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Allowance for Credit Losses (Impaired Loans) (Details) - USD ($)	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Nov. 03, 2020
Recorded investment:
Total impaired loans with related allowance				$ 636,000
Total impaired loans with no related allowance				12,039,000
Unpaid principal balance:
Total impaired loans with an allowance recorded				685,000
Total impaired loans with no related allowance recorded				13,255,000
Related Allowance				365,000
Average recorded investment:
Total impaired loans with an allowance recorded	$ 731,000		$ 748,000	645,000
Total impaired loans with no related allowance recorded	22,297,000		51,852,000	65,192,000
Interest Recognized
Total impaired loans with no related allowance recorded	92,000		274,000	368,000
Total trouble debt restructuring		$ 5,835,000		7,788,000
Loans		$ 7,589,235,000		6,894,946,000
Period of charge off for past due unsecured commercial loans		90 days
Subsequent Event
Interest Recognized
Amount of loans with some degree of payment deferral					$ 1,799,000,000
Percentage of loan payments on deferral period					83.00%
Percentage of loan sort of deferral program for relief					17.00%
Paycheck protection program loans
Interest Recognized
Period of time PPP loans cover payroll and certain other costs		168 days
Interest rate on receivable		1.00%
Paycheck protection program loans | Subsequent Event
Interest Recognized
Number of loans approved and closed					4,944
Amount of loans approved and closed					$ 455,347,000
Paycheck protection program loans | Loans made prior to June 5, 2020
Interest Recognized
Term of receivable (in years)		2 years
Paycheck protection program loans | Loans made after June 5, 2020
Interest Recognized
Term of receivable (in years)		5 years
Commercial, financial and agricultural
Interest Recognized
Loans		$ 4,426,866,000		3,379,837,000
Commercial, financial and agricultural | Domestic | Commercial
Recorded investment:
Total impaired loans with related allowance				510,000
Total impaired loans with no related allowance				1,425,000
Unpaid principal balance:
Total impaired loans with an allowance recorded				516,000
Total impaired loans with no related allowance recorded				1,516,000
Related Allowance				249,000
Average recorded investment:
Total impaired loans with an allowance recorded	602,000		616,000	514,000
Total impaired loans with no related allowance recorded	8,002,000		17,945,000	16,194,000
Interest Recognized
Total impaired loans with no related allowance recorded	1,000		2,000	3,000
Total trouble debt restructuring				32,000
Loans		1,877,979,000		1,292,660,000
Commercial, financial and agricultural | Domestic | Commercial Real Estate: farmland and commercial
Recorded investment:
Total impaired loans with no related allowance				1,208,000
Unpaid principal balance:
Total impaired loans with no related allowance recorded				1,841,000
Average recorded investment:
Total impaired loans with no related allowance recorded	3,222,000		22,389,000	36,632,000
Interest Recognized
Loans		2,110,950,000		1,896,747,000
Commercial, financial and agricultural | Domestic | Commercial Real Estate: multifamily
Recorded investment:
Total impaired loans with no related allowance				165,000
Unpaid principal balance:
Total impaired loans with no related allowance recorded				168,000
Average recorded investment:
Total impaired loans with no related allowance recorded	653,000		656,000	565,000
Interest Recognized
Loans		437,937,000		190,430,000
Real estate - construction
Interest Recognized
Loans		1,943,430,000		2,185,883,000
Real estate - construction | Domestic | Commercial Real Estate: other construction and land development
Recorded investment:
Total impaired loans with related allowance				126,000
Total impaired loans with no related allowance				812,000
Unpaid principal balance:
Total impaired loans with an allowance recorded				169,000
Total impaired loans with no related allowance recorded				1,133,000
Related Allowance				116,000
Average recorded investment:
Total impaired loans with an allowance recorded	129,000		132,000	131,000
Total impaired loans with no related allowance recorded	1,674,000		1,767,000	2,151,000
Interest Recognized
Loans		1,943,430,000		2,185,883,000
Real estate - mortgage
Interest Recognized
Loans		1,033,646,000		1,140,377,000
Real estate - mortgage | Domestic | Residential First Lien
Recorded investment:
Total impaired loans with no related allowance				6,278,000
Unpaid principal balance:
Total impaired loans with no related allowance recorded				6,445,000
Average recorded investment:
Total impaired loans with no related allowance recorded	6,427,000		6,698,000	7,136,000
Interest Recognized
Total impaired loans with no related allowance recorded	78,000		231,000	305,000
Total trouble debt restructuring		4,149,000		5,608,000
Loans		410,015,000		433,901,000
Real estate - mortgage | Domestic | Residential Junior Lien
Recorded investment:
Total impaired loans with no related allowance				692,000
Unpaid principal balance:
Total impaired loans with no related allowance recorded				692,000
Average recorded investment:
Total impaired loans with no related allowance recorded	914,000		1,028,000	976,000
Interest Recognized
Total impaired loans with no related allowance recorded	10,000		32,000	44,000
Total trouble debt restructuring		557,000		692,000
Loans		623,631,000		706,476,000
Consumer
Interest Recognized
Loans		41,110,000		47,800,000
Consumer | Domestic
Recorded investment:
Total impaired loans with no related allowance				1,195,000
Unpaid principal balance:
Total impaired loans with no related allowance recorded				1,196,000
Average recorded investment:
Total impaired loans with no related allowance recorded	1,131,000		1,088,000	1,211,000
Interest Recognized
Total impaired loans with no related allowance recorded				2,000
Total trouble debt restructuring		888,000		1,192,000
Loans		41,110,000		47,800,000
Foreign
Interest Recognized
Loans		144,183,000		141,049,000
Foreign | Foreign
Recorded investment:
Total impaired loans with no related allowance				264,000
Unpaid principal balance:
Total impaired loans with no related allowance recorded				264,000
Average recorded investment:
Total impaired loans with no related allowance recorded	274,000		281,000	327,000
Interest Recognized
Total impaired loans with no related allowance recorded	$ 3,000		$ 9,000	14,000
Total trouble debt restructuring		241,000		264,000
Loans		$ 144,183,000		$ 141,049,000

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