hedges:
Change in fair value of cash flow hedges, Pre Tax Amount	0	43,556
Less: net cash flow hedge losses reclassified into interest income, Pre Tax Amount	8,274	3,112
Net change in fair value of cash flow hedges, Pre Tax Amount	(8,274)	40,444
Defined benefit pension plans:
Change in actuarial net loss, Pre Tax Amount	0	0
Less: amortization of actuarial net loss, Pre-Tax Amount	(3,537)	0
Net change in other comprehensive income for defined benefit postretirement plans, Pre Tax Amount	592	0
Total other comprehensive income, Pre Tax Amount	(103,825)	74,124
Unrealized gains (losses) on securities available for sale:
Change in fair value of securities available for sale, Tax (Expense) Benefit	20,982	(7,515)
Less: reclassification adjustments for gains included in net income, Tax (Expense) Benefit	(257)	(27)
Less: net accretion of prior service cost, Pre-Tax Amount	2,945	0
Net change in fair value of securities available for sale, Tax (Expense) Benefit	21,239	(7,488)
Unrealized gains (losses) on cash flow hedges:
Change in fair value of cash flow hedges, Tax (Expense) Benefit	0	(12,244)
Less: net cash flow hedge losses reclassified into interest income, Tax (Expense) Benefit	(2,326)	(875)
Net change in fair value of cash flow hedges, Tax (Expense) Benefit	2,326	(11,369)
Defined benefit pension plans:
Change in actuarial net loss, Tax (Expense) Benefit	0	0
Less: amortization of actuarial net loss, Tax (Expense) Benefit	994	0
Less: net accretion of prior service cost, After Tax Amount	(828)	0
Net change in other comprehensive income for defined benefit postretirement plans, Tax (Expense) Benefit	(166)	0
Total other comprehensive income, Tax (Expense) Benefit	23,399	(18,857)
Unrealized gains (losses) on securities available for sale:
Change in fair value of securities available for sale, After Tax Amount	(73,997)	26,287
Less: reclassification adjustment for gains included in net income, After Tax Amount	907	95
Net change in fair value of securities available for sale, After Tax Amount	(74,904)	26,192
Unrealized gains (losses) on cash flow hedges:
Change in fair value of cash flow hedges, After Tax Amount	0	31,312
Less: net cash flow hedge losses reclassified into interest income, Amount After Tax	5,948	2,237
Net change in fair value of cash flow hedges, Amount After Tax	(5,948)	29,075
Defined benefit pension plans:
Amortization of actuarial net loss, Amount After Tax	0	0
Less: amortization of actuarial net loss, After Tax Amount	(2,543)	0
Less: net accretion of prior service cost, After Tax Amount	2,117	0
Net change in other comprehensive income for defined benefit postretirement plans, Amount After Tax	426	0
Net current-period other comprehensive income	(80,426)	$ 55,267
Total realized gain, net of tax	41,200
Balance of gain after amortization, net of tax	$ 23,900