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Other Comprehensive Income (Loss) - Components of Other Comprehensive Income (loss) Pre-Tax and After-Tax (Detail) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
After-tax:
Other comprehensive income (loss)	$ (36,421)	$ 26,068	$ (8,946)	$ 5,160	$ 11,955	$ 11,984	$ (19,299)	$ 29,099
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member]
OCI, Pre-tax:
Pre-tax, Unrealized holding gains arising during the period	5,518			6,815			28,953	38,685
Pre-tax, Less: Reclassification adjustments (gains) losses realized in net income (loss)	(53,827)			22			(54,294)	(87)
Pre-tax, Other comprehensive income (loss)	(48,309)			6,837			(25,341)	38,598
OCI, Tax:
Tax, Unrealized holding gains arising during the period	1,359			1,671			7,320	9,520
Income taxes	(13,247)			6			(13,362)	(21)
Tax, Other comprehensive income (loss)	(11,888)			1,677			(6,042)	9,499
After-tax:
After-tax, Unrealized holding gains arising during the period	4,159			5,144			21,633	29,165
After-tax, Less: Reclassification adjustments (gains) realized in net income (loss)	(40,580)			16			(40,932)	(66)
Other comprehensive income (loss)	$ (36,421)			$ 5,160			(19,299)	$ 29,099
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member] \| Cumulative Effect, Period Of Adoption, Adjustment [Member]
OCI, Pre-tax:
Pre-tax, Less: Reclassification adjustments (gains) losses realized in net income (loss)							791
OCI, Tax:
Income taxes							194
After-tax:
After-tax, Less: Reclassification adjustments (gains) realized in net income (loss)							597
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member] \| Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
OCI, Pre-tax:
Pre-tax, Other comprehensive income (loss)							(24,550)
OCI, Tax:
Tax, Other comprehensive income (loss)							(5,848)
After-tax:
Other comprehensive income (loss)							$ (18,702)