Defined Benefit and Other Postretirement Benefits - Net Periodic Benefit Cost for Plan and Continuing plan (Details) - The Continuing 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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Net periodic benefit cost [Abstract]
Service cost	$ 53	$ 69	$ 159	$ 208
Interest cost	90	83	270	249
Expected return on plan assets	(51)	(57)	(152)	(171)
Recognized net loss due to lump sum settlements	47	41	141	121
Recognized net actuarial loss	92	143	278	428
Net periodic benefit cost	$ 231	$ 279	$ 696	$ 835