Comprehensive Income/(Loss) - Components of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 2,533,062	$ 2,426,662
Amounts reclassified from accumulated other comprehensive income/(loss)	(111)	189
Total other comprehensive (loss) income	(8,229)	28,017
Ending Balance	2,538,362	2,475,457
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	6,515	(535)
Other comprehensive income/(loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income/(loss)	(52)	75
Total other comprehensive (loss) income	(52)	75
Ending Balance	6,463	(460)
Accumulated Unrealized Gains (Losses) on Debt Securities Available for Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(233,208)	(261,881)
Other comprehensive income/(loss) before reclassifications	(8,118)	27,828
Amounts reclassified from accumulated other comprehensive income/(loss)	(59)	114
Total other comprehensive (loss) income	(8,177)	27,942
Ending Balance	(241,385)	(233,939)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(226,693)	(262,416)
Other comprehensive income/(loss) before reclassifications	(8,118)	27,828
Amounts reclassified from accumulated other comprehensive income/(loss)	(111)	189
Total other comprehensive (loss) income	(8,229)	28,017
Ending Balance	$ (234,922)	$ (234,399)