CONSOLIDATED STATEMENTS OF INCOME (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Interest income:
Interest and fees on loans	$ 185,527	$ 188,197	$ 192,710
Interest and dividends on taxable investments	50,247	54,995	65,165
Interest and dividends on nontaxable investments	20,446	20,967	23,525
Total interest income	256,220	264,159	281,400
Interest expense:
Interest on deposits	8,191	10,732	18,162
Interest on borrowings	1,124	12,813	30,098
Interest on subordinated debt held by unconsolidated subsidiary trusts	2,477	2,520	2,716
Total interest expense	11,792	26,065	50,976
Net interest income	244,428	238,094	230,424
Provision for loan losses	7,178	7,992	9,108
Net interest income after provision for loan losses	237,250	230,102	221,316
Noninterest income:
Deposit service fees	52,756	49,357	46,064
Other banking services	5,814	5,245	4,069
Employee benefit services	42,580	38,596	35,946
Wealth management services	17,870	15,550	12,876
Gain on sales of investment securities, net	0	80,768	291
Loss on debt extinguishments	0	(87,336)	0
Total noninterest income	119,020	102,180	99,246
Noninterest expenses:
Salaries and employee benefits	123,077	121,629	112,034
Occupancy and equipment	27,948	27,045	25,799
Data processing and communications	29,294	27,186	23,696
Amortization of intangible assets	4,287	4,469	4,607
Legal and professional fees	7,247	7,008	7,950
Office supplies and postage	6,270	6,122	5,742
Business development and marketing	7,125	6,815	5,919
FDIC insurance premiums	3,899	3,829	3,804
Acquisition expenses	123	2,181	5,747
Other expenses	17,310	14,971	16,459
Total noninterest expenses	226,580	221,255	211,757
Income before income taxes	129,690	111,027	108,805
Income taxes	38,337	32,198	31,737
Net income	$ 91,353	$ 78,829	$ 77,068
Basic earnings per share (in dollars per share)	$ 2.24	$ 1.96	$ 1.95
Diluted earnings per share (in dollars per share)	$ 2.22	$ 1.94	$ 1.93