Comprehensive Income/(Loss) - Components of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 2,538,362	$ 2,475,457	$ 2,533,062	$ 2,426,662
Amounts reclassified from accumulated other comprehensive income/(loss)	(126)	81	(237)	270
Total other comprehensive loss	(286)	(30,228)	(8,515)	(2,211)
Ending Balance	2,544,279	2,464,998	2,544,279	2,464,998
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			6,515	(535)
Other comprehensive loss before reclassifications			0	0
Amounts reclassified from accumulated other comprehensive income/(loss)	(77)	75	(130)	150
Total other comprehensive loss			(130)	150
Ending Balance	6,385	(385)	6,385	(385)
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 loss before reclassifications			(8,278)	(2,481)
Amounts reclassified from accumulated other comprehensive income/(loss)	(49)	6	(107)	120
Total other comprehensive loss			(8,385)	(2,361)
Ending Balance	(241,593)	(264,242)	(241,593)	(264,242)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(234,922)	(234,399)	(226,693)	(262,416)
Other comprehensive loss before reclassifications			(8,278)	(2,481)
Amounts reclassified from accumulated other comprehensive income/(loss)			(237)	270
Total other comprehensive loss	(286)	(30,228)	(8,515)	(2,211)
Ending Balance	$ (235,208)	$ (264,627)	$ (235,208)	$ (264,627)