Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Change in Allowance for Loan Losses (Details) - USD ($) $ in Thousands		3 Months Ended			6 Months Ended
		Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	[1]		$ 84,277		$ 84,277
FASB ASU 2016-13 Adoption Adjustment			19,949
PCL	[1]	$ 18,185		$ 2,486	38,766	$ 4,097
Balance at end of period	[1]	119,188			119,188
Allowance for Credit Losses, ACL [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		100,564	84,277	79,005	84,277	79,290
FASB ASU 2016-13 Adoption Adjustment					(1,217)
Loans charged-off		(1,870)		(2,937)	(7,415)	(6,970)
Recoveries		2,309		1,845	4,777	3,982
Net (charge-offs) recoveries		439		(1,092)	(2,638)	(2,988)
PCL		18,185		2,486	38,766	4,097
Balance at end of period		119,188	100,564	80,399	119,188	80,399
Allowance for Credit Losses, ACL [Member] | Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period			8,260		8,260
Balance at end of period		11,940			11,940
Allowance for Credit Losses, ACL [Member] | Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member] | Commercial LHFI [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		9,079	6,371		6,371
FASB ASU 2016-13 Adoption Adjustment					(188)
Loans charged-off		(7)			(7)
Recoveries		92			186
PCL		2,776			5,578
Balance at end of period		11,940	9,079		11,940
Allowance for Credit Losses, ACL [Member] | Other Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		11,711	5,888		5,888
FASB ASU 2016-13 Adoption Adjustment					4,188
Loans charged-off		(36)			(100)
Recoveries		83			146
PCL		958			2,594
Balance at end of period		12,716	11,711		12,716
Allowance for Credit Losses, ACL [Member] | Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period			8,932		8,932
Allowance for Credit Losses, ACL [Member] | Secured by 1-4 Family Residential Properties [Member] | Other Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		8,042	3,044		3,044
FASB ASU 2016-13 Adoption Adjustment					2,891
Loans charged-off					(19)
Recoveries		13			106
PCL		2,844			4,877
Balance at end of period		10,899	8,042		10,899
Allowance for Credit Losses, ACL [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		28,127	26,158		26,158
FASB ASU 2016-13 Adoption Adjustment					(8,179)
Loans charged-off					(2,448)
Recoveries		445			506
PCL		7,845			20,380
Balance at end of period		36,417	28,127		36,417
Allowance for Credit Losses, ACL [Member] | Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		5,273	4,024		4,024
FASB ASU 2016-13 Adoption Adjustment					(765)
Loans charged-off					(8)
Recoveries		12			18
PCL		2,315			4,331
Balance at end of period		7,600	5,273		7,600
Allowance for Credit Losses, ACL [Member] | Commercial and Industrial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		14,564	25,992		25,992
FASB ASU 2016-13 Adoption Adjustment					(8,964)
Loans charged-off		(297)			(1,279)
Recoveries		191			733
PCL		(1,908)			(3,932)
Balance at end of period		12,550	14,564		12,550
Allowance for Credit Losses, ACL [Member] | Other Construction [Member] | Other Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		7,711	1,889		1,889
FASB ASU 2016-13 Adoption Adjustment					3,202
Recoveries		21			40
PCL		3,071			5,672
Balance at end of period		10,803	7,711		10,803
Allowance for Credit Losses, ACL [Member] | Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		6,596	3,379		3,379
FASB ASU 2016-13 Adoption Adjustment					2,059
Loans charged-off		(671)			(1,361)
Recoveries		468			941
PCL		4			1,379
Balance at end of period		6,397	6,596		6,397
Allowance for Credit Losses, ACL [Member] | State and Other Political Subdivision Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		3,441	2,229		2,229
FASB ASU 2016-13 Adoption Adjustment					2,455
PCL		(27)			(1,270)
Balance at end of period		3,414	3,441		3,414
Allowance for Credit Losses, ACL [Member] | Other Commercial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		6,020	5,303		5,303
FASB ASU 2016-13 Adoption Adjustment					2,084
Loans charged-off		(859)			(2,193)
Recoveries		984			2,101
PCL		307			(843)
Balance at end of period		6,452	6,020		6,452
Allowance for Credit Losses, ACL [Member] | Other Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period			5,303		5,303
Allowance for Credit Losses, ACL [Member] | ASU 2016-13 [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
LHFI					(3,039)
Allowance for loan losses, acquired loans transfer					815
Acquired loans ACL adjustment					1,007
Allowance for Loan Losses, LHFI [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period			9,335,628		9,335,628	79,290
Loans charged-off						(6,970)
Recoveries						3,982
PCL						4,097
Balance at end of period		9,659,806		80,399	9,659,806	80,399
Allowance for Loan Losses, LHFI [Member] | Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period			1,162,791		1,162,791	7,390
Loans charged-off						(35)
Recoveries						577
PCL						(1,001)
Balance at end of period		482,309		6,931	482,309	6,931
Allowance for Loan Losses, LHFI [Member] | Other Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at end of period		562,828			562,828
Allowance for Loan Losses, LHFI [Member] | Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period			1,855,913		1,855,913	8,641
Loans charged-off						(281)
Recoveries						336
PCL						(337)
Balance at end of period				8,359		8,359
Allowance for Loan Losses, LHFI [Member] | Secured by 1-4 Family Residential Properties [Member] | Other Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at end of period		1,250,697			1,250,697
Allowance for Loan Losses, LHFI [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period			2,475,245		2,475,245	22,376
Loans charged-off						(34)
Recoveries						30
PCL						1,876
Balance at end of period		2,610,392		24,248	2,610,392	24,248
Allowance for Loan Losses, LHFI [Member] | Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period			724,480		724,480	3,450
Recoveries						16
PCL						255
Balance at end of period		884,815		3,721	884,815	3,721
Allowance for Loan Losses, LHFI [Member] | Commercial and Industrial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period			1,477,896		1,477,896	27,359
Loans charged-off						(2,831)
Recoveries						329
PCL						2,761
Balance at end of period		1,413,255		27,618	1,413,255	27,618
Allowance for Loan Losses, LHFI [Member] | Other Construction [Member] | Other Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at end of period		794,968			794,968
Allowance for Loan Losses, LHFI [Member] | Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period			175,738		175,738	2,890
Loans charged-off						(1,269)
Recoveries						954
PCL						673
Balance at end of period		164,560		3,248	164,560	3,248
Allowance for Loan Losses, LHFI [Member] | State and Other Political Subdivision Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period			967,944		967,944	990
PCL						(552)
Balance at end of period		931,536		438	931,536	438
Allowance for Loan Losses, LHFI [Member] | Other Commercial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at end of period		$ 564,446			564,446
Allowance for Loan Losses, LHFI [Member] | Other Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period			$ 495,621		$ 495,621	6,194
Loans charged-off						(2,520)
Recoveries						1,740
PCL						422
Balance at end of period				$ 5,836		$ 5,836

[1] Effective January 1, 2020, Trustmark adopted FASB ASU 2016-13 using the modified retrospective approach.  Therefore, prior period balances are presented under legacy GAAP and may not be comparable to current period presentation.