Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Change in Allowance for Loan Losses (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	[1]	$ 84,277
FASB ASU 2016-13 Adoption Adjustment		19,949
PCL	[1]	20,581	$ 1,611
Balance at end of period	[1]	100,564
Allowance for Credit Losses, ACL [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		84,277	79,290
FASB ASU 2016-13 Adoption Adjustment		(1,217)
Loans charged-off		(5,545)	(4,033)
Recoveries		2,468	2,137
Net (charge-offs) recoveries		(3,077)	(1,896)
PCL		20,581	1,611
Balance at end of period		100,564	79,005
Allowance for Credit Losses, ACL [Member] | Commercial and Industrial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		25,992
FASB ASU 2016-13 Adoption Adjustment		(8,964)
Loans charged-off		(982)
Recoveries		542
PCL		(2,024)
Balance at end of period		14,564
Allowance for Credit Losses, ACL [Member] | Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		3,379
FASB ASU 2016-13 Adoption Adjustment		2,059
Loans charged-off		(690)
Recoveries		473
PCL		1,375
Balance at end of period		6,596
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		2,229
FASB ASU 2016-13 Adoption Adjustment		2,455
PCL		(1,243)
Balance at end of period		3,441
Allowance for Credit Losses, ACL [Member] | Other Commercial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		5,303
FASB ASU 2016-13 Adoption Adjustment		2,084
Loans charged-off		(1,334)
Recoveries		1,117
PCL		(1,150)
Balance at end of period		6,020
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] | Loans Secured by Real Estate [Member] | Construction, Land Development and Other Land [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		8,260
Balance at end of period		9,079
Allowance for Credit Losses, ACL [Member] | Loans Secured by Real Estate [Member] | Construction, Land Development and Other Land [Member] | Commercial LHFI [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		6,371
FASB ASU 2016-13 Adoption Adjustment		(188)
Recoveries		94
PCL		2,802
Balance at end of period		9,079
Allowance for Credit Losses, ACL [Member] | Loans Secured by Real Estate [Member] | Other Secured by 1-4 Family Residential Properties [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		5,888
FASB ASU 2016-13 Adoption Adjustment		4,188
Loans charged-off		(64)
Recoveries		63
PCL		1,636
Balance at end of period		11,711
Allowance for Credit Losses, ACL [Member] | Loans Secured by Real Estate [Member] | Secured by 1-4 Family Residential Properties [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		8,932
Allowance for Credit Losses, ACL [Member] | Loans Secured by Real Estate [Member] | Secured by Nonfarm, Nonresidential Properties [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		26,158
FASB ASU 2016-13 Adoption Adjustment		(8,179)
Loans charged-off		(2,448)
Recoveries		61
PCL		12,535
Balance at end of period		28,127
Allowance for Credit Losses, ACL [Member] | Loans Secured by Real Estate [Member] | Other Real Estate Secured [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		4,024
FASB ASU 2016-13 Adoption Adjustment		(765)
Loans charged-off		(8)
Recoveries		6
PCL		2,016
Balance at end of period		5,273
Allowance for Credit Losses, ACL [Member] | Other Loans Secured by Real Estate [Member] | Secured by 1-4 Family Residential Properties [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		3,044
FASB ASU 2016-13 Adoption Adjustment		2,891
Loans charged-off		(19)
Recoveries		93
PCL		2,033
Balance at end of period		8,042
Allowance for Credit Losses, ACL [Member] | Other Loans Secured by Real Estate [Member] | Other Construction [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		1,889
FASB ASU 2016-13 Adoption Adjustment		3,202
Recoveries		19
PCL		2,601
Balance at end of period		7,711
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	79,290
Loans charged-off			(4,033)
Recoveries			2,137
PCL			1,611
Balance at end of period		9,567,920	79,005
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			(1,859)
Recoveries			137
PCL			3,565
Balance at end of period		1,476,777	29,202
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			(793)
Recoveries			534
PCL			517
Balance at end of period		173,678	3,148
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			(198)
Balance at end of period		938,637	792
Allowance for Loan Losses, LHFI [Member] | Other Commercial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at end of period		576,379
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			(1,185)
Recoveries			919
PCL			(526)
Balance at end of period			5,402
Allowance for Loan Losses, LHFI [Member] | Loans Secured by Real Estate [Member] | Construction, Land Development and Other Land [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		1,162,791	7,390
Loans charged-off			(35)
Recoveries			306
PCL			(137)
Balance at end of period		486,265	7,524
Allowance for Loan Losses, LHFI [Member] | Loans Secured by Real Estate [Member] | Other Secured by 1-4 Family Residential Properties [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at end of period		584,981
Allowance for Loan Losses, LHFI [Member] | Loans Secured by Real Estate [Member] | Secured by 1-4 Family Residential Properties [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		1,855,913	8,641
Loans charged-off			(161)
Recoveries			216
PCL			(315)
Balance at end of period			8,381
Allowance for Loan Losses, LHFI [Member] | Loans Secured by Real Estate [Member] | Secured by Nonfarm, Nonresidential Properties [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		2,475,245	22,376
Recoveries			15
PCL			(912)
Balance at end of period		2,575,422	21,479
Allowance for Loan Losses, LHFI [Member] | Loans Secured by Real Estate [Member] | Other Real Estate Secured [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		724,480	3,450
Recoveries			10
PCL			(383)
Balance at end of period		838,573	$ 3,077
Allowance for Loan Losses, LHFI [Member] | Other Loans Secured by Real Estate [Member] | Secured by 1-4 Family Residential Properties [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at end of period		1,267,084
Allowance for Loan Losses, LHFI [Member] | Other Loans Secured by Real Estate [Member] | Other Construction [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at end of period		$ 650,124

[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.