Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Allowance for credit losses	$ 48,050	$ 9,991
Purchase accounting adjustments—loans	18,084	32,333
Reserves and other accruals	17,115	14,663
Investments	2,045	2,323
Derivatives	5,009	182
Employee benefit plans	2,504	3,581
Lease liabilities	34,955	38,181
Other (1)	821	633
Total deferred tax assets	128,583	101,887
Deferred tax liabilities:
Unrealized gains on available-for-sale securities	(18,997)	(8,651)
Unrealized gains on equity investments	0	(10,031)
Accelerated depreciation	(4,233)	(1,745)
Other (2)	(1,278)	(841)
Right of use assets	(31,707)	(34,907)
Deferred loan costs	(783)	(2,843)
Intangibles	(22,373)	(24,625)
Total deferred tax liabilities	(79,371)	(83,643)
Net deferred tax asset	$ 49,212	$ 18,244