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Allowance for Credit Losses (By Loan Class) (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Allowance for probable loan losses
Balance at the beginning of the period	$ 94,554	$ 58,203		$ 61,384	$ 60,278	$ 61,384
Losses charged to allowance	(1,887)	(4,293)			(7,209)	(19,172)
Recoveries credited to allowance	728	672			2,154	2,285
Net (losses) recoveries charged to allowance	(1,159)	(3,621)			(5,055)	(16,887)
Provision charged (credited) to operations	8,770	5,278			36,595	15,363
Balance at the end of the period	102,165	59,860	$ 58,203		102,165	59,860
Watch - List Impaired
Allowance for probable loan losses
Losses charged to allowance				(9,500)
ASU 2016-13 | 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						(11,011)
Recoveries credited to allowance						1,631
Net (losses) recoveries charged to allowance						(9,380)
Provision charged (credited) to operations						7,755
Balance at the end of the period		10,971				10,971
Commercial, financial and agricultural | Domestic | Commercial
Allowance for probable loan losses
Balance at the beginning of the period	20,079	11,780			11,145
Losses charged to allowance	(1,735)	(3,231)			(6,696)
Recoveries credited to allowance	523	585			1,793
Net (losses) recoveries charged to allowance	(1,212)	(2,646)			(4,903)
Provision charged (credited) to operations	3,277	1,837			11,655
Balance at the end of the period	22,144	10,971	11,780		22,144	10,971
Commercial, financial and agricultural | Domestic | Commercial | ASU 2016-13 | 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	18,711	17,029		19,353	16,533	19,353
Losses charged to allowance		(468)			(55)	(7,347)
Recoveries credited to allowance	86	10			107	308
Net (losses) recoveries charged to allowance	86	(458)			52	(7,039)
Provision charged (credited) to operations	4,892	230			11,396	4,487
Balance at the end of the period	23,689	16,801	17,029		23,689	16,801
Commercial, financial and agricultural | Domestic | Commercial Real Estate: farmland and commercial | ASU 2016-13 | 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	1,864	1,849		1,808	1,786	1,808
Provision charged (credited) to operations	2,639	350			3,072	391
Balance at the end of the period	4,503	2,199	1,849		4,503	2,199
Commercial, financial and agricultural | Domestic | Commercial Real Estate: multifamily | ASU 2016-13 | 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	40,924	15,273		15,123	18,152	15,123
Losses charged to allowance					(19)
Recoveries credited to allowance	14	4			15	80
Net (losses) recoveries charged to allowance	14	4			(4)	80
Provision charged (credited) to operations	(2,855)	1,573			6,544	1,647
Balance at the end of the period	38,083	16,850	15,273		38,083	16,850
Real estate - construction | Domestic | Commercial Real Estate: other construction and land development | ASU 2016-13 | 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	3,208	3,627		3,467	3,762	3,467
Losses charged to allowance	(87)	(164)			(123)	(166)
Recoveries credited to allowance	8	3			10	14
Net (losses) recoveries charged to allowance	(79)	(161)			(113)	(152)
Provision charged (credited) to operations	400	263			1,460	414
Balance at the end of the period	3,529	3,729	3,627		3,529	3,729
Real estate - mortgage | Domestic | Residential First Lien | ASU 2016-13 | 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	8,964	7,312		7,719	7,535	7,719
Losses charged to allowance	(3)	(214)			(124)	(314)
Recoveries credited to allowance	51	65			171	222
Net (losses) recoveries charged to allowance	48	(149)			47	(92)
Provision charged (credited) to operations	293	852			2,152	388
Balance at the end of the period	9,305	8,015	7,312		9,305	8,015
Real estate - mortgage | Domestic | Residential Junior Lien | ASU 2016-13 | 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	288	488		447	542	447
Losses charged to allowance	(62)	(216)			(192)	(334)
Recoveries credited to allowance	46	5			58	30
Net (losses) recoveries charged to allowance	(16)	(211)			(134)	(304)
Provision charged (credited) to operations	2	199			91	333
Balance at the end of the period	274	476	488		274	476
Consumer | Domestic | ASU 2016-13 | 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	516	845		$ 871	823	871
Provision charged (credited) to operations	122	(26)			225	(52)
Balance at the end of the period	$ 638	$ 819	$ 845		638	$ 819
Foreign | Foreign | ASU 2016-13 | Adjustment
Allowance for probable loan losses
Balance at the beginning of the period					$ (410)

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Amount, after recovery, of writeoff of financing receivable, charged against allowance for credit loss.

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Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

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