Comprehensive Income/(Loss) - Components of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 2,464,998	$ 2,467,951	$ 2,426,662	$ 2,693,166
Amounts reclassified from accumulated other comprehensive loss	128	55	399	173
Total other comprehensive loss	(33,279)	(90,579)	(35,490)	(261,520)
Ending Balance	2,447,941	2,395,652	2,447,941	2,395,652
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(535)	(398)
Other comprehensive loss before reclassifications			0	0
Amounts reclassified from accumulated other comprehensive loss	76	55	226	165
Total other comprehensive loss			226	165
Ending Balance	(309)	(233)	(309)	(233)
Accumulated Unrealized Gains (Losses) on Debt Securities Available for Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(261,881)	(4,722)
Other comprehensive loss before reclassifications			(35,889)	(261,693)
Amounts reclassified from accumulated other comprehensive loss	52	0	173	8
Total other comprehensive loss			(35,716)	(261,685)
Ending Balance	(297,597)	(266,407)	(297,597)	(266,407)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(264,627)	(176,061)	(262,416)	(5,120)
Other comprehensive loss before reclassifications			(35,889)	(261,693)
Amounts reclassified from accumulated other comprehensive loss			399	173
Total other comprehensive loss	(33,279)	(90,579)	(35,490)	(261,520)
Ending Balance	$ (297,906)	$ (266,640)	$ (297,906)	$ (266,640)