Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets [Abstract]
Loan purchase accounting	$ 9,341	$ 26,049
Other real estate	25,750	32,664
Allowance for loan losses	31,618	30,451
Deferred compensation	21,893	21,102
Realized built in losses	18,699	16,215
Securities	9,256	13,016
Pension and other postretirement benefit plans	15,545	14,433
Nonaccrual loans	2,093	3,137
Unrealized losses on securities available for sale	3,629
Stock-based compensation	3,031	3,137
Other	13,731	15,820
Gross deferred tax asset	154,586	176,024
Valuation allowance	(8,650)	(8,650)
Deferred tax asset net of valuation allowance	145,936	167,374
Deferred tax liabilities [Abstract]
Goodwill and other identifiable intangibles	25,666	26,651
Premises and equipment	19,391	21,257
Unrealized gains on securities available for sale		2,241
Mortgage servicing rights	12,159	9,924
Securities	1,697	1,474
Leases	60	106
Other	6,891	4,764
Gross deferred tax liability	65,864	66,417
Net deferred tax asset	$ 80,072	$ 100,957