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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, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
After-tax:
Other comprehensive income (loss)	$ 5,160	$ 11,955	$ 11,984	$ (737)	$ (1,849)	$ (4,050)	$ 29,099	$ (6,636)
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member]
OCI, Pre-tax:
Pre-tax, Unrealized holding gains (losses) arising during the period	6,815			(989)			38,685	(11,511)
Pre-tax, Less: Reclassification adjustments (gains) losses realized in net income	22			10			(87)	2,807
Pre-tax, Other comprehensive income (loss)	6,837			(979)			38,598	(8,704)
OCI, Tax:
Tax, Unrealized holding gains (losses) arising during the period	1,671			(244)			9,520	(2,751)
Income taxes	6			2			(21)	683
Tax, Other comprehensive income (loss)	1,677			(242)			9,499	(2,068)
After-tax:
After-tax, Unrealized holding gains (losses) arising during the period	5,144			(745)			29,165	(8,760)
After-tax, Less: Reclassification adjustments (gains) losses realized in net income	16			8			(66)	2,124
Other comprehensive income (loss)	$ 5,160			$ (737)			$ 29,099	$ (6,636)