Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Summary of Balance in Allowance for Loan Losses (Details) - USD ($) $ in Thousands		Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually Evaluated for Credit Loss				$ 5,500
Total	[1]	$ 122,010		84,277
Allowance for Credit Losses, ACL [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually Evaluated for Credit Loss		4,622		5,459
Collectively Evaluated for Credit Loss		117,388		78,818
Total		122,010	$ 119,188	84,277	$ 83,226	$ 80,399	$ 79,290
Allowance for Loan Losses, LHFI [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually Evaluated for Credit Loss		41,167		41,217
Collectively Evaluated for Credit Loss		9,806,561		9,294,411
Total		9,847,728		9,335,628	83,226		79,290
Commercial and Industrial Loans [Member] | Allowance for Credit Losses, ACL [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually Evaluated for Credit Loss		578		707
Collectively Evaluated for Credit Loss		12,580		25,285
Total		13,158	12,550	25,992
Commercial and Industrial Loans [Member] | Allowance for Loan Losses, LHFI [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually Evaluated for Credit Loss		14,882		23,458
Collectively Evaluated for Credit Loss		1,383,586		1,454,438
Total		1,398,468		1,477,896	28,867		27,359
Consumer Loans [Member] | Allowance for Credit Losses, ACL [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Collectively Evaluated for Credit Loss		6,040		3,379
Total		6,040	6,397	3,379
Consumer Loans [Member] | Allowance for Loan Losses, LHFI [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually Evaluated for Credit Loss				21
Collectively Evaluated for Credit Loss		163,933		175,717
Total		163,933		175,738	3,312		2,890
State and Other Political Subdivision Loans [Member] | Allowance for Credit Losses, ACL [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually Evaluated for Credit Loss		1,749		1,809
Collectively Evaluated for Credit Loss		1,209		420
Total		2,958	3,414	2,229
State and Other Political Subdivision Loans [Member] | Allowance for Loan Losses, LHFI [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually Evaluated for Credit Loss		4,019		4,079
Collectively Evaluated for Credit Loss		931,330		963,865
Total		935,349		967,944	472		990
Other Commercial Loans [Member] | Allowance for Credit Losses, ACL [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually Evaluated for Credit Loss		2,295
Collectively Evaluated for Credit Loss		5,419
Total		7,714	6,452	5,303
Other Commercial Loans [Member] | Allowance for Loan Losses, LHFI [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually Evaluated for Credit Loss		5,725
Collectively Evaluated for Credit Loss		587,487
Total		593,212
Other Loans [Member] | Allowance for Credit Losses, ACL [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually Evaluated for Credit Loss				553
Collectively Evaluated for Credit Loss				4,750
Total				5,303
Other Loans [Member] | Allowance for Loan Losses, LHFI [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually Evaluated for Credit Loss				784
Collectively Evaluated for Credit Loss				494,837
Total				495,621	5,674		6,194
Loans Secured by Real Estate [Member] | Construction, Land Development and Other Land [Member] | Allowance for Credit Losses, ACL [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Collectively Evaluated for Credit Loss		10,905		8,260
Total		10,905		8,260
Loans Secured by Real Estate [Member] | Construction, Land Development and Other Land [Member] | Allowance for Loan Losses, LHFI [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually Evaluated for Credit Loss		1,024		626
Collectively Evaluated for Credit Loss		493,495		1,162,165
Total		494,519		1,162,791
Loans Secured by Real Estate [Member] | Other Secured by 1-4 Family Residential Properties [Member] | Allowance for Credit Losses, ACL [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Collectively Evaluated for Credit Loss		11,358
Total		11,358	12,716	5,888
Loans Secured by Real Estate [Member] | Other Secured by 1-4 Family Residential Properties [Member] | Allowance for Loan Losses, LHFI [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually Evaluated for Credit Loss		474
Collectively Evaluated for Credit Loss		546,674
Total		547,148
Loans Secured by Real Estate [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Allowance for Credit Losses, ACL [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually Evaluated for Credit Loss				2,355
Collectively Evaluated for Credit Loss		43,762		23,803
Total		43,762	36,417	26,158
Loans Secured by Real Estate [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Allowance for Loan Losses, LHFI [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually Evaluated for Credit Loss		13,529		6,717
Collectively Evaluated for Credit Loss		2,694,098		2,468,528
Total		2,707,627		2,475,245	25,202		22,376
Loans Secured by Real Estate [Member] | Other Real Estate Secured [Member] | Allowance for Credit Losses, ACL [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Collectively Evaluated for Credit Loss		7,172		4,024
Total		7,172	7,600	4,024
Loans Secured by Real Estate [Member] | Other Real Estate Secured [Member] | Allowance for Loan Losses, LHFI [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually Evaluated for Credit Loss		61		68
Collectively Evaluated for Credit Loss		887,731		724,412
Total		887,792		724,480	3,554		3,450
Loans Secured by Real Estate [Member] | Secured by 1-4 Family Residential Properties [Member] | Allowance for Credit Losses, ACL [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually Evaluated for Credit Loss				35
Collectively Evaluated for Credit Loss				8,897
Total				8,932
Loans Secured by Real Estate [Member] | Secured by 1-4 Family Residential Properties [Member] | Allowance for Loan Losses, LHFI [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually Evaluated for Credit Loss				5,464
Collectively Evaluated for Credit Loss				1,850,449
Total				1,855,913	$ 8,444		$ 8,641
Other Loans Secured by Real Estate [Member] | Other Construction [Member] | Allowance for Credit Losses, ACL [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Collectively Evaluated for Credit Loss		10,209
Total		10,209	10,803	1,889
Other Loans Secured by Real Estate [Member] | Other Construction [Member] | Allowance for Loan Losses, LHFI [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Collectively Evaluated for Credit Loss		891,428
Total		891,428
Other Loans Secured by Real Estate [Member] | Secured by 1-4 Family Residential Properties [Member] | Allowance for Credit Losses, ACL [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Collectively Evaluated for Credit Loss		8,734
Total		8,734	$ 10,899	$ 3,044
Other Loans Secured by Real Estate [Member] | Secured by 1-4 Family Residential Properties [Member] | Allowance for Loan Losses, LHFI [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually Evaluated for Credit Loss		1,453
Collectively Evaluated for Credit Loss		1,226,799
Total		$ 1,228,252

[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.