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, 2018	Dec. 31, 2017	Dec. 31, 2016
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		$ 10,102	$ 75,900
Service cost		277	253	$ 428
Interest cost		332	1,461	3,355
Actuarial (gain) loss		(827)	391
Benefits paid		(705)	(68,583)
Settlement loss		0	680
Benefit obligation, end of year		9,179	10,102	75,900
Change in plan assets [ Roll Forward]
Fair value of plan assets, beginning of year		4,596	54,734
Actual return on plan assets		(314)	481
Employer contributions		377	17,964
Benefit payments for the Plan		(705)	(68,583)
Fair value of plan assets, end of year		3,954	4,596	54,734
Funded status at end of year - net liability		(5,225)	(5,506)
Amounts recognized in accumulated other comprehensive income (loss) [Abstract]
Net loss		2,170	3,187
Supplemental Retirement Plan [Member]
Change in benefit obligation [Roll Forward]
Benefit obligation, beginning of year		57,930	58,495
Service cost		116	141	295
Interest cost		1,865	2,103	2,223
Actuarial (gain) loss		(3,228)	642
Benefits paid		(3,426)	(3,451)
Benefit obligation, end of year		53,257	57,930	58,495
Change in plan assets [ Roll Forward]
Fair value of plan assets, beginning of year		0	0
Employer contributions		3,426	3,451
Benefit payments for the Plan		(3,426)	(3,451)
Fair value of plan assets, end of year		0	0	$ 0
Funded status at end of year - net liability		(53,257)	(57,930)
Amounts recognized in accumulated other comprehensive income (loss) [Abstract]
Net loss		13,403	17,514
Prior service cost		859	1,109
Amounts recognized		$ 14,262	$ 18,623