Defined Benefit and Other Postretirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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		$ 277	253	$ 428
Interest cost		332	1,461	3,355
Expected return on plan assets		(227)	(317)	(2,897)
Recognized net loss due to defined benefit plan termination	$ 17,600	0	17,662	0
Recognized net loss due to lump sum settlements		161	0	3,906
Recognized net actuarial loss		571	1,414	2,749
Net periodic benefit cost		1,114	20,473	7,541
Other changes in plan assets and benefit obligation recognized in other comprehensive income (loss), before taxes:
Net (gain) loss - Total recognized in other comprehensive income (loss)		(1,017)	(18,168)	(3,572)
Total recognized in net periodic benefit cost and other comprehensive income (loss)		$ 97	$ 2,305	$ 3,969
Weighted-average assumptions as of end of year [Abstract]
Discount rate for benefit obligation (in hundredths)		3.97%	3.32%	3.71%
Discount rate for net periodic benefit cost (in hundredths)		3.32%	3.71%	3.86%
Expected long-term return on plan assets (in hundredths)		5.00%	5.00%	4.25%
Supplemental Retirement Plan [Member]
Net periodic benefit cost [Abstract]
Service cost		$ 116	$ 141	$ 295
Interest cost		1,865	2,103	2,223
Amortization of prior service credits		250	250	250
Recognized net actuarial loss		884	866	864
Net periodic benefit cost		3,115	3,360	3,632
Other changes in plan assets and benefit obligation recognized in other comprehensive income (loss), before taxes:
Net loss (gain)		(4,111)	(224)	(810)
Amortization of prior service cost		(250)	(250)	(250)
Net (gain) loss - Total recognized in other comprehensive income (loss)		(4,361)	(474)	(1,060)
Total recognized in net periodic benefit cost and other comprehensive income (loss)		$ (1,246)	$ 2,886	$ 2,572
Weighted-average assumptions as of end of year [Abstract]
Discount rate for benefit obligation (in hundredths)		3.97%	3.32%	3.71%
Discount rate for net periodic benefit cost (in hundredths)		3.32%	3.71%	3.86%