Comprehensive Income/(Loss) - Components of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 2,467,951	$ 2,780,836	$ 2,693,166	$ 2,756,737
Amounts reclassified from accumulated other comprehensive loss	55	473	173	1,396
Total other comprehensive gain (loss)	(90,579)	(8,123)	(261,520)	(26,896)
Ending Balance	2,395,652	2,723,983	2,395,652	2,723,983
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(398)	(15,502)
Other comprehensive loss before reclassifications				0
Amounts reclassified from accumulated other comprehensive loss	55	482	165	1,431
Total other comprehensive gain (loss)			165	1,431
Ending Balance	(233)	(14,071)	(233)	(14,071)
Accumulated Unrealized Gains (Losses) on Debt Securities Available for Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(4,722)	46,861
Other comprehensive loss before reclassifications			(261,693)	(28,292)
Amounts reclassified from accumulated other comprehensive loss		(9)	8	(35)
Total other comprehensive gain (loss)			(261,685)	(28,327)
Ending Balance	(266,407)	18,534	(266,407)	18,534
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(176,061)	12,586	(5,120)	31,359
Other comprehensive loss before reclassifications			(261,693)	(28,292)
Amounts reclassified from accumulated other comprehensive loss			173	1,396
Total other comprehensive gain (loss)	(90,579)	(8,123)	(261,520)	(26,896)
Ending Balance	$ (266,640)	$ 4,463	$ (266,640)	$ 4,463