Defined Benefit and Other Postretirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net periodic benefit cost [Abstract]
Recognized net loss due to defined benefit plan termination		$ 17,644
Trustmark Capital Accumulation Plan [Member] | Trustmark Corporation Pension Plan for Certain Employees of Acquired Financial Institutions [Member]
Net periodic benefit cost [Abstract]
Service cost		253	$ 428	$ 513
Interest cost		1,461	3,355	3,461
Expected return on plan assets		(317)	(2,897)	(5,187)
Recognized net loss due to defined benefit plan termination	$ 17,600	17,662	0	0
Recognized net loss due to lump sum settlements		0	3,906	2,221
Recognized net actuarial loss		1,414	2,749	3,878
Net periodic benefit cost		20,473	7,541	4,886
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)		(18,168)	(3,572)	(3,173)
Total recognized in net periodic benefit cost and other comprehensive income (loss)		$ 2,305	$ 3,969	$ 1,713
Weighted-average assumptions as of end of year [Abstract]
Discount rate for benefit obligation (in hundredths)		3.32%	3.71%	3.86%
Discount rate for net periodic benefit cost (in hundredths)		3.71%	3.86%	3.57%
Expected long-term return on plan assets (in hundredths)		5.00%	4.25%	7.00%
Supplemental Retirement Plan [Member]
Net periodic benefit cost [Abstract]
Service cost		$ 141	$ 295	$ 431
Interest cost		2,103	2,223	2,082
Amortization of prior service credits		250	250	250
Recognized net actuarial loss		866	864	992
Net periodic benefit cost		3,360	3,632	3,755
Other changes in plan assets and benefit obligation recognized in other comprehensive income (loss), before taxes:
Net loss (gain)		(224)	(810)	(2,694)
Net (gain) loss - Total recognized in other comprehensive income (loss)		(474)	(1,060)	(2,944)
Total recognized in net periodic benefit cost and other comprehensive income (loss)		2,886	2,572	811
Amortization of prior service cost		$ (250)	$ (250)	$ (250)
Weighted-average assumptions as of end of year [Abstract]
Discount rate for benefit obligation (in hundredths)		3.32%	3.71%	3.86%
Discount rate for net periodic benefit cost (in hundredths)		3.71%	3.86%	3.57%