Defined Benefit and Other Postretirement Benefits - Net Periodic Benefit Cost for Plan and Continuing plan (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net periodic benefit cost [Abstract]
Recognized net loss due to defined benefit plan termination		$ 17,644		$ 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	$ 70	63	$ 139	126
Interest cost	83	627	166	1,292
Expected return on plan assets	(57)	(108)	(114)	(216)
Recognized net loss due to lump sum settlements	40	0	80	0
Recognized net loss due to defined benefit plan termination	0	17,644	0	17,644
Recognized net actuarial loss	143	565	285	1,130
Net periodic benefit cost	$ 279	$ 18,791	$ 556	$ 19,976