Defined Benefit and Other Postretirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Trustmark Capital Accumulation Plan [Member]
Net periodic benefit cost [Abstract]
Service cost			$ 428	$ 513	$ 495
Interest cost			3,355	3,461	5,299
Expected return on plan assets			(2,897)	(5,187)	(6,245)
Recognized net loss due to BancTrust termination			0	0	1,355
Recognized net loss due to lump sum settlements			3,906	2,221	905
Recognized net actuarial loss (gain)			2,749	3,878	(283)
Net periodic benefit cost			7,541	4,886	1,526
Other changes in plan assets and benefit obligation recognized in other comprehensive income (loss), before taxes:
Net (gain) loss - Total recognized in other comprehensive income (loss)			(3,572)	(3,173)	12,664
Total recognized in net periodic benefit cost and other comprehensive income (loss)			$ 3,969	$ 1,713	$ 14,190
Weighted-average assumptions as of end of year [Abstract]
Discount rate for benefit obligation (in hundredths)	3.71%		3.71%	3.86%	3.57%
Discount rate for net periodic benefit cost (in hundredths)			3.86%	3.57%	4.30%
Expected long-term return on plan assets (in hundredths)	2.50%	6.00%	4.25%	7.00%	7.50%
Supplemental Retirement Plan [Member]
Net periodic benefit cost [Abstract]
Service cost			$ 295	$ 431	$ 296
Interest cost			2,223	2,082	2,198
Amortization of prior service credits			250	250	250
Recognized net actuarial loss (gain)			864	992	661
Net periodic benefit cost			3,632	3,755	3,405
Other changes in plan assets and benefit obligation recognized in other comprehensive income (loss), before taxes:
Net loss (gain)			(810)	(2,694)	6,733
Amortization of prior service cost			(250)	(250)	(250)
Net (gain) loss - Total recognized in other comprehensive income (loss)			(1,060)	(2,944)	6,483
Total recognized in net periodic benefit cost and other comprehensive income (loss)			$ 2,572	$ 811	$ 9,888
Weighted-average assumptions as of end of year [Abstract]
Discount rate for benefit obligation (in hundredths)	3.71%		3.71%	3.86%	3.57%
Discount rate for net periodic benefit cost (in hundredths)			3.86%	3.57%	4.30%