Comprehensive Income/(Loss) - Components of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 2,785,522	$ 2,586,060	$ 2,756,737	$ 2,593,921
Amounts reclassified from accumulated other comprehensive income/(loss)	453	402	923	(776)
Total other comprehensive gain (loss)	2,783	(625)	(18,773)	39,315
Ending Balance	2,780,836	2,569,521	2,780,836	2,569,521
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)	477	569	949	1,137
Total other comprehensive gain (loss)			949	1,137
Ending Balance	(14,553)	(16,331)	(14,553)	(16,331)
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			(19,696)	40,091
Amounts reclassified from accumulated other comprehensive income/(loss)	(24)	(162)	(26)	(1,906)
Total other comprehensive gain (loss)			(19,722)	38,185
Ending Balance	27,139	56,829	27,139	56,829
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)		(5)		(7)
Total other comprehensive gain (loss)				(7)
Ending Balance		18		18
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	9,803	41,141	31,359	1,201
Other comprehensive income/(loss) before reclassifications			(19,696)	40,091
Amounts reclassified from accumulated other comprehensive income/(loss)			923	(776)
Total other comprehensive gain (loss)	2,783	(625)	(18,773)	39,315
Ending Balance	$ 12,586	$ 40,516	$ 12,586	$ 40,516