Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Summary of Trustmark's Portfolio Segments, Loan Classes, Loan Pools and the ACL Methodology and Loss Drivers (Details)	9 Months Ended
	Sep. 30, 2021
Loans Secured by Real Estate [Member] | Construction, Land Development and Other Land [Member] | 1 -4 Family Residential Construction [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	DCF
Loss Drivers	Prime Rate, National GDP
Loans Secured by Real Estate [Member] | Construction, Land Development and Other Land [Member] | Lots and Development [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	DCF
Loss Drivers	Prime Rate, Southern Unemployment
Loans Secured by Real Estate [Member] | Construction, Land Development and Other Land [Member] | Unimproved Land [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	DCF
Loss Drivers	Prime Rate, Southern Unemployment
Loans Secured by Real Estate [Member] | Construction, Land Development and Other Land [Member] | All Other Consumer [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	DCF
Loss Drivers	Southern Unemployment
Loans Secured by Real Estate [Member] | Other Secured by 1-4 Family Residential Properties [Member] | All Other Consumer [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	DCF
Loss Drivers	Southern Unemployment
Loans Secured by Real Estate [Member] | Other Secured by 1-4 Family Residential Properties [Member] | Consumer 1-4 Family - 1st Liens [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	DCF
Loss Drivers	Prime Rate, Southern Unemployment
Loans Secured by Real Estate [Member] | Other Secured by 1-4 Family Residential Properties [Member] | Nonresidential Owner- Occupied [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	DCF
Loss Drivers	Southern Unemployment, National GDP
Loans Secured by Real Estate [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Nonresidential Owner- Occupied [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	DCF
Loss Drivers	Southern Unemployment, National GDP
Loans Secured by Real Estate [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Nonowner-Occupied - Hotel/Motel [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	DCF
Loss Drivers	Southern Vacancy Rate, Southern Unemployment
Loans Secured by Real Estate [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Nonowner-Occupied - Office [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	DCF
Loss Drivers	Southern Vacancy Rate, Southern Unemployment
Loans Secured by Real Estate [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Nonowner-Occupied- Retail [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	DCF
Loss Drivers	Southern Vacancy Rate, Southern Unemployment
Loans Secured by Real Estate [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Nonowner-Occupied- Senior Living/ Nursing Homes [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	DCF
Loss Drivers	Southern Vacancy Rate, Southern Unemployment
Loans Secured by Real Estate [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Nonowner-occupied - All Other [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	DCF
Loss Drivers	Southern Vacancy Rate, Southern Unemployment
Loans Secured by Real Estate [Member] | Other Real Estate Secured [Member] | Nonresidential Owner- Occupied [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	DCF
Loss Drivers	Southern Unemployment, National GDP
Loans Secured by Real Estate [Member] | Other Real Estate Secured [Member] | Nonowner-occupied - All Other [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	DCF
Loss Drivers	Southern Vacancy Rate, Southern Unemployment
Loans Secured by Real Estate [Member] | Other Real Estate Secured [Member] | Nonresidential Nonowner- Occupied - Apartments [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	DCF
Loss Drivers	Southern Vacancy Rate, Southern Unemployment
Other Loans Secured by Real Estate [Member] | Other Construction [Member] | Other Construction [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	WARM
Loss Drivers	Prime Rate, National Unemployment
Other Loans Secured by Real Estate [Member] | Secured by 1-4 Family Residential Properties [Member] | Trustmark Mortgage [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	WARM
Loss Drivers	Southern Unemployment
Commercial and Industrial Loans [Member] | Commercial and Industrial Loans [Member] | Commercial and Industrial - Non-Working Capital [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	DCF
Loss Drivers	Trustmark historical data
Commercial and Industrial Loans [Member] | Commercial and Industrial Loans [Member] | Commercial and Industrial - Working Capital [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	DCF
Loss Drivers	Trustmark historical data
Commercial and Industrial Loans [Member] | Commercial and Industrial Loans [Member] | Credit Cards [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	WARM
Loss Drivers	Trustmark call report data
Consumer Loans [Member] | Consumer Loans [Member] | All Other Consumer [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	DCF
Loss Drivers	Southern Unemployment
Consumer Loans [Member] | Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	WARM
Loss Drivers	Trustmark call report data
Consumer Loans [Member] | Consumer Loans [Member] | Overdrafts [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	Loss Rate
Loss Drivers	Trustmark historical data
State and Other Political Subdivision Loans [Member] | State and Other Political Subdivision Loans [Member] | Obligations of State and Political Subdivisions [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	DCF
Loss Drivers	Moody's Bond Default Study
Other Commercial Loans [Member] | Other Commercial Loans [Member] | Commercial and Industrial - Non-Working Capital [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	DCF
Loss Drivers	Trustmark historical data
Other Commercial Loans [Member] | Other Commercial Loans [Member] | Commercial and Industrial - Working Capital [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	DCF
Loss Drivers	Trustmark historical data
Other Commercial Loans [Member] | Other Commercial Loans [Member] | Other Loans [Member]
Financing Receivable Recorded Investment [Line Items]
Methodology	DCF
Loss Drivers	Prime Rate, Southern Unemployment