Loans Recorded investment by loan grade (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Impaired [Line Items]
Total loans	$ 7,177,785,000	$ 6,501,404,000	$ 5,692,132,000
Loans and Leases Receivable, Recorded Investment	7,198,438,000	6,520,625,000
Five Rated [Member]
Financing Receivable, Impaired [Line Items]
Total loans	103,370,000	27,473,000
Six Rated [Member]
Financing Receivable, Impaired [Line Items]
Total loans	139,000	979,000
Impaired [Member]
Financing Receivable, Impaired [Line Items]
Total loans	108,499,000	77,528,000
Purchased credit impaired [Member]
Financing Receivable, Impaired [Line Items]
Total loans	10,422,000	13,469,000
Pass [Member]
Financing Receivable, Impaired [Line Items]
Total loans	3,906,691,000	3,430,022,000
Recorded Investment [Member]
Financing Receivable, Impaired [Line Items]
Total loans	4,129,121,000	3,549,471,000
Commercial Financial And Agricultural [Member]
Financing Receivable, Impaired [Line Items]
Loans and Leases Receivable, Recorded Investment	1,595,517,000	1,189,503,000
Commercial Financial And Agricultural [Member] | Five Rated [Member]
Financing Receivable, Impaired [Line Items]
Total loans	14,638,000	11,981,000
Commercial Financial And Agricultural [Member] | Six Rated [Member]
Financing Receivable, Impaired [Line Items]
Total loans		3,000
Commercial Financial And Agricultural [Member] | Impaired [Member]
Financing Receivable, Impaired [Line Items]
Total loans	28,880,000	33,125,000
Commercial Financial And Agricultural [Member] | Purchased credit impaired [Member]
Financing Receivable, Impaired [Line Items]
Total loans	337,000	966,000
Commercial Financial And Agricultural [Member] | Pass [Member]
Financing Receivable, Impaired [Line Items]
Total loans	1,551,662,000	1,143,428,000
Commercial Financial And Agricultural [Member] | Recorded Investment [Member]
Financing Receivable, Impaired [Line Items]
Total loans	1,595,517,000	1,189,503,000
Commercial Real Estate Portfolio Segment [Member]
Financing Receivable, Impaired [Line Items]
Loans and Leases Receivable, Recorded Investment	1,754,206,000	1,614,984,000
Commercial Real Estate Portfolio Segment [Member] | Five Rated [Member]
Financing Receivable, Impaired [Line Items]
Total loans	87,439,000	6,796,000
Commercial Real Estate Portfolio Segment [Member] | Six Rated [Member]
Financing Receivable, Impaired [Line Items]
Total loans	117,000	945,000
Commercial Real Estate Portfolio Segment [Member] | Impaired [Member]
Financing Receivable, Impaired [Line Items]
Total loans	70,357,000	41,791,000
Commercial Real Estate Portfolio Segment [Member] | Purchased credit impaired [Member]
Financing Receivable, Impaired [Line Items]
Total loans	7,461,000	9,182,000
Commercial Real Estate Portfolio Segment [Member] | Pass [Member]
Financing Receivable, Impaired [Line Items]
Total loans	1,588,832,000	1,556,270,000
Commercial Real Estate Portfolio Segment [Member] | Recorded Investment [Member]
Financing Receivable, Impaired [Line Items]
Total loans	1,754,206,000	1,614,984,000
Construction Real Estate Remaining Commercial [Member]
Financing Receivable, Impaired [Line Items]
Loans and Leases Receivable, Recorded Investment	227,563,000	234,463,000
Construction Real Estate Remaining Commercial [Member] | Five Rated [Member]
Financing Receivable, Impaired [Line Items]
Total loans	164,000	4,857,000
Construction Real Estate Remaining Commercial [Member] | Six Rated [Member]
Financing Receivable, Impaired [Line Items]
Total loans		1,000
Construction Real Estate Remaining Commercial [Member] | Impaired [Member]
Financing Receivable, Impaired [Line Items]
Total loans	3,110,000	453,000
Construction Real Estate Remaining Commercial [Member] | Purchased credit impaired [Member]
Financing Receivable, Impaired [Line Items]
Total loans	1,002,000	1,044,000
Construction Real Estate Remaining Commercial [Member] | Pass [Member]
Financing Receivable, Impaired [Line Items]
Total loans	223,287,000	228,108,000
Construction Real Estate Remaining Commercial [Member] | Recorded Investment [Member]
Financing Receivable, Impaired [Line Items]
Total loans	227,563,000	234,463,000
Residential Real Estate Commercial [Member]
Financing Receivable, Impaired [Line Items]
Loans and Leases Receivable, Recorded Investment	527,383,000	480,420,000
Residential Real Estate Commercial [Member] | Five Rated [Member]
Financing Receivable, Impaired [Line Items]
Total loans	798,000	3,839,000
Residential Real Estate Commercial [Member] | Six Rated [Member]
Financing Receivable, Impaired [Line Items]
Total loans	22,000	30,000
Residential Real Estate Commercial [Member] | Impaired [Member]
Financing Receivable, Impaired [Line Items]
Total loans	4,557,000	2,025,000
Residential Real Estate Commercial [Member] | Purchased credit impaired [Member]
Financing Receivable, Impaired [Line Items]
Total loans	1,510,000	1,754,000
Residential Real Estate Commercial [Member] | Pass [Member]
Financing Receivable, Impaired [Line Items]
Total loans	520,496,000	472,772,000
Residential Real Estate Commercial [Member] | Recorded Investment [Member]
Financing Receivable, Impaired [Line Items]
Total loans	527,383,000	480,420,000
Leases
Financing Receivable, Impaired [Line Items]
Loans and Leases Receivable, Recorded Investment	24,452,000	30,101,000
Leases | Five Rated [Member]
Financing Receivable, Impaired [Line Items]
Total loans	331,000
Leases | Impaired [Member]
Financing Receivable, Impaired [Line Items]
Total loans	1,595,000	134,000
Leases | Purchased credit impaired [Member]
Financing Receivable, Impaired [Line Items]
Total loans	112,000	523,000
Leases | Pass [Member]
Financing Receivable, Impaired [Line Items]
Total loans	22,414,000	29,444,000
Leases | Recorded Investment [Member]
Financing Receivable, Impaired [Line Items]
Total loans	$ 24,452,000	30,101,000
Commercial Portfolio Segment [Member] | Purchased credit impaired [Member] | Financial Asset Acquired with Credit Deterioration [Member] | Nonperforming Financial Instruments [Member]
Financing Receivable, Impaired [Line Items]
Total loans		5,000
Loans and Leases Receivable, Recorded Investment		$ 6,000