Comprehensive Income/(Loss) - Components of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 2,790,281	$ 2,533,062
Amounts reclassified from accumulated other comprehensive income/(loss)	(214)	(111)
Total other comprehensive income (loss)	27,922	(8,229)
Ending Balance	3,781,579	2,538,362
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	5,124	6,515
Other comprehensive income before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income/(loss)	(128)	(52)
Total other comprehensive income (loss)	(128)	(52)
Ending Balance	4,996	6,463
Accumulated Unrealized Gains (Losses) on Debt Securities Available for Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(223,756)	(233,208)
Other comprehensive income before reclassifications	28,136	(8,118)
Amounts reclassified from accumulated other comprehensive income/(loss)	(86)	(59)
Total other comprehensive income (loss)	28,050	(8,177)
Ending Balance	(195,706)	(241,385)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(218,632)	(226,693)
Other comprehensive income before reclassifications	28,136	(8,118)
Amounts reclassified from accumulated other comprehensive income/(loss)	(214)	(111)
Total other comprehensive income (loss)	27,922	(8,229)
Ending Balance	$ (190,710)	$ (234,922)