Comprehensive Income/(Loss) - Components of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 2,547,316	$ 2,785,522	$ 2,693,166	$ 2,756,737
Amounts reclassified from accumulated other comprehensive loss	62	453	117	923
Total other comprehensive gain (loss)	(64,749)	2,783	(170,941)	(18,773)
Ending Balance	2,467,951	2,780,836	2,467,951	2,780,836
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(398)	(15,502)
Other comprehensive loss before reclassifications				0
Amounts reclassified from accumulated other comprehensive loss	55	477	109	949
Total other comprehensive gain (loss)			109	949
Ending Balance	(289)	(14,553)	(289)	(14,553)
Accumulated Unrealized Gains (Losses) on Debt Securities Available for Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(4,722)	46,861
Other comprehensive loss before reclassifications			(171,058)	(19,696)
Amounts reclassified from accumulated other comprehensive loss	7	(24)	8	(26)
Total other comprehensive gain (loss)			(171,050)	(19,722)
Ending Balance	(175,772)	27,139	(175,772)	27,139
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(111,312)	9,803	(5,120)	31,359
Other comprehensive loss before reclassifications			(171,058)	(19,696)
Amounts reclassified from accumulated other comprehensive loss			117	923
Total other comprehensive gain (loss)	(64,749)	2,783	(170,941)	(18,773)
Ending Balance	$ (176,061)	$ 12,586	$ (176,061)	$ 12,586