Income Taxes - Schedule of Significant Components of Net Deferred Tax Assets (Liabilities) (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred Tax Assets
Allowance for loan losses, net of recapture	$ 34,971	$ 14,179
Benefit plan accruals	20,076	19,673
Alternative minimum tax credit	0	1,272
Net operating loss carryforwards	18,982	25,336
Deferred gain on securities	2,102	3,754
Acquired loans	11,989	16,784
Operating lease liabilities	24,245	26,503
Tax credit investments and other partnerships	1,054	1,765
Other real estate owned	28	141
Other, net	460	591
Total deferred tax assets	113,907	109,998
Deferred Tax Liabilities
Purchase accounting	(18,232)	(17,564)
Loan servicing rights	(6,582)	(6,289)
Premises and equipment	(14,008)	(12,167)
Prepaid expenses	(955)	(973)
Operating lease right-of-use assets	(21,569)	(25,448)
Unrealized gains on available-for-sale investment securities	(40,756)	(15,751)
Unrealized gains on hedges	(1,080)	(78)
Other, net	(1,555)	(2,023)
Total deferred tax liabilities	(104,737)	(80,293)
Net deferred tax assets	$ 9,170	$ 29,705