Comprehensive Income/(Loss) - Components of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 2,023,139	$ 1,403,026	$ 1,978,827	$ 1,395,321
Amounts reclassified from accumulated other comprehensive income/(loss)	515	538	988	926
Total other comprehensive (loss) gain	22,211	(6,276)	41,984	(20,794)
Ending Balance	2,074,116	1,524,106	2,074,116	1,524,106
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(16,542)	(18,626)
Other comprehensive income/(loss) before reclassifications			0
Amounts reclassified from accumulated other comprehensive income/(loss)	585	589	1,129	1,026
Total other comprehensive (loss) gain			1,129	1,026
Ending Balance	(15,413)	(17,600)	(15,413)	(17,600)
Accumulated Unrealized Gains (Losses) on Debt Securities Available for Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(21,522)	(13,250)
Other comprehensive income/(loss) before reclassifications			40,996	(21,720)
Amounts reclassified from accumulated other comprehensive income/(loss)	3		(30)
Total other comprehensive (loss) gain			40,966	(21,720)
Adoption of Accounting Standard ASU 2016-01		(1,063)		(1,063)
Ending Balance	19,444	(36,033)	19,444	(36,033)
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			193	381
Other comprehensive income/(loss) before reclassifications			0
Amounts reclassified from accumulated other comprehensive income/(loss)	(73)	(51)	(111)	(100)
Total other comprehensive (loss) gain			(111)	(100)
Ending Balance	82	281	82	281
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(18,098)	(47,076)	(37,871)	(31,495)
Other comprehensive income/(loss) before reclassifications			40,996	(21,720)
Amounts reclassified from accumulated other comprehensive income/(loss)			988	926
Total other comprehensive (loss) gain	22,211	(6,276)	41,984	(20,794)
Adoption of Accounting Standard ASU 2016-01		(1,063)		(1,063)
Ending Balance	$ 4,113	$ (53,352)	$ 4,113	$ (53,352)