Defined Benefit and Other Postretirement Benefits - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Recognized net loss due to defined benefit plan termination					$ 17,644
Trustmark Capital Accumulation Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Years of service required to vest			3 years
Defined benefit plan, additional asset contributions on termination basis	$ 17,600
Trustmark's minimum required contribution to the Trustmark Corporation Pension Plan for Certain Employees of Acquired Financial Institutions			$ 157
Trustmark's contribution to the Trustmark Corporation Pension Plan for Certain Employees of Acquired Financial Institutions			285
Trustmark Capital Accumulation Plan [Member] | Scenario Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Trustmark's minimum required contribution to the Trustmark Corporation Pension Plan for Certain Employees of Acquired Financial Institutions		$ 175
Trustmark Capital Accumulation Plan [Member] | Trustmark Corporation Pension Plan for Certain Employees of Acquired Financial Institutions [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, additional asset contributions on termination basis			387	$ 377
Recognized net loss due to defined benefit plan termination	$ 17,600		0	0	17,662
Estimated future benefit payments [Abstract]
Accumulated other comprehensive loss expected to be recognized during next fiscal year as components of net periodic benefit cost			(324)
Supplemental Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, additional asset contributions on termination basis			3,426	3,426
Estimated future benefit payments [Abstract]
Accumulated other comprehensive loss expected to be recognized during next fiscal year as components of net periodic benefit cost			(957)
Accumulated other comprehensive loss expected to be recognized during next fiscal year as prior service cost			149
Defined Contribution Plan [Member]
Other Benefit Plans - Defined Contribution Plan [Abstract]
Trustmarks contribution to defined contribution plan			$ 8,200	$ 7,900	$ 7,500
Contributions up to a maximum of eligible compensation			6.00%
Trustmark contributions to the plan			100.00%
Period when associates may become eligible to make elective deferral contributions after employment			30 days
Eligible associates must complete number of years of service			1 year