Defined Benefit and Other Postretirement Benefits - Plan Benefit Obligation, Plan Assets and Funded Status of the Plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Trustmark Capital Accumulation Plan [Member]
Change in benefit obligation [Roll Forward]
Benefit obligation, beginning of year	$ 101,904	$ 128,974
Service cost	513	495	$ 595
Interest cost	3,461	5,299	4,758
Actuarial (gain) loss	(2,807)	11,675
Benefits paid	(11,668)	(7,041)
Settlement loss from BancTrust termination	0	1,355
Benefit obligation, end of year	91,403	101,904	128,974
Change in plan assets [ Roll Forward]
Fair value of plan assets, beginning of year	86,287	126,142
Actual return on plan assets	(547)	4,632
Employer contributions	65	1,407
Benefits paid	(11,668)	(7,041)
Fair value of plan assets, end of year	74,137	86,287	126,142
Funded status at end of year - net liability	(17,266)	(15,617)
Amounts recognized in accumulated other comprehensive income (loss) [Abstract]
Net loss	24,927	28,100
BancTrust Pension Plan [Member]
Change in benefit obligation [Roll Forward]
Benefits paid	0	(38,853)
Change in plan assets [ Roll Forward]
Benefits paid	0	(38,853)
Supplemental Retirement Plan [Member]
Change in benefit obligation [Roll Forward]
Benefit obligation, beginning of year	59,744	52,489
Service cost	431	296	597
Interest cost	2,082	2,198	1,943
Actuarial (gain) loss	(1,702)	7,392
Benefits paid	(2,789)	(2,631)
Benefit obligation, end of year	57,766	59,744	52,489
Change in plan assets [ Roll Forward]
Fair value of plan assets, beginning of year	0	0
Employer contributions	2,789	2,631
Benefits paid	(2,789)	(2,631)
Fair value of plan assets, end of year	0	0	$ 0
Funded status at end of year - net liability	(57,766)	(59,744)
Amounts recognized in accumulated other comprehensive income (loss) [Abstract]
Net loss	18,548	21,242
Prior service cost	1,609	1,860
Amounts recognized	$ 20,157	$ 23,102