Defined Benefit and Other Postretirement Benefits - Additional Information (Details) - USD ($)	6 Months Ended		12 Months Ended
	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Trustmark Capital Accumulation Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Years of service required to vest				3 years
Expected rate of return on plan assets	2.50%	6.00%		4.25%	7.00%	7.50%
Increase in periodic benefit costs	$ 1,300,000
Trustmark's expected minimum required contribution to the Plan				$ 0	$ 0
Estimated future benefit payments [Abstract]
Accumulated other comprehensive loss expected to be recognized during next fiscal year as components of net periodic benefit cost				(18,400,000)
BancTrust Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pre-tax gain recognized in OCI as a result of the termination of the plan						$ 1,200,000
Spin-Off Plan [Member] | Scenario Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Trustmark's expected minimum required contribution to the Plan			$ 0
Supplemental Retirement Plan [Member]
Estimated future benefit payments [Abstract]
Accumulated other comprehensive loss expected to be recognized during next fiscal year as components of net periodic benefit cost				(866,000)
Accumulated other comprehensive loss expected to be recognized during next fiscal year as prior service cost				250,000
Defined Contribution Plan [Member]
Other Benefit Plans - Defined Contribution Plan [Abstract]
Trustmarks contribution to defined contribution plan				$ 7,200,000	$ 7,000,000	$ 6,700,000
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