Defined Benefit and Other Postretirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 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	$ 63	$ 126	$ 127
Interest cost	43	61	86	121
Expected return on plan assets	(32)	(39)	(65)	(77)
Recognized net loss due to lump sum settlements	0	40	0	40
Recognized net actuarial loss	148	82	297	163
Net periodic benefit cost	222	207	444	374
Supplemental Retirement Plan [Member]
Net periodic benefit cost [Abstract]
Service cost	19	20	38	39
Interest cost	277	387	570	801
Amortization of prior service cost	28	38	56	75
Recognized net actuarial loss	295	236	601	482
Net periodic benefit cost	$ 619	$ 681	$ 1,265	$ 1,397