Defined Benefit and Other Postretirement Benefits - Weighted-Average Asset Allocation (Details) - Trustmark Capital Accumulation Plan [Member] - Trustmark Corporation Pension Plan for Certain Employees of Acquired Financial Institutions [Member]	Dec. 31, 2019	Dec. 31, 2018
Asset target allocations [Abstract]
Weighted-average asset allocation (in hundredths)	100.00%	100.00%
Money Market Funds [Member]
Asset target allocations [Abstract]
Weighted-average asset allocation (in hundredths)	2.00%	8.00%
Exchange Traded Equity Securities Funds [Member]
Asset target allocations [Abstract]
Weighted-average asset allocation (in hundredths)	46.00%	52.00%
Exchange Traded Fixed Income Funds [Member]
Asset target allocations [Abstract]
Weighted-average asset allocation (in hundredths)	41.00%	29.00%
International Exchange Traded Funds [Member]
Asset target allocations [Abstract]
Weighted-average asset allocation (in hundredths)	11.00%	11.00%