Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets [Abstract]
Loan purchase accounting	$ 3,638	$ 9,341
Other real estate	13,403	25,750
Allowance for loan losses	20,203	31,618
Deferred compensation	15,184	21,893
Realized built-in losses	12,932	18,699
Securities	4,869	9,256
Pension and other postretirement benefit plans	5,453	15,545
Interest on nonaccrual loans	774	2,093
Unrealized losses on securities available for sale	5,874	3,629
Stock-based compensation	2,067	3,031
Federal carryovers	4,569
Other	8,964	13,731
Gross deferred tax asset	97,930	154,586
Valuation allowance		(8,650)
Deferred tax asset net of valuation allowance	97,930	145,936
Deferred tax liabilities [Abstract]
Goodwill and other identifiable intangibles	16,729	25,666
Premises and equipment	11,877	19,391
Mortgage servicing rights	9,964	12,159
Securities	1,345	1,697
Leases		60
Other	5,040	6,891
Gross deferred tax liability	44,955	65,864
Net deferred tax asset	$ 52,975	$ 80,072