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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				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
After-tax:
Other comprehensive income	$ 26,068	$ (8,946)	$ 11,955	$ 11,984	$ 17,122	$ 23,939
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member]
OCI, Pre-tax:
Pre-tax, Unrealized holding gains arising during the period	34,661		15,825		23,435	31,870
Pre-tax, Less: Reclassification adjustments (gains) losses realized in net income	(168)		36		(467)	(109)
Pre-tax, Other comprehensive income	34,493		15,861		22,968	31,761
OCI, Tax:
Tax, Unrealized holding gains arising during the period	8,466		3,896		5,961	7,849
Income taxes	(41)		10		(115)	(27)
Tax, Other comprehensive income	8,425		3,906		5,846	7,822
After-tax:
After-tax, Unrealized holding gains arising during the period	26,195		11,929		17,474	24,021
After-tax, Less: Reclassification adjustments (gains) realized in net income	(127)		26		(352)	(82)
Other comprehensive income	$ 26,068		$ 11,955		17,122	$ 23,939
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					791
OCI, Tax:
Income taxes					194
After-tax:
After-tax, Less: Reclassification adjustments (gains) realized in net income					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					23,759
OCI, Tax:
Tax, Other comprehensive income					6,040
After-tax:
Other comprehensive income					$ 17,719