Defined Benefit and Other Postretirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Trustmark Capital Accumulation Plan [Member]
Net periodic benefit cost [Abstract]
Service cost	$ 513	$ 495	$ 595
Interest cost	3,461	5,299	4,758
Expected return on plan assets	(5,187)	(6,245)	(7,720)
Recognized net loss due to BancTrust termination	0	1,355	0
Recognized net loss due to lump sum settlements	2,221	905	2,225
Recognized net actuarial (gain) loss	3,878	(283)	5,516
Net periodic benefit cost	4,886	1,526	5,374
Other changes in plan assets and benefit obligation recognized in other comprehensive income (loss), before taxes:
Total recognized in other comprehensive income (loss)	(3,173)	12,664	(29,742)
Total recognized in net periodic benefit cost and other comprehensive income (loss)	$ 1,713	$ 14,190	$ (24,368)
Weighted-average assumptions as of end of year [Abstract]
Discount rate for benefit obligation (in hundredths)	3.86%	3.57%	4.30%
Discount rate for net periodic benefit cost (in hundredths)	3.57%	4.30%	3.50%
Expected long-term return on plan assets (in hundredths)	7.00%	7.50%	7.50%
Supplemental Retirement Plan [Member]
Net periodic benefit cost [Abstract]
Service cost	$ 431	$ 296	$ 597
Interest cost	2,082	2,198	1,943
Amortization of prior service credits	250	250	250
Recognized net actuarial (gain) loss	992	661	1,038
Net periodic benefit cost	3,755	3,405	3,828
Other changes in plan assets and benefit obligation recognized in other comprehensive income (loss), before taxes:
Net loss (gain)	(2,694)	6,733	(5,224)
Amortization of prior service cost	(250)	(250)	(250)
Total recognized in other comprehensive income (loss)	(2,944)	6,483	(5,474)
Total recognized in net periodic benefit cost and other comprehensive income (loss)	$ 811	$ 9,888	$ (1,646)
Weighted-average assumptions as of end of year [Abstract]
Discount rate for benefit obligation (in hundredths)	3.86%	3.57%	4.30%
Discount rate for net periodic benefit cost (in hundredths)	3.57%	4.30%	3.50%