Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets [Abstract]
Loan purchase accounting	$ 845	$ 1,564
Other real estate	5,845	7,284
Allowance for loan losses	21,774	20,638
Deferred compensation	16,498	15,607
Financing and operating lease liabilities	10,469
Realized built-in losses	11,431	12,182
Securities	3,028	3,929
Pension and other postretirement benefit plans	5,194	4,108
Interest on nonaccrual loans	942	806
Unrealized losses on securities available for sale		10,679
Stock-based compensation	2,527	2,192
Federal carryovers		1,606
Other	8,790	9,179
Gross deferred tax asset	87,343	89,774
Deferred tax liabilities [Abstract]
Goodwill and other identifiable intangibles	15,336	16,229
Premises and equipment	11,913	12,109
Financing and operating lease right-of-use assets	10,127
Mortgage servicing rights	11,002	14,415
Securities	2,115	1,519
Other	5,192	5,600
Gross deferred tax liability	55,685	49,872
Net deferred tax asset	$ 31,658	$ 39,902