Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets [Abstract]
Loan purchase accounting	$ 26,049	$ 38,471
Other real estate	32,664	36,086
Allowance for loan losses	30,451	31,241
Deferred compensation	21,102	20,825
Realized built in losses	16,215	15,907
Securities	13,016	15,443
Pension and other postretirement benefit plans	14,433	14,904
Nonaccrual loans	3,137	3,372
Stock-based compensation	3,137	3,077
Other	15,820	17,833
Gross deferred tax asset	176,024	197,159
Valuation allowance	(8,650)	(8,650)
Deferred tax asset net of valuation allowance	167,374	188,509
Deferred tax liabilities [Abstract]
Goodwill and other identifiable intangibles	26,651	28,010
Premises and equipment	21,257	20,813
Unrealized gains on securities available for sale	2,241	8,627
Mortgage servicing rights	9,924	7,747
Securities	1,474	1,353
Leases	106	625
Other	4,764	3,900
Gross deferred tax liability	66,417	71,075
Net deferred tax asset	$ 100,957	$ 117,434