Investment in Securities - Gross Unrealized Losses and Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Bonds available-for-sale securities, less than 12 months, unrealized (losses)	$ (20,021)	$ (5,276)
Bonds available-for-sale securities, less than 12 months, fair value	166,640	107,003
Bonds available-for-sale securities, 12 months or more, unrealized (losses)	(116,424)	(7,735)
Bonds available-for-sale securities, 12 months or more, fair value	1,601,681	141,647
Bonds available-for-sale securities, total, unrealized (losses)	(136,445)	(13,011)
Bonds available-for-sale securities, total, fair value	1,768,321	248,650
U.S. treasury and government
Schedule of Available-for-sale Securities [Line Items]
Bonds available-for-sale securities, less than 12 months, unrealized (losses)		0
Bonds available-for-sale securities, less than 12 months, fair value		0
Bonds available-for-sale securities, 12 months or more, unrealized (losses)		(5)
Bonds available-for-sale securities, 12 months or more, fair value		8,299
Bonds available-for-sale securities, total, unrealized (losses)		(5)
Bonds available-for-sale securities, total, fair value		8,299
U.S. states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Bonds available-for-sale securities, less than 12 months, unrealized (losses)	(8)	(9)
Bonds available-for-sale securities, less than 12 months, fair value	1,471	1,733
Bonds available-for-sale securities, 12 months or more, unrealized (losses)	0	0
Bonds available-for-sale securities, 12 months or more, fair value	121	0
Bonds available-for-sale securities, total, unrealized (losses)	(8)	(9)
Bonds available-for-sale securities, total, fair value	1,592	1,733
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Bonds available-for-sale securities, less than 12 months, unrealized (losses)	(20,013)	(5,257)
Bonds available-for-sale securities, less than 12 months, fair value	165,169	94,942
Bonds available-for-sale securities, 12 months or more, unrealized (losses)	(115,698)	(7,538)
Bonds available-for-sale securities, 12 months or more, fair value	1,587,397	132,626
Bonds available-for-sale securities, total, unrealized (losses)	(135,711)	(12,795)
Bonds available-for-sale securities, total, fair value	1,752,566	227,568
Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Bonds available-for-sale securities, less than 12 months, unrealized (losses)	0	(9)
Bonds available-for-sale securities, less than 12 months, fair value	0	10,169
Bonds available-for-sale securities, 12 months or more, unrealized (losses)	(7)	(192)
Bonds available-for-sale securities, 12 months or more, fair value	618	722
Bonds available-for-sale securities, total, unrealized (losses)	(7)	(201)
Bonds available-for-sale securities, total, fair value	618	10,891
Collateralized debt securities
Schedule of Available-for-sale Securities [Line Items]
Bonds available-for-sale securities, less than 12 months, unrealized (losses)	0	(1)
Bonds available-for-sale securities, less than 12 months, fair value	0	159
Bonds available-for-sale securities, 12 months or more, unrealized (losses)	(719)	0
Bonds available-for-sale securities, 12 months or more, fair value	13,545	0
Bonds available-for-sale securities, total, unrealized (losses)	(719)	(1)
Bonds available-for-sale securities, total, fair value	$ 13,545	$ 159

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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