Income Taxes - Provision for income taxes - (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Current:
Federal	$ 41,527	$ 8,577	$ 22,153
State	2,327	1,609	1,135
Total current tax expense	43,854	10,186	23,288
Deferred:
Federal	6,344	26,156	973
State	704	(351)	1,094
Total deferred tax expense (benefit)	7,048	25,805	2,067
Provision for income taxes	$ 50,902	35,991	25,355
Federal statutory income tax rate (as a percent)	35.00%
Difference between the provision for income taxes and taxes computed by applying the federal statutory income tax rate to income before provision for income taxes
Income taxes at federal statutory rate	$ 52,631	39,000	26,101
Increase (reduction) of taxes resulting from:
State income taxes, net of federal tax benefit	1,970	818	1,450
Tax-exempt interest	(1,842)	(1,778)	(1,792)
Income tax credits	(1,781)	(1,257)	(1,094)
Dividends received deduction	(8)	(5)	(4)
Other, net	(68)	(787)	694
Provision for income taxes	50,902	35,991	25,355
Components of the net deferred tax asset
Allowance for loan losses	13,680	13,611
Other-than-temporary impairment on securities	295	120
Share-based compensation	1,224	1,762
Pension plan and post-retirement benefits	1,449	1,063
Deferred compensation	2,832	2,523
Purchase accounting adjustments	45,066	53,052
Other real estate owned	1,468	1,926
Tax deductible goodwill	1,361	1,624
Net operating loss carryforwards	15,757	17,127
Cash flow hedge	274	326
Other	563	1,024
Total deferred tax assets	83,969	94,158
Gain on FDIC assisted transaction deferred for tax purposes	4,493	9,856
Unrealized gains on investment securities available for sale	1,595	3,369
Depreciation	6,589	6,388
Intangible assets	13,328	15,584
Deferred loan fees	5,404	3,445
Prepaid expense	524	537
Mortgage servicing rights	9,375	7,715
Other	3,098	3,094
Total deferred tax liabilities	44,406	49,988
Net deferred tax asset (liability) before valuation allowance	39,563	44,170
Less, valuation allowance	(1,736)	(1,478)
Net deferred tax assets	37,827	42,692
Parent company
Deferred:
Provision for income taxes	(2,975)	(3,628)	(2,360)
Increase (reduction) of taxes resulting from:
Provision for income taxes	$ (2,975)	$ (3,628)	$ (2,360)