Income Taxes - Provision for income taxes - (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	$ 59,082	$ 53,915	$ 52,631
Increase (reduction) of taxes resulting from:
State income taxes, net of federal tax benefit	2,032	2,527	1,970
Non- deductible merger expenses	586	686
Tax-exempt interest	(2,840)	(1,956)	(1,842)
Income tax credits	(1,951)	(2,068)	(1,781)
Dividends received deduction	(12)	(12)	(8)
DTA, Revaluation due to tax law change	26,558
Other, net	2,204	332	68
Provision for income taxes	81,251	52,760	50,902
Components of the net deferred tax asset
Allowance for loan losses	11,011	14,913
Other-than-temporary impairment on securities	257	295
Share-based compensation	3,935	3,915
Pension plan and post-retirement benefits	607	1,450
Deferred compensation	13,048	3,155
Purchase accounting adjustments	33,888	34,499
FDIC Acquisition	3,190
Other real estate owned	838	962
Tax deductible goodwill	32	682
Net operating loss carryforwards	14,066	14,645
Cash flow hedge	54	190
Unrealized losses on investment securities available for sale	3,472	1,053
Other	643	688
Total deferred tax assets	85,041	76,447
Gain on FDIC assisted transaction deferred for tax purposes		3,540
Depreciation	5,870	7,510
Intangible assets	14,745	11,239
Deferred loan fees	7,840	7,911
Prepaid expense	474	523
Mortgage servicing rights	6,818	10,380
Other	372	2,219
Total deferred tax liabilities	36,119	43,322
Net deferred tax assets before valuation allowance	48,922	33,125
Less, valuation allowance	(3,020)	(2,002)
Net deferred tax assets	45,902	31,123
Parent company
Increase (reduction) of taxes resulting from:
Provision for income taxes	$ (5,053)	$ (4,270)	$ (2,975)