Defined Benefit and Other Postretirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Trustmark Capital Accumulation Plan [Member]
Net periodic benefit cost [Abstract]
Service cost	$ 108	$ 131
Interest cost	830	862
Expected return on plan assets	(1,022)	(1,296)
Recognized net loss due to lump sum settlements	423	417
Recognized net actuarial loss	661	967
Net periodic benefit cost	1,000	1,081
Supplemental Retirement Plan [Member]
Net periodic benefit cost [Abstract]
Service cost	74	108
Interest cost	546	524
Amortization of prior service cost	63	63
Recognized net actuarial loss	221	253
Net periodic benefit cost	$ 904	$ 948