Accumulated Other Comprehensive Income (Loss) - Schedule of AOCI (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 2,852,453	$ 2,689,570
Other comprehensive income (loss) before reclassifications	86,255	47,415
Amounts reclassified from AOCI to income	(4,305)	155
Ending balance	2,823,435	2,751,872
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	56,207	(44,950)
Ending balance	138,157	2,620
Unrealized Gains and Losses on Available- for-Sale Debt Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	56,131	(37,348)
Other comprehensive income (loss) before reclassifications	80,369	47,711
Amounts reclassified from AOCI to income	(4,000)	79
Ending balance	132,500	10,442
Unrealized Gains and Losses on Held-to- Maturity Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0	(8,515)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from AOCI to income	0	351
Ending balance	0	(8,164)
Gains and Losses on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	240	1,099
Other comprehensive income (loss) before reclassifications	5,886	(296)
Amounts reclassified from AOCI to income	(325)	(290)
Ending balance	5,801	513
Defined Benefit Pension Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(164)	(186)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from AOCI to income	20	15
Ending balance	$ (144)	$ (171)