Accumulated Other Comprehensive Income (Loss) - Schedule of AOCI (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 2,823,435	$ 2,751,872	$ 2,852,453	$ 2,689,570
Other comprehensive income (loss) before reclassifications	12,547	39,862	98,802	87,277
Amounts reclassified from AOCI to income	(1,661)	(445)	(5,966)	(290)
Ending balance	2,864,255	2,803,139	2,864,255	2,803,139
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	138,157	2,620	56,207	(44,950)
Ending balance	149,043	42,037	149,043	42,037
Unrealized Gains and Losses on Available- for-Sale Debt Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	132,500	10,442	56,131	(37,348)
Other comprehensive income (loss) before reclassifications	12,733	39,151	93,102	86,862
Amounts reclassified from AOCI to income	(422)	(892)	(4,422)	(813)
Ending balance	144,811	48,701	144,811	48,701
Unrealized Gains and Losses on Held-to- Maturity Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0	(8,164)	0	(8,515)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from AOCI to income	0	435	0	786
Ending balance	0	(7,729)	0	(7,729)
Gains and Losses on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	5,801	513	240	1,099
Other comprehensive income (loss) before reclassifications	(186)	711	5,700	415
Amounts reclassified from AOCI to income	(1,260)	(4)	(1,585)	(294)
Ending balance	4,355	1,220	4,355	1,220
Defined Benefit Pension Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(144)	(171)	(164)	(186)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from AOCI to income	21	16	41	31
Ending balance	$ (123)	$ (155)	$ (123)	$ (155)