Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Summary of LHFI by Loan Type and Credit Quality Indicator (Details) - USD ($) $ in Thousands		Sep. 30, 2020	Dec. 31, 2019
Financing Receivable [Abstract]
Loans held for investment (LHFI)	[1]	$ 9,847,728	$ 9,335,628
Current		9,829,499	9,269,775
Financing Receivable, Recorded Investment, Past Due		18,229	12,627
Nonaccrual		53,856	53,226
Past Due 90 Days or More [Member]
Financing Receivable [Abstract]
Financing Receivable, Recorded Investment, Past Due		8,456	642	[2]
Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		494,519	1,162,791
Current		493,204	1,161,258
Financing Receivable, Recorded Investment, Past Due		1,315	636
Nonaccrual		786	897
Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member] | Past Due 90 Days or More [Member]
Financing Receivable [Abstract]
Financing Receivable, Recorded Investment, Past Due		186
Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			1,855,913
Current			1,832,698
Financing Receivable, Recorded Investment, Past Due			6,405
Nonaccrual			16,810
Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member] | Past Due 90 Days or More [Member]
Financing Receivable [Abstract]
Financing Receivable, Recorded Investment, Past Due	[2]		211
Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		2,707,627	2,475,245
Current		2,706,481	2,465,847
Financing Receivable, Recorded Investment, Past Due		1,146	1,698
Nonaccrual		10,272	7,700
Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member] | Past Due 90 Days or More [Member]
Financing Receivable [Abstract]
Financing Receivable, Recorded Investment, Past Due		881
Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		887,792	724,480
Current		887,670	723,440
Financing Receivable, Recorded Investment, Past Due		122	8
Nonaccrual		565	1,032
Commercial and Industrial Loans [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		1,398,468	1,477,896
Current		1,395,466	1,455,453
Financing Receivable, Recorded Investment, Past Due		3,002	668
Nonaccrual		16,813	21,775
Commercial and Industrial Loans [Member] | Past Due 90 Days or More [Member]
Financing Receivable [Abstract]
Financing Receivable, Recorded Investment, Past Due		1,922	39	[2]
Consumer Loans [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		163,933	175,738
Current		162,684	172,650
Financing Receivable, Recorded Investment, Past Due		1,249	2,980
Nonaccrual		96	108
Consumer Loans [Member] | Past Due 90 Days or More [Member]
Financing Receivable [Abstract]
Financing Receivable, Recorded Investment, Past Due		124	392	[2]
State and Other Political Subdivision Loans [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		935,349	967,944
Current		935,172	963,789
Financing Receivable, Recorded Investment, Past Due		177	76
Nonaccrual		4,019	4,079
State and Other Political Subdivision Loans [Member] | Past Due 90 Days or More [Member]
Financing Receivable [Abstract]
Financing Receivable, Recorded Investment, Past Due		177
Other Loans [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			495,621
Current			494,640
Financing Receivable, Recorded Investment, Past Due			156
Nonaccrual			825
Commercial LHFI [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		7,939,010	7,348,158
Commercial LHFI [Member] | Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			1,090,914
Commercial LHFI [Member] | Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			123,033
Commercial LHFI [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		2,706,913	2,475,090
Commercial LHFI [Member] | Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			723,785
Commercial LHFI [Member] | Commercial and Industrial Loans [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		1,398,468	1,477,896
Commercial LHFI [Member] | State and Other Political Subdivision Loans [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		935,349	967,944
Commercial LHFI [Member] | Other Loans [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			489,496
Commercial LHFI [Member] | Pass Categories 1-6 [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			7,178,617
Commercial LHFI [Member] | Pass Categories 1-6 [Member] | Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			1,075,146
Commercial LHFI [Member] | Pass Categories 1-6 [Member] | Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			116,592
Commercial LHFI [Member] | Pass Categories 1-6 [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		2,529,290	2,430,761
Commercial LHFI [Member] | Pass Categories 1-6 [Member] | Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			721,238
Commercial LHFI [Member] | Pass Categories 1-6 [Member] | Commercial and Industrial Loans [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		1,325,439	1,407,837
Commercial LHFI [Member] | Pass Categories 1-6 [Member] | State and Other Political Subdivision Loans [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		926,437	957,948
Commercial LHFI [Member] | Pass Categories 1-6 [Member] | Other Loans [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			469,095
Commercial LHFI [Member] | Special Mention Category 7 [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			9,049
Commercial LHFI [Member] | Special Mention Category 7 [Member] | Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			45
Commercial LHFI [Member] | Special Mention Category 7 [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		51,564
Commercial LHFI [Member] | Special Mention Category 7 [Member] | Commercial and Industrial Loans [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		4,021	909
Commercial LHFI [Member] | Special Mention Category 7 [Member] | State and Other Political Subdivision Loans [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		4,000	4,650
Commercial LHFI [Member] | Special Mention Category 7 [Member] | Other Loans [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			3,445
Commercial LHFI [Member] | Substandard Category 8 [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			159,163
Commercial LHFI [Member] | Substandard Category 8 [Member] | Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			15,726
Commercial LHFI [Member] | Substandard Category 8 [Member] | Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			6,355
Commercial LHFI [Member] | Substandard Category 8 [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		125,302	44,001
Commercial LHFI [Member] | Substandard Category 8 [Member] | Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			2,547
Commercial LHFI [Member] | Substandard Category 8 [Member] | Commercial and Industrial Loans [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		68,545	68,262
Commercial LHFI [Member] | Substandard Category 8 [Member] | State and Other Political Subdivision Loans [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		4,912	5,346
Commercial LHFI [Member] | Substandard Category 8 [Member] | Other Loans [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			16,926
Commercial LHFI [Member] | Doubtful Category 9 [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			1,329
Commercial LHFI [Member] | Doubtful Category 9 [Member] | Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			42
Commercial LHFI [Member] | Doubtful Category 9 [Member] | Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			41
Commercial LHFI [Member] | Doubtful Category 9 [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		757	328
Commercial LHFI [Member] | Doubtful Category 9 [Member] | Commercial and Industrial Loans [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		463	888
Commercial LHFI [Member] | Doubtful Category 9 [Member] | Other Loans [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			30
Consumer LHFI [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		1,908,718	9,335,628
Current			1,961,967
Nonaccrual			16,057
Subtotal			1,987,470
Consumer LHFI [Member] | Past Due 30-89 Days [Member]
Financing Receivable [Abstract]
Financing Receivable, Recorded Investment, Past Due			8,842
Consumer LHFI [Member] | Past Due 90 Days or More [Member]
Financing Receivable [Abstract]
Financing Receivable, Recorded Investment, Past Due			604
Consumer LHFI [Member] | Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			1,162,791
Current			71,413
Nonaccrual			132
Subtotal			71,877
Consumer LHFI [Member] | Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member] | Past Due 30-89 Days [Member]
Financing Receivable [Abstract]
Financing Receivable, Recorded Investment, Past Due			332
Consumer LHFI [Member] | Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			1,855,913
Current			1,710,930
Nonaccrual			15,817
Subtotal			1,732,880
Consumer LHFI [Member] | Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member] | Past Due 30-89 Days [Member]
Financing Receivable [Abstract]
Financing Receivable, Recorded Investment, Past Due			5,922
Consumer LHFI [Member] | Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member] | Past Due 90 Days or More [Member]
Financing Receivable [Abstract]
Financing Receivable, Recorded Investment, Past Due			211
Consumer LHFI [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		714	2,475,245
Current			155
Subtotal			155
Consumer LHFI [Member] | Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			724,480
Current			695
Subtotal			695
Consumer LHFI [Member] | Commercial and Industrial Loans [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			1,477,896
Consumer LHFI [Member] | Consumer Loans [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		163,933	175,738
Current			172,649
Nonaccrual			108
Subtotal			175,738
Consumer LHFI [Member] | Consumer Loans [Member] | Past Due 30-89 Days [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		1,125
Financing Receivable, Recorded Investment, Past Due			2,588
Consumer LHFI [Member] | Consumer Loans [Member] | Past Due 90 Days or More [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)		$ 123
Financing Receivable, Recorded Investment, Past Due			393
Consumer LHFI [Member] | State and Other Political Subdivision Loans [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			967,944
Consumer LHFI [Member] | Other Loans [Member]
Financing Receivable [Abstract]
Loans held for investment (LHFI)			495,621
Current			6,125
Subtotal			$ 6,125

[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.

[2] Past due 90 days or more but still accruing interest.