Investment Securities - Gross Unrealized Losses on Investment Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available for Sale, Less than 12 Months, Fair Value	$ 331,795	$ 336,433
Debt Securities, Available for Sale, Less than 12 more, Unrealized Loss	(4,806)	(6,148)
Debt Securities, Available for Sale, 12 Months or more, Fair Value	71,152	415,349
Debt Securities, Available for Sale, 12 Months or more, Unrealized Loss	(511)	(16,061)
Debt Securities, Available for Sale, Fair Value	402,947	751,782
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	5,317	22,209
Debt Securities Held to Maturity, Less than 12 months, Fair Value		229,596
Debt Securities Held to Maturity, Less than 12 months, Unrealized Loss		(2,733)
Debt Securities Held to Maturity, 12 months or more, Fair Value		162,690
Debt Securities Held to Maturity, 12 months or more, Unrealized Loss		(5,242)
Debt Securities Held to Maturity, Fair Value		392,286
Debt Securities Held to Maturity, Unrealized Loss		(7,975)
Obligations of U.S. Government Agencies
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available for Sale, Less than 12 Months, Fair Value	38,295	171,309
Debt Securities, Available for Sale, Less than 12 more, Unrealized Loss	(374)	(3,588)
Debt Securities, Available for Sale, 12 Months or more, Fair Value	25,171	394,630
Debt Securities, Available for Sale, 12 Months or more, Unrealized Loss	(73)	(14,490)
Debt Securities, Available for Sale, Fair Value	63,466	565,939
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	447	18,078
Debt Securities Held to Maturity, Less than 12 months, Fair Value	19,749	223,810
Debt Securities Held to Maturity, Less than 12 months, Unrealized Loss	(94)	(2,619)
Debt Securities Held to Maturity, 12 months or more, Fair Value	78,604	158,648
Debt Securities Held to Maturity, 12 months or more, Unrealized Loss	(388)	(5,126)
Debt Securities Held to Maturity, Fair Value	98,353	382,458
Debt Securities Held to Maturity, Unrealized Loss	(482)	(7,745)
Obligations of States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available for Sale, Less than 12 Months, Fair Value		63,738
Debt Securities, Available for Sale, Less than 12 more, Unrealized Loss		(1,541)
Debt Securities, Available for Sale, 12 Months or more, Fair Value		20,719
Debt Securities, Available for Sale, 12 Months or more, Unrealized Loss		(1,571)
Debt Securities, Available for Sale, Fair Value		84,457
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total		3,112
Debt Securities Held to Maturity, Less than 12 months, Fair Value		5,786
Debt Securities Held to Maturity, Less than 12 months, Unrealized Loss		(114)
Debt Securities Held to Maturity, 12 months or more, Fair Value		4,042
Debt Securities Held to Maturity, 12 months or more, Unrealized Loss		(116)
Debt Securities Held to Maturity, Fair Value		9,828
Debt Securities Held to Maturity, Unrealized Loss		(230)
Asset-backed Securities
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available for Sale, Less than 12 Months, Fair Value	293,500	101,386
Debt Securities, Available for Sale, Less than 12 more, Unrealized Loss	(4,432)	(1,019)
Debt Securities, Available for Sale, 12 Months or more, Fair Value	45,981	0
Debt Securities, Available for Sale, 12 Months or more, Unrealized Loss	(438)
Debt Securities, Available for Sale, Fair Value	339,481	101,386
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	$ 4,870	$ 1,019

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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