Accumulated Other Comprehensive Income (Loss) - Schedule of AOCI (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 2,864,255	$ 2,803,139	$ 2,852,453	$ 2,689,570
Other comprehensive income (loss) before reclassifications	5,226	15,818	104,028	103,094
Amounts reclassified from AOCI to income	(4,985)	659	(10,951)	370
Ending balance	2,921,149	2,832,530	2,921,149	2,832,530
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	149,043	42,037	56,207	(44,950)
Ending balance	149,284	58,514	149,284	58,514
Unrealized Gains and Losses on Available- for-Sale Debt Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	144,811	48,701	56,131	(37,348)
Other comprehensive income (loss) before reclassifications	5,234	15,876	98,336	102,737
Amounts reclassified from AOCI to income	(3,837)	(335)	(8,259)	(1,147)
Ending balance	146,208	64,242	146,208	64,242
Unrealized Gains and Losses on Held-to- Maturity Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0	(7,729)	0	(8,515)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from AOCI to income	0	992	0	1,778
Ending balance	0	(6,737)	0	(6,737)
Gains and Losses on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	4,355	1,220	240	1,099
Other comprehensive income (loss) before reclassifications	(8)	(58)	5,692	357
Amounts reclassified from AOCI to income	(1,169)	(13)	(2,754)	(307)
Ending balance	3,178	1,149	3,178	1,149
Defined Benefit Pension Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(123)	(155)	(164)	(186)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from AOCI to income	21	15	62	46
Ending balance	$ (102)	$ (140)	$ (102)	$ (140)