Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Loan Portfolio Held for Investment (Details) - USD ($) $ in Thousands		Sep. 30, 2020	Dec. 31, 2019
Loan Portfolio [Abstract]
Total LHFI	[1]	$ 9,847,728	$ 9,335,628
Less allowance for credit losses (ACL), LHFI	[1]	122,010	84,277
Net LHFI		9,725,718	9,251,351
Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI		494,519	1,162,791
Secured by 1-4 Family Residential Properties [Member] | Other Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI	[2]	1,228,252
Other Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI	[2]	547,148	1,855,913
Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI		887,792	724,480
Other Construction [Member] | Other Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI	[2]	891,428
Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Loan Portfolio [Abstract]
Total LHFI		2,707,627	2,475,245
Commercial and Industrial Loans [Member]
Loan Portfolio [Abstract]
Total LHFI		1,398,468	1,477,896
Consumer Loans [Member]
Loan Portfolio [Abstract]
Total LHFI		163,933	175,738
State and Other Political Subdivision Loans [Member]
Loan Portfolio [Abstract]
Total LHFI		935,349	967,944
Other Commercial Loans [Member]
Loan Portfolio [Abstract]
Total LHFI	[2]	$ 593,212	$ 495,621

[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] In accordance with the guidance in FASB ASC Topic 326, Trustmark redefined its LHFI portfolio segments and related loan classes based on the level at which risk is monitored within the ACL methodology. The other loans secured by real estate portfolio segment and related loan classes were separated from the loans secured by real estate portfolio segment. The other construction loans were segregated from the construction, land development and other land loans. The other loans secured by 1-4 family residential properties were segregated from the loans secured by 1-4 family residential properties and the loans secured by 1-4 family residential properties were redefined in the other loans secured by real estate portfolio segment. Other loans were redefined as other commercial loans.