Income Taxes - Provision for income taxes - (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current:
Federal	$ 37,187	$ 41,527	$ 8,577
State	3,325	2,327	1,609
Total current tax expense	40,512	43,854	10,186
Deferred:
Federal	11,684	6,344	26,156
State	564	704	(351)
Total deferred tax expense (benefit)	12,248	7,048	25,805
Provision for income taxes	$ 52,760	50,902	35,991
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	$ 53,915	52,631	39,000
Increase (reduction) of taxes resulting from:
State income taxes, net of federal tax benefit	2,527	1,970	818
Non- deductible merger expenses	686
Tax-exempt interest	(1,956)	(1,842)	(1,778)
Income tax credits	(2,068)	(1,781)	(1,257)
Dividends received deduction	(12)	(8)	(5)
Other, net	(332)	(68)	(787)
Provision for income taxes	52,760	50,902	35,991
Components of the net deferred tax asset
Allowance for loan losses	14,913	13,680
Other-than-temporary impairment on securities	295	295
Share-based compensation	3,915	1,224
Pension plan and post-retirement benefits	1,450	1,449
Deferred compensation	3,155	2,832
Purchase accounting adjustments	34,499	45,066
Other real estate owned	962	1,468
Tax deductible goodwill	682	1,361
Net operating loss carryforwards	14,645	15,757
Cash flow hedge	190	274
Unrealized losses on investment securities available for sale	1,053
Other	688	563
Total deferred tax assets	76,447	83,969
Gain on FDIC assisted transaction deferred for tax purposes	3,540	4,493
Unrealized gains on investment securities available for sale		1,595
Depreciation	7,510	6,589
Intangible assets	11,239	13,328
Deferred loan fees	7,911	5,404
Prepaid expense	523	524
Mortgage servicing rights	10,380	9,375
Other	2,219	3,098
Total deferred tax liabilities	43,322	44,406
Net deferred tax assets before valuation allowance	33,125	39,563
Less, valuation allowance	(2,002)	(1,736)
Net deferred tax assets	31,123	37,827
Parent company
Deferred:
Provision for income taxes	(4,270)	(2,975)	(3,628)
Increase (reduction) of taxes resulting from:
Provision for income taxes	$ (4,270)	$ (2,975)	$ (3,628)