Defined Benefit and Other Postretirement Benefits - Net Periodic Benefit Cost for Plan and Continuing plan (Details) - Trustmark Capital Accumulation Plan [Member] - Trustmark Corporation Pension Plan for Certain Employees of Acquired Financial Institutions [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Net periodic benefit cost [Abstract]
Service cost	$ 64	$ 53	$ 191	$ 159
Interest cost	60	90	181	270
Expected return on plan assets	(39)	(51)	(116)	(152)
Recognized net loss due to lump sum settlements	40	47	80	141
Recognized net actuarial loss	82	92	245	278
Net periodic benefit cost	$ 207	$ 231	$ 581	$ 696