Consolidated Statements of Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest income
Loans and leases	$ 955,637	$ 876,472	$ 874,892
Investment securities:
U. S. Treasury	17,657	11,837	15,353
Government agency	634	2,883	6,843
Residential mortgage-backed securities	98,341	79,336	65,815
Corporate bonds	3,877	1,783	0
Other	698	912	239
Total investment securities interest and dividend income	121,207	96,751	88,250
Overnight investments	26,846	14,534	6,067
Total interest income	1,103,690	987,757	969,209
Interest expense
Deposits	16,196	18,169	21,230
Short-term borrowings	4,838	1,965	4,660
Long-term obligations	22,760	22,948	18,414
Total interest expense	43,794	43,082	44,304
Net interest income (loss)	1,059,896	944,675	924,905
Provision for loan and lease losses	25,692	32,941	20,664
Net interest income after provision for loan and lease losses	1,034,204	911,734	904,241
Noninterest income
Gain on acquisitions	134,745	5,831	42,930
Cardholder services	95,365	83,417	77,342
Merchant services	103,962	95,774	84,207
Service charges on deposit accounts	101,201	89,359	90,546
Wealth management services	86,719	80,221	82,865
Securities gains (losses)	4,293	26,673	10,817
Other service charges and fees	28,321	27,011	23,987
Mortgage income	23,251	20,348	18,168
Insurance commissions	12,465	11,150	11,757
ATM income	9,143	7,283	7,119
Adjustments to FDIC receivable	(6,232)	(9,725)	(19,009)
Net impact from FDIC shared-loss termination	(45)	16,559	0
Other	47,841	34,170	36,359
Total noninterest income	641,029	488,071	467,088
Noninterest expense
Salaries and wages	475,214	428,351	429,742
Employee benefits	113,231	104,518	113,309
Occupancy expense	104,690	102,609	98,191
Equipment expense	97,478	92,501	92,639
Merchant processing	78,537	71,150	62,473
Cardholder processing	30,573	29,207	25,296
FDIC insurance expense	22,191	20,967	18,340
Foreclosure-related expenses	14,407	13,379	12,311
Merger-related expenses	9,015	5,341	14,174
Other	186,199	180,715	172,440
Total noninterest expense	1,131,535	1,048,738	1,038,915
Income before income taxes	543,698	351,067	332,414
Income taxes	219,946	125,585	122,028
Net income	$ 323,752	$ 225,482	$ 210,386
Average shares outstanding (in shares)	12,010,405	12,010,405	12,010,405
Cash dividends (in dollars per share)	$ 1.25	$ 1.20	$ 1.20
Net income per share	$ 26.96	$ 18.77	$ 17.52