Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Summary of Balance in Allowance for Loan Losses (Details) - USD ($) $ in Thousands		Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually Evaluated for Credit Loss				$ 5,500
Total	[1]	$ 119,188		84,277
Allowance for Credit Losses, ACL [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually Evaluated for Credit Loss		2,318		5,459
Collectively Evaluated for Credit Loss		116,870		78,818
Total		119,188	$ 100,564	84,277	$ 80,399	$ 79,005	$ 79,290
Allowance for Loan Losses, LHFI [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually Evaluated for Credit Loss		42,129		41,217
Collectively Evaluated for Credit Loss		9,617,677		9,294,411
Total		9,659,806		9,335,628	80,399		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		21		707
Collectively Evaluated for Credit Loss		12,529		25,285
Total		12,550	14,564	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		16,363		23,458
Collectively Evaluated for Credit Loss		1,396,892		1,454,438
Total		1,413,255		1,477,896	27,618		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		6,397		3,379
Total		6,397	6,596	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		164,560		175,717
Total		164,560		175,738	3,248		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		1,779		1,809
Collectively Evaluated for Credit Loss		1,635		420
Total		3,414	3,441	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		4,049		4,079
Collectively Evaluated for Credit Loss		927,487		963,865
Total		931,536		967,944	438		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		518
Collectively Evaluated for Credit Loss		5,934
Total		6,452	6,020	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		5,764
Collectively Evaluated for Credit Loss		558,682
Total		564,446
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	5,836		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		11,940		8,260
Total		11,940		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		1,948		626
Collectively Evaluated for Credit Loss		480,361		1,162,165
Total		482,309		1,162,791	6,931		7,390
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		12,716
Total		12,716	11,711	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		496
Collectively Evaluated for Credit Loss		562,332
Total		562,828
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		36,417		23,803
Total		36,417	28,127	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		11,966		6,717
Collectively Evaluated for Credit Loss		2,598,426		2,468,528
Total		2,610,392		2,475,245	24,248		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		7,600		4,024
Total		7,600	5,273	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		63		68
Collectively Evaluated for Credit Loss		884,752		724,412
Total		884,815		724,480	3,721		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	$ 8,359		$ 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		10,803
Total		10,803	7,711	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		794,968
Total		794,968
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		10,899
Total		10,899	$ 8,042	$ 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		1,480
Collectively Evaluated for Credit Loss		1,249,217
Total		$ 1,250,697

[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.