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, 2018	Dec. 31, 2017
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)	8.00%	6.00%
Exchange Traded Funds [Member] | Equity Securities [Member]
Asset target allocations [Abstract]
Weighted-average asset allocation (in hundredths)	52.00%	48.00%
Exchange Traded Funds [Member] | Fixed Income [Member]
Asset target allocations [Abstract]
Weighted-average asset allocation (in hundredths)	29.00%	32.00%
Exchange Traded Funds [Member] | International [Member]
Asset target allocations [Abstract]
Weighted-average asset allocation (in hundredths)	11.00%	14.00%