Income Taxes - Schedule of Significant Components of Net Deferred Tax Assets (Liabilities) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Deferred Tax Assets
Allowance for credit losses, net of recapture	$ 28,791	$ 34,971
Benefit plan accruals	15,579	20,076
Net operating loss carryforwards	14,823	18,982
Deferred gain on securities	1,334	2,102
Acquired loans	8,949	11,989
Operating lease liabilities	22,959	24,245
Tax credit investments and other partnerships	494	1,054
Other, net	942	488
Total deferred tax assets	93,871	113,907
Deferred Tax Liabilities
Purchase accounting	(18,524)	(18,232)
Loan servicing rights	(7,179)	(6,582)
Premises and equipment	(14,550)	(14,008)
Prepaid expenses	(953)	(955)
Operating lease right-of-use assets	(21,106)	(21,569)
Unrealized gains on available-for-sale investment securities	(14,590)	(40,756)
Unrealized gains on hedges	(628)	(1,080)
Other, net	(1,561)	(1,555)
Total deferred tax liabilities	(79,091)	(104,737)
Net deferred tax assets	$ 14,780	$ 9,170