Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Assets
Cash and due from banks	$ 230,809	$ 209,381	$ 205,973
Money market and other interest-earning investments	31,932	46,138	61,947
Total cash and cash equivalents	262,741	255,519	267,920
Trading securities, at fair value	5,235	4,982	4,838
Investment securities - available-for-sale, at fair value:
Total investment securities - available-for-sale	2,814,338	2,797,174	2,648,901
Investment securities - held-to-maturity, at amortized cost (fair value $749,363; $784,172; and $939,855, respectively)	695,139	745,090	865,957
Federal Home Loan Bank/Federal Reserve Bank stock, at cost	109,715	101,716	90,742
Loans held for sale, at fair value	27,425	90,682	44,422
Loans:
Commercial	2,001,621	1,917,099	1,893,700
Commercial real estate	3,259,998	3,130,853	2,943,525
Residential real estate	2,099,374	2,087,530	2,099,770
Consumer credit, net of unearned income	1,871,047	1,875,030	1,893,163
Total loans	9,232,040	9,010,512	8,830,158
Allowance for loan losses	(50,986)	(49,808)	(51,804)
Net loans	9,181,054	8,960,704	8,778,354
Premises and equipment, net	413,933	429,622	231,656
Accrued interest receivable	79,830	81,381	79,536
Goodwill	655,018	655,018	655,523
Other intangible assets	31,876	37,677	44,237
Company-owned life insurance	354,875	352,956	350,193
Net deferred tax assets	146,780	181,863	179,448
Loan servicing rights	25,023	25,561	25,756
Assets held for sale	11,725	5,970	4,867
Other real estate owned and repossessed personal property	11,071	18,546	24,254
Other assets	131,503	115,776	123,658
Total assets	14,957,281	14,860,237	14,420,262
Deposits:
Noninterest-bearing demand	3,011,156	3,016,093	2,883,917
Interest-bearing:
NOW	2,639,813	2,596,595	2,456,963
Savings	2,924,689	2,954,709	2,616,365
Money market	672,391	707,748	1,015,336
Time	1,435,665	1,468,108	1,479,021
Total deposits	10,683,714	10,743,253	10,451,602
Federal funds purchased and interbank borrowings	227,029	213,003	263,536
Securities sold under agreements to repurchase	298,094	367,052	354,123
Federal Home Loan Bank advances	1,515,628	1,353,092	1,099,240
Other borrowings	219,167	218,939	218,656
Accrued expenses and other liabilities	127,055	150,481	221,988
Total liabilities	13,070,687	13,045,820	12,609,145
Shareholders' Equity
Preferred stock, 2,000 shares authorized, no shares issued or outstanding
Common stock, $1.00 per share stated value, 300,000 shares authorized, 135,516; 135,159; and 135,005 shares issued and outstanding, respectively	135,516	135,159	135,005
Capital surplus	1,352,411	1,348,338	1,342,393
Retained earnings	429,787	390,292	357,336
Accumulated other comprehensive income (loss), net of tax	(31,120)	(59,372)	(23,617)
Total shareholders' equity	1,886,594	1,814,417	1,811,117
Total liabilities and shareholders' equity	14,957,281	14,860,237	14,420,262
U.S. Treasury [Member]
Investment securities - available-for-sale, at fair value:
Total investment securities - available-for-sale	5,634	7,103	12,269
U.S. Government-Sponsored Entities and Agencies [Member]
Investment securities - available-for-sale, at fair value:
Total investment securities - available-for-sale	580,624	493,956	540,775
Mortgage-Backed Securities - Agency [Member]
Investment securities - available-for-sale, at fair value:
Total investment securities - available-for-sale	1,462,111	1,525,019	1,336,605
States and Political Subdivisions [Member]
Investment securities - available-for-sale, at fair value:
Total investment securities - available-for-sale	431,874	436,684	417,163
Other Debt Securities Including Pooled Trust Preferred Securities [Member]
Investment securities - available-for-sale, at fair value:
Total investment securities - available-for-sale	$ 334,095	$ 334,412	$ 342,089