Defined Benefit and Other Postretirement Benefits - Net Periodic Benefit Cost for Plan and Continuing plan (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net periodic benefit cost [Abstract]
Recognized net loss due to defined benefit plan termination				$ 17,644
Trustmark Capital Accumulation Plan [Member] | Trustmark Corporation Pension Plan for Certain Employees of Acquired Financial Institutions [Member]
Net periodic benefit cost [Abstract]
Service cost	$ 69	$ 64	$ 208	190
Interest cost	83	102	249	1,394
Expected return on plan assets	(57)	(51)	(171)	(267)
Recognized net loss due to lump sum settlements	41	0	121	0
Recognized net loss due to defined benefit plan termination	0	0	0	17,644
Recognized net actuarial loss	143	142	428	1,272
Net periodic benefit cost	$ 279	$ 257	$ 835	$ 20,233