Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Change in Allowance for Loan Losses (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	[1]			$ 84,277
PCL	[1]	$ 1,760	$ 3,039	40,526	$ 7,136
Balance at end of period	[1]	122,010		122,010
Allowance for Credit Losses, ACL [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		119,188	80,399	84,277	79,290
Loans charged-off		(1,263)	(2,892)	(8,678)	(9,862)
Recoveries		2,325	2,680	7,102	6,662
Net (charge-offs) recoveries		1,062	(212)	(1,576)	(3,200)
PCL		1,760	3,039	40,526	7,136
Balance at end of period		122,010	83,226	122,010	83,226
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
Balance at end of period		10,905		10,905
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		11,940		6,371
Loans charged-off				(7)
Recoveries		443		629
PCL		(1,478)		4,100
Balance at end of period		10,905		10,905
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		12,716		5,888
Loans charged-off		(18)		(118)
Recoveries		75		221
PCL		(1,415)		1,179
Balance at end of period		11,358		11,358
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
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		10,899		3,044
Loans charged-off				(19)
Recoveries		18		124
PCL		(2,183)		2,694
Balance at end of period		8,734		8,734
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		36,417		26,158
Loans charged-off		(115)		(2,563)
Recoveries		18		524
PCL		7,442		27,822
Balance at end of period		43,762		43,762
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		7,600		4,024
Loans charged-off				(8)
Recoveries		42		60
PCL		(470)		3,861
Balance at end of period		7,172		7,172
Allowance for Credit Losses, ACL [Member] | Commercial and Industrial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		12,550		25,992
Loans charged-off		(71)		(1,350)
Recoveries		447		1,180
PCL		232		(3,700)
Balance at end of period		13,158		13,158
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		10,803		1,889
Recoveries		30		70
PCL		(624)		5,048
Balance at end of period		10,209		10,209
Allowance for Credit Losses, ACL [Member] | Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		6,397		3,379
Loans charged-off		(384)		(1,745)
Recoveries		375		1,316
PCL		(348)		1,031
Balance at end of period		6,040		6,040
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		3,414		2,229
PCL		(456)		(1,726)
Balance at end of period		2,958		2,958
Allowance for Credit Losses, ACL [Member] | Other Commercial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		6,452		5,303
Loans charged-off		(675)		(2,868)
Recoveries		877		2,978
PCL		1,060		217
Balance at end of period		7,714		7,714
Allowance for Credit Losses, ACL [Member] | Other Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				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 Credit Losses, ACL [Member] | ASU 2016-13 [Member] | Cumulative Effect Period Of Adoption Adjustment [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				(1,217)
Allowance for Credit Losses, ACL [Member] | ASU 2016-13 [Member] | Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member] | Commercial LHFI [Member] | Cumulative Effect Period Of Adoption Adjustment [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				(188)
Allowance for Credit Losses, ACL [Member] | ASU 2016-13 [Member] | Other Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member] | Cumulative Effect Period Of Adoption Adjustment [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				4,188
Allowance for Credit Losses, ACL [Member] | ASU 2016-13 [Member] | Secured by 1-4 Family Residential Properties [Member] | Other Loans Secured by Real Estate [Member] | Cumulative Effect Period Of Adoption Adjustment [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				2,891
Allowance for Credit Losses, ACL [Member] | ASU 2016-13 [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member] | Cumulative Effect Period Of Adoption Adjustment [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				(8,179)
Allowance for Credit Losses, ACL [Member] | ASU 2016-13 [Member] | Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member] | Cumulative Effect Period Of Adoption Adjustment [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				(765)
Allowance for Credit Losses, ACL [Member] | ASU 2016-13 [Member] | Commercial and Industrial Loans [Member] | Cumulative Effect Period Of Adoption Adjustment [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				(8,964)
Allowance for Credit Losses, ACL [Member] | ASU 2016-13 [Member] | Other Construction [Member] | Other Loans Secured by Real Estate [Member] | Cumulative Effect Period Of Adoption Adjustment [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				3,202
Allowance for Credit Losses, ACL [Member] | ASU 2016-13 [Member] | Consumer Loans [Member] | Cumulative Effect Period Of Adoption Adjustment [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				2,059
Allowance for Credit Losses, ACL [Member] | ASU 2016-13 [Member] | State and Other Political Subdivision Loans [Member] | Cumulative Effect Period Of Adoption Adjustment [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				2,455
Allowance for Credit Losses, ACL [Member] | ASU 2016-13 [Member] | Other Commercial Loans [Member] | Cumulative Effect Period Of Adoption Adjustment [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				2,084
Allowance for Loan Losses, LHFI [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				9,335,628	79,290
Loans charged-off					(9,862)
Recoveries					6,662
PCL					7,136
Balance at end of period		9,847,728	83,226	9,847,728	83,226
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
Balance at end of period		494,519		494,519
Allowance for Loan Losses, LHFI [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					7,390
Loans charged-off					(35)
Recoveries					807
PCL					(461)
Balance at end of period			7,701		7,701
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		547,148		547,148
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	8,641
Loans charged-off					(310)
Recoveries					530
PCL					(417)
Balance at end of period			8,444		8,444
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		1,228,252		1,228,252
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	22,376
Loans charged-off					(261)
Recoveries					285
PCL					2,802
Balance at end of period		2,707,627	25,202	2,707,627	25,202
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	3,450
Recoveries					22
PCL					82
Balance at end of period		887,792	3,554	887,792	3,554
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	27,359
Loans charged-off					(3,090)
Recoveries					980
PCL					3,618
Balance at end of period		1,398,468	28,867	1,398,468	28,867
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		891,428		891,428
Allowance for Loan Losses, LHFI [Member] | Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				175,738	2,890
Loans charged-off					(1,712)
Recoveries					1,432
PCL					702
Balance at end of period		163,933	3,312	163,933	3,312
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	990
PCL					(518)
Balance at end of period		935,349	472	935,349	472
Allowance for Loan Losses, LHFI [Member] | Other Commercial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at end of period		$ 593,212		593,212
Allowance for Loan Losses, LHFI [Member] | Other Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				$ 495,621	6,194
Loans charged-off					(4,454)
Recoveries					2,606
PCL					1,328
Balance at end of period			$ 5,674		$ 5,674

[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.