Comprehensive Income/(Loss) - Components of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 2,756,737	$ 2,593,921
Amounts reclassified from accumulated other comprehensive income/(loss)	470	(1,176)
Total other comprehensive gain (loss)	(21,556)	39,940
Ending Balance	2,785,522	2,586,060
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(15,502)	(17,468)
Amounts reclassified from accumulated other comprehensive income/(loss)	472	569
Total other comprehensive gain (loss)	472	569
Ending Balance	(15,030)	(16,899)
Accumulated Unrealized Gains (Losses) on Debt Securities Available for Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	46,861	18,644
Other comprehensive income/(loss) before reclassifications	(22,026)	41,116
Amounts reclassified from accumulated other comprehensive income/(loss)	(2)	(1,743)
Total other comprehensive gain (loss)	(22,028)	39,373
Ending Balance	24,833	58,017
Accumulated Unrealized Gains on Debt Securities Transferred from Available For Sale to Held to Maturity [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		25
Amounts reclassified from accumulated other comprehensive income/(loss)		(2)
Total other comprehensive gain (loss)		(2)
Ending Balance		23
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	31,359	1,201
Other comprehensive income/(loss) before reclassifications	(22,026)	41,116
Amounts reclassified from accumulated other comprehensive income/(loss)	470	(1,176)
Total other comprehensive gain (loss)	(21,556)	39,940
Ending Balance	$ 9,803	$ 41,141