Investment Securities - Summary of Amortized Cost and Approximate Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,911,234		$ 3,138,534
Allowance for Credit Losses	(3,849)	$ (3,439)	(578)	$ (253)	$ (411)	$ 0
Gross Unrealized Gains	1,594		442
Gross Unrealized Losses	(162,251)		(177,383)
Fair Value	2,746,728		2,961,015
Equity securities	26,479		26,485
Total investment securities, at fair value	2,773,207		2,987,500
Available-for-sale debt securities, accrued interest	19,700		16,700
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	111,915		169,170
Allowance for Credit Losses	(1,755)	(1,563)	(578)	(253)	(411)	0
Gross Unrealized Gains	4		0
Gross Unrealized Losses	(5,448)		(8,050)
Fair Value	104,716		160,542
Agency-guaranteed residential collateralized mortgage obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	133,425		147,481
Allowance for Credit Losses	0		0
Gross Unrealized Gains	0		0
Gross Unrealized Losses	(14,956)		(13,617)
Fair Value	118,469		133,864
Collateralized loan obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	826,764		896,992
Allowance for Credit Losses	0		0
Gross Unrealized Gains	817		88
Gross Unrealized Losses	(11,725)		(24,342)
Fair Value	815,856		872,738
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	136,111		142,222
Allowance for Credit Losses	0		0
Gross Unrealized Gains	0		1
Gross Unrealized Losses	(4,923)		(5,866)
Fair Value	131,188		136,357
Corporate notes
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	638,230		657,086
Allowance for Credit Losses	(2,094)	$ (1,876)	0	$ 0	$ 0	$ 0
Gross Unrealized Gains	86		45
Gross Unrealized Losses	(57,325)		(61,878)
Fair Value	578,897		595,253
Private label collateralized mortgage obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,064,789		1,125,583
Allowance for Credit Losses	0		0
Gross Unrealized Gains	687		308
Gross Unrealized Losses	(67,874)		(63,630)
Fair Value	$ 997,602		$ 1,062,261