Defined Benefit and Other Postretirement Benefits (Details) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2012	Dec. 31, 2011
Net periodic benefit cost [Abstract]
Years of service required to vest (in years)			3Y
Trustmark Capital Accumulation Plan [Member]
Net periodic benefit cost [Abstract]
Service cost	$ 134,000	$ 124,000	$ 413,000	$ 398,000
Interest cost	947,000	1,115,000	2,837,000	3,345,000
Expected return on plan assets	(1,438,000)	(1,470,000)	(4,238,000)	(4,412,000)
Recognized net actuarial loss	1,303,000	1,026,000	3,921,000	3,100,000
Net periodic benefit cost	946,000	795,000	2,933,000	2,431,000
Contribution					1,600,000	1,000,000
Supplemental Retirement Plan [Member]
Net periodic benefit cost [Abstract]
Service cost	170,000	147,000	510,000	441,000
Interest cost	517,000	569,000	1,550,000	1,707,000
Amortization of prior service cost	63,000	59,000	188,000	177,000
Recognized net actuarial loss	215,000	124,000	645,000	372,000
Net periodic benefit cost	$ 965,000	$ 899,000	$ 2,893,000	$ 2,697,000