Defined Benefit and Other Postretirement Benefits - Plan Benefit Obligation, Plan Assets and Funded Status of the Plan (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Trustmark Capital Accumulation Plan [Member]
Change in plan assets [ Roll Forward]
Employer contributions	$ 17,600
Trustmark Capital Accumulation Plan [Member] | Trustmark Corporation Pension Plan for Certain Employees of Acquired Financial Institutions [Member]
Change in benefit obligation [Roll Forward]
Benefit obligation, beginning of year		$ 75,900	$ 91,403
Service cost		253	428	$ 513
Interest cost		1,461	3,355	3,461
Actuarial loss (gain)		391	(893)
Benefits paid		(68,583)	(18,393)
Settlement loss		680	0
Benefit obligation, end of year		10,102	75,900	91,403
Change in plan assets [ Roll Forward]
Fair value of plan assets, beginning of year		54,734	74,137
Actual return on plan assets		481	(1,079)
Employer contributions		17,964	69
Benefits paid		(68,583)	(18,393)
Fair value of plan assets, end of year		4,596	54,734	74,137
Funded status at end of year - net liability		(5,506)	(21,166)
Amounts recognized in accumulated other comprehensive income (loss) [Abstract]
Net loss		3,187	21,355
Supplemental Retirement Plan [Member]
Change in benefit obligation [Roll Forward]
Benefit obligation, beginning of year		58,495	57,766
Service cost		141	295	431
Interest cost		2,103	2,223	2,082
Actuarial loss (gain)		642	1,537
Benefits paid		(3,451)	(3,326)
Benefit obligation, end of year		57,930	58,495	57,766
Change in plan assets [ Roll Forward]
Fair value of plan assets, beginning of year		0	0
Employer contributions		3,451	3,326
Benefits paid		(3,451)	(3,326)
Fair value of plan assets, end of year		0	0	$ 0
Funded status at end of year - net liability		(57,930)	(58,495)
Amounts recognized in accumulated other comprehensive income (loss) [Abstract]
Net loss		17,514	17,739
Prior service cost		1,109	1,359
Amounts recognized		$ 18,623	$ 19,098