Comprehensive Income/(Loss) - Components of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 2,593,921	$ 1,978,827
Amounts reclassified from accumulated other comprehensive income/(loss)	(1,176)	471
Total other comprehensive (loss) gain	39,940	19,773
Ending Balance	2,586,060	2,023,139
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(17,468)	(16,542)
Amounts reclassified from accumulated other comprehensive income/(loss)	569	543
Total other comprehensive (loss) gain	569	543
Ending Balance	(16,899)	(15,999)
Accumulated Unrealized Gains (Losses) on Debt Securities Available for Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	18,644	(21,522)
Other comprehensive income before reclassifications	41,116	19,302
Amounts reclassified from accumulated other comprehensive income/(loss)	(1,743)	(33)
Total other comprehensive (loss) gain	39,373	19,269
Ending Balance	58,017	(2,253)
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	193
Amounts reclassified from accumulated other comprehensive income/(loss)	(2)	(39)
Total other comprehensive (loss) gain	(2)	(39)
Ending Balance	23	154
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	1,201	(37,871)
Other comprehensive income before reclassifications	41,116	19,302
Amounts reclassified from accumulated other comprehensive income/(loss)	(1,176)	471
Total other comprehensive (loss) gain	39,940	19,773
Ending Balance	$ 41,141	$ (18,098)