Loans Recorded investment by loan grade (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Total loans	$ 6,501,404,000	$ 5,692,132,000	$ 5,372,483,000
Loans and Leases Receivable, Recorded Investment	6,520,625,000	5,709,798,000
Five Rated [Member]
Financing Receivable, Impaired [Line Items]
Total loans	27,473,000	16,048,000
Six Rated [Member]
Financing Receivable, Impaired [Line Items]
Total loans	979,000	485,000
Impaired [Member]
Financing Receivable, Impaired [Line Items]
Total loans	77,528,000	48,218,000
Purchased credit impaired [Member]
Financing Receivable, Impaired [Line Items]
Total loans	13,469,000	3,961,000
Pass [Member]
Financing Receivable, Impaired [Line Items]
Total loans	3,430,022,000	2,905,752,000
Recorded Investment [Member]
Financing Receivable, Impaired [Line Items]
Total loans	3,549,471,000	2,974,464,000
Commercial Financial And Agricultural [Member]
Financing Receivable, Impaired [Line Items]
Loans and Leases Receivable, Recorded Investment	1,189,503,000	1,077,389,000
Commercial Financial And Agricultural [Member] | Five Rated [Member]
Financing Receivable, Impaired [Line Items]
Total loans	11,981,000	11,509,000
Commercial Financial And Agricultural [Member] | Six Rated [Member]
Financing Receivable, Impaired [Line Items]
Total loans	3,000	444,000
Commercial Financial And Agricultural [Member] | Impaired [Member]
Financing Receivable, Impaired [Line Items]
Total loans	33,125,000	15,194,000
Commercial Financial And Agricultural [Member] | Purchased credit impaired [Member]
Financing Receivable, Impaired [Line Items]
Total loans	966,000	148,000
Commercial Financial And Agricultural [Member] | Pass [Member]
Financing Receivable, Impaired [Line Items]
Total loans	1,143,428,000	1,050,094,000
Commercial Financial And Agricultural [Member] | Recorded Investment [Member]
Financing Receivable, Impaired [Line Items]
Total loans	1,189,503,000	1,077,389,000
Commercial Real Estate Portfolio Segment [Member]
Financing Receivable, Impaired [Line Items]
Loans and Leases Receivable, Recorded Investment	1,614,984,000	1,287,795,000
Commercial Real Estate Portfolio Segment [Member] | Five Rated [Member]
Financing Receivable, Impaired [Line Items]
Total loans	6,796,000	2,707,000
Commercial Real Estate Portfolio Segment [Member] | Six Rated [Member]
Financing Receivable, Impaired [Line Items]
Total loans	945,000
Commercial Real Estate Portfolio Segment [Member] | Impaired [Member]
Financing Receivable, Impaired [Line Items]
Total loans	41,791,000	28,426,000
Commercial Real Estate Portfolio Segment [Member] | Purchased credit impaired [Member]
Financing Receivable, Impaired [Line Items]
Total loans	9,182,000	3,059,000
Commercial Real Estate Portfolio Segment [Member] | Pass [Member]
Financing Receivable, Impaired [Line Items]
Total loans	1,556,270,000	1,253,603,000
Commercial Real Estate Portfolio Segment [Member] | Recorded Investment [Member]
Financing Receivable, Impaired [Line Items]
Total loans	1,614,984,000	1,287,795,000
Construction Real Estate Remaining Commercial [Member]
Financing Receivable, Impaired [Line Items]
Loans and Leases Receivable, Recorded Investment	234,463,000	176,101,000
Construction Real Estate Remaining Commercial [Member] | Five Rated [Member]
Financing Receivable, Impaired [Line Items]
Total loans	4,857,000	1,560,000
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	453,000	1,866,000
Construction Real Estate Remaining Commercial [Member] | Purchased credit impaired [Member]
Financing Receivable, Impaired [Line Items]
Total loans	1,044,000	503,000
Construction Real Estate Remaining Commercial [Member] | Pass [Member]
Financing Receivable, Impaired [Line Items]
Total loans	228,108,000	172,172,000
Construction Real Estate Remaining Commercial [Member] | Recorded Investment [Member]
Financing Receivable, Impaired [Line Items]
Total loans	234,463,000	176,101,000
Residential Real Estate Commercial [Member]
Financing Receivable, Impaired [Line Items]
Loans and Leases Receivable, Recorded Investment	480,420,000	430,880,000
Residential Real Estate Commercial [Member] | Five Rated [Member]
Financing Receivable, Impaired [Line Items]
Total loans	3,839,000	272,000
Residential Real Estate Commercial [Member] | Six Rated [Member]
Financing Receivable, Impaired [Line Items]
Total loans	30,000	41,000
Residential Real Estate Commercial [Member] | Impaired [Member]
Financing Receivable, Impaired [Line Items]
Total loans	2,025,000	2,732,000
Residential Real Estate Commercial [Member] | Purchased credit impaired [Member]
Financing Receivable, Impaired [Line Items]
Total loans	1,754,000	251,000
Residential Real Estate Commercial [Member] | Pass [Member]
Financing Receivable, Impaired [Line Items]
Total loans	472,772,000	427,584,000
Residential Real Estate Commercial [Member] | Recorded Investment [Member]
Financing Receivable, Impaired [Line Items]
Total loans	480,420,000	430,880,000
Leases
Financing Receivable, Impaired [Line Items]
Loans and Leases Receivable, Recorded Investment	30,101,000	2,299,000
Leases | Impaired [Member]
Financing Receivable, Impaired [Line Items]
Total loans	134,000
Leases | Purchased credit impaired [Member]
Financing Receivable, Impaired [Line Items]
Total loans	523,000	0
Leases | Pass [Member]
Financing Receivable, Impaired [Line Items]
Total loans	29,444,000	2,299,000
Leases | Recorded Investment [Member]
Financing Receivable, Impaired [Line Items]
Total loans	30,101,000	2,299,000
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	475,000
Loans and Leases Receivable, Recorded Investment	$ 6,000	$ 475,000