Defined Benefit and Other Postretirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Trustmark Capital Accumulation Plan [Member] | Trustmark Corporation Pension Plan for Certain Employees of Acquired Financial Institutions [Member]
Net periodic benefit cost [Abstract]
Service cost	$ 63	$ 64
Interest cost	43	60
Expected return on plan assets	(33)	(38)
Recognized net actuarial loss	149	81
Net periodic benefit cost	222	167
Supplemental Retirement Plan [Member]
Net periodic benefit cost [Abstract]
Service cost	19	19
Interest cost	293	414
Amortization of prior service cost	28	37
Recognized net actuarial loss	306	246
Net periodic benefit cost	$ 646	$ 716