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Defined Benefit and Other Postretirement Benefits - Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Trustmark Capital Accumulation Plan [Member] | Trustmark Corporation Pension Plan for Certain Employees of Acquired Financial Institutions [Member]        
Net periodic benefit cost [Abstract]        
Service cost $ 9 $ 10 $ 27 $ 30
Interest cost 68 62 206 185
Expected return on plan assets (31) (24) (94) (72)
Recognized net (gain) loss due to lump sum settlements (29) 0 (79) (13)
Recognized net actuarial (gain) loss (1) 0 (5) 0
Net periodic benefit cost 16 48 55 130
Supplemental Retirement Plan [Member]        
Net periodic benefit cost [Abstract]        
Service cost 3 11 11 33
Interest cost 476 457 1,446 1,393
Amortization of prior service cost 4 28 11 83
Recognized net actuarial (gain) loss 62 84 198 263
Net periodic benefit cost $ 545 $ 580 $ 1,666 $ 1,772