Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets [Abstract]
Loan purchase accounting	$ 1,564	$ 3,638
Other real estate	7,284	13,403
Allowance for loan losses	20,638	20,203
Deferred compensation	15,607	15,184
Realized built-in losses	12,182	12,932
Securities	3,929	4,869
Pension and other postretirement benefit plans	4,108	5,453
Interest on nonaccrual loans	806	774
Unrealized losses on securities available for sale	10,679	5,874
Stock-based compensation	2,192	2,067
Federal carryovers	1,606	4,569
Other	9,179	8,964
Gross deferred tax asset	89,774	97,930
Deferred tax liabilities [Abstract]
Goodwill and other identifiable intangibles	16,229	16,729
Premises and equipment	12,109	11,877
Mortgage servicing rights	14,415	9,964
Securities	1,519	1,345
Other	5,600	5,040
Gross deferred tax liability	49,872	44,955
Net deferred tax asset	$ 39,902	$ 52,975