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, 2019	Dec. 31, 2018	Dec. 31, 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		$ 211	$ 277	253
Interest cost		361	332	1,461
Expected return on plan assets		(202)	(227)	(317)
Recognized net loss due to defined benefit plan termination	$ 17,600	0	0	17,662
Recognized net loss due to lump sum settlements		312	161	0
Recognized net actuarial loss		373	571	1,414
Net periodic benefit cost		1,055	1,114	20,473
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)		(277)	(1,017)	(18,168)
Total recognized in net periodic benefit cost and other comprehensive income (loss)		$ 778	$ 97	$ 2,305
Weighted-average assumptions as of end of year [Abstract]
Discount rate for benefit obligation (in hundredths)		2.84%	3.97%	3.32%
Discount rate for net periodic benefit cost (in hundredths)		3.97%	3.32%	3.71%
Expected long-term return on plan assets (in hundredths)		5.00%	5.00%	5.00%
Supplemental Retirement Plan [Member]
Net periodic benefit cost [Abstract]
Service cost		$ 109	$ 116	$ 141
Interest cost		2,044	1,865	2,103
Amortization of prior service credits		250	250	250
Recognized net actuarial loss		627	884	866
Net periodic benefit cost		3,030	3,115	3,360
Other changes in plan assets and benefit obligation recognized in other comprehensive income (loss), before taxes:
Net loss (gain)		4,872	(4,111)	(224)
Amortization of prior service cost		(250)	(250)	(250)
Net (gain) loss - Total recognized in other comprehensive income (loss)		4,622	(4,361)	(474)
Total recognized in net periodic benefit cost and other comprehensive income (loss)		$ 7,652	$ (1,246)	$ 2,886
Weighted-average assumptions as of end of year [Abstract]
Discount rate for benefit obligation (in hundredths)		2.84%	3.97%	3.32%
Discount rate for net periodic benefit cost (in hundredths)		3.97%	3.32%	3.71%