Investment Securities - Gross Unrealized Losses on Debt Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available for Sale, Less than 12 Months, Fair Value	$ 246,082	$ 274,950
Debt Securities, Available for Sale, Less than 12 Months, Unrealized Loss	(1,736)	(4,845)
Debt Securities, Available for Sale, 12 Months or more, Fair Value	296,841	123,833
Debt Securities, Available for Sale, 12 Months or more, Unrealized Loss	(8,111)	(2,489)
Debt Securities, Available for Sale, Fair Value	542,923	398,783
Debt Securities, Available-for-sale, Unrealized Loss	(9,847)	(7,334)
Obligations of U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available for Sale, Less than 12 Months, Fair Value	152,656	36,709
Debt Securities, Available for Sale, Less than 12 Months, Unrealized Loss	(228)	(309)
Debt Securities, Available for Sale, 12 Months or more, Fair Value	0	23,852
Debt Securities, Available for Sale, 12 Months or more, Unrealized Loss	0	(111)
Debt Securities, Available for Sale, Fair Value	152,656	60,561
Debt Securities, Available-for-sale, Unrealized Loss	(228)	(420)
Debt Securities Held to Maturity, Less than 12 months, Fair Value		18,813
Debt Securities Held to Maturity, Less than 12 months, Unrealized Loss		(142)
Debt Securities Held to Maturity, 12 months or more, Fair Value		62,952
Debt Securities Held to Maturity, 12 months or more, Unrealized Loss		(338)
Debt Securities Held to Maturity, Fair Value		81,765
Debt Securities Held to Maturity, Unrealized Loss		(480)
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available for Sale, Less than 12 Months, Fair Value	963	778
Debt Securities, Available for Sale, Less than 12 Months, Unrealized Loss	(13)	(1)
Debt Securities, Available for Sale, 12 Months or more, Fair Value	0	0
Debt Securities, Available for Sale, 12 Months or more, Unrealized Loss	0	0
Debt Securities, Available for Sale, Fair Value	963	778
Debt Securities, Available-for-sale, Unrealized Loss	(13)	(1)
Asset backed securities
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available for Sale, Less than 12 Months, Fair Value	92,463	237,463
Debt Securities, Available for Sale, Less than 12 Months, Unrealized Loss	(1,495)	(4,535)
Debt Securities, Available for Sale, 12 Months or more, Fair Value	296,841	99,981
Debt Securities, Available for Sale, 12 Months or more, Unrealized Loss	(8,111)	(2,378)
Debt Securities, Available for Sale, Fair Value	389,304	337,444
Debt Securities, Available-for-sale, Unrealized Loss	$ (9,606)	$ (6,913)

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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