Consolidated Statements of Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest income:
Loans, including fees	$ 534,804	$ 458,094	$ 433,872
Securities:
Taxable	92,979	103,025	112,601
Tax-exempt	222,620	210,918	194,793
Interest-bearing deposits	41,608	16,103	8,123
Federal funds sold and resell agreements	936	272	107
Total interest income	892,947	788,412	749,496
Interest expense:
Deposits	17,188	7,248	9,024
Federal funds purchased and repurchase agreements	1,522	204	167
Junior subordinated deferrable interest debentures	3,955	3,281	2,725
Other long-term borrowings	3,860	1,343	948
Total interest expense	26,525	12,076	12,864
Net interest income	866,422	776,336	736,632
Provision for loan losses	35,460	51,673	51,845
Net interest income after provision for loan losses	830,962	724,663	684,787
Non-interest income:
Trust and investment management fees	110,675	104,240	105,512
Service charges on deposit accounts	84,182	81,203	81,350
Insurance commissions and fees	46,169	47,154	48,926
Interchange and debit card transaction fees	23,232	21,369	19,666
Other charges, commissions and fees	39,931	39,623	37,551
Net gain (loss) on securities transactions	(4,941)	14,975	69
Other	37,222	41,144	35,656
Total non-interest income	336,470	349,708	328,730
Non-interest expense:
Salaries and wages	337,068	318,665	310,504
Employee benefits	74,575	72,615	69,746
Net occupancy	75,971	71,627	65,690
Technology, furniture and equipment	74,335	71,208	64,373
Deposit insurance	20,128	17,428	14,519
Intangible amortization	1,703	2,429	3,325
Other	175,289	178,988	165,561
Total non-interest expense	759,069	732,960	693,718
Income before income taxes	408,363	341,411	319,799
Income taxes	44,214	37,150	40,471
Net income	364,149	304,261	279,328
Preferred stock dividends	8,063	8,063	8,063
Net income available to common shareholders	$ 356,086	$ 296,198	$ 271,265
Earnings per common share:
Basic (in dollars per share)	$ 5.56	$ 4.73	$ 4.31
Diluted (in dollars per share)	$ 5.51	$ 4.70	$ 4.28