Allowance for Credit Losses (By Loan Class) (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Allowance for probable loan losses
Balance at the beginning of the period	$ 85,273	$ 67,030	$ 61,384	$ 60,278	$ 61,384
Losses charged to allowance	(2,342)	(12,044)		(5,322)	(14,879)
Recoveries credited to allowance	634	552		1,426	1,613
Net losses (recoveries) charged to allowance	(1,708)	(11,492)		(3,896)	(13,266)
Provision charged to operations	10,989	2,665		27,825	10,085
Balance at the end of the period	94,554	58,203	67,030	94,554	58,203
Watch - List Impaired
Allowance for probable loan losses
Losses charged to allowance			(9,500)
ASU 2016-13 | Restatement Adjustment
Allowance for probable loan losses
Balance at the beginning of the period				10,347
Commercial Real Estate: farmland and commercial | Watch - List Impaired
Allowance for probable loan losses
Losses charged to allowance		(9,500)
Commercial, financial and agricultural | Commercial
Allowance for probable loan losses
Balance at the beginning of the period			12,596		12,596
Losses charged to allowance					(7,780)
Recoveries credited to allowance					1,046
Net losses (recoveries) charged to allowance					(6,734)
Provision charged to operations					5,918
Balance at the end of the period		11,780			11,780
Commercial, financial and agricultural | Domestic | Commercial
Allowance for probable loan losses
Balance at the beginning of the period	19,067	11,412		11,145
Losses charged to allowance	(2,142)	(5,016)		(4,961)
Recoveries credited to allowance	599	408		1,270
Net losses (recoveries) charged to allowance	(1,543)	(4,608)		(3,691)
Provision charged to operations	2,555	4,976		8,378
Balance at the end of the period	20,079	11,780	11,412	20,079	11,780
Commercial, financial and agricultural | Domestic | Commercial | ASU 2016-13 | Restatement Adjustment
Allowance for probable loan losses
Balance at the beginning of the period				4,247
Commercial, financial and agricultural | Domestic | Commercial Real Estate: farmland and commercial
Allowance for probable loan losses
Balance at the beginning of the period	15,004	25,880	19,353	16,533	19,353
Losses charged to allowance		(6,878)		(55)	(6,879)
Recoveries credited to allowance	12	15		21	298
Net losses (recoveries) charged to allowance	12	(6,863)		(34)	(6,581)
Provision charged to operations	3,695	(1,988)		6,504	4,257
Balance at the end of the period	18,711	17,029	25,880	18,711	17,029
Commercial, financial and agricultural | Domestic | Commercial Real Estate: farmland and commercial | ASU 2016-13 | Restatement Adjustment
Allowance for probable loan losses
Balance at the beginning of the period				(4,292)
Commercial, financial and agricultural | Domestic | Commercial Real Estate: multifamily
Allowance for probable loan losses
Balance at the beginning of the period	1,758	2,345	1,808	1,786	1,808
Provision charged to operations	106	(496)		433	41
Balance at the end of the period	1,864	1,849	2,345	1,864	1,849
Commercial, financial and agricultural | Domestic | Commercial Real Estate: multifamily | ASU 2016-13 | Restatement Adjustment
Allowance for probable loan losses
Balance at the beginning of the period				(355)
Real estate - construction | Domestic | Commercial Real Estate: other construction and land development
Allowance for probable loan losses
Balance at the beginning of the period	37,421	14,789	15,123	18,152	15,123
Losses charged to allowance	(19)			(19)
Recoveries credited to allowance	1	56		1	76
Net losses (recoveries) charged to allowance	(18)	56		(18)	76
Provision charged to operations	3,521	428		9,399	74
Balance at the end of the period	40,924	15,273	14,789	40,924	15,273
Real estate - construction | Domestic | Commercial Real Estate: other construction and land development | ASU 2016-13 | Restatement Adjustment
Allowance for probable loan losses
Balance at the beginning of the period				13,391
Real estate - mortgage | Domestic | Residential First Lien
Allowance for probable loan losses
Balance at the beginning of the period	2,782	3,546	3,467	3,762	3,467
Losses charged to allowance		(1)		(36)	(2)
Recoveries credited to allowance	1	10		2	11
Net losses (recoveries) charged to allowance	1	9		(34)	9
Provision charged to operations	425	72		1,060	151
Balance at the end of the period	3,208	3,627	3,546	3,208	3,627
Real estate - mortgage | Domestic | Residential First Lien | ASU 2016-13 | Restatement Adjustment
Allowance for probable loan losses
Balance at the beginning of the period				(1,580)
Real estate - mortgage | Domestic | Residential Junior Lien
Allowance for probable loan losses
Balance at the beginning of the period	8,452	7,745	7,719	7,535	7,719
Losses charged to allowance	(121)	(94)		(121)	(100)
Recoveries credited to allowance	17	55		120	157
Net losses (recoveries) charged to allowance	(104)	(39)		(1)	57
Provision charged to operations	616	(394)		1,859	(464)
Balance at the end of the period	8,964	7,312	7,745	8,964	7,312
Real estate - mortgage | Domestic | Residential Junior Lien | ASU 2016-13 | Restatement Adjustment
Allowance for probable loan losses
Balance at the beginning of the period				(429)
Consumer | Domestic
Allowance for probable loan losses
Balance at the beginning of the period	323	462	447	542	447
Losses charged to allowance	(60)	(55)		(130)	(118)
Recoveries credited to allowance	4	8		12	25
Net losses (recoveries) charged to allowance	(56)	(47)		(118)	(93)
Provision charged to operations	21	73		89	134
Balance at the end of the period	288	488	462	288	488
Consumer | Domestic | ASU 2016-13 | Restatement Adjustment
Allowance for probable loan losses
Balance at the beginning of the period				(225)
Foreign | Foreign
Allowance for probable loan losses
Balance at the beginning of the period	466	851	871	823	871
Provision charged to operations	50	(6)		103	(26)
Balance at the end of the period	$ 516	$ 845	$ 851	516	$ 845
Foreign | Foreign | ASU 2016-13 | Restatement Adjustment
Allowance for probable loan losses
Balance at the beginning of the period				$ (410)