Defined Benefit and Other Postretirement Benefits (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Years of service required to vest	3 years
Trustmark Capital Accumulation Plan [Member]
Net periodic benefit cost [Abstract]
Service cost	$ 150,000	$ 140,000
Interest cost	990,000	945,000
Expected return on plan assets	(1,541,000)	(1,400,000)
Recognized net actuarial loss	1,395,000	1,309,000
Net periodic benefit cost	994,000	994,000
Reduction in expenses -BancTrust	120,000
Period of average interest rate before MAP-21	2 years
Period of average interest rate after MAP-21	25 years
Pension funding contributions cap percentage (in hundredths)	115.00%
Pension funding contributions floor percentage (in hundredths)	85.00%
Minimum required contribution	1,500,000
Voluntary contribution			1,500,000
Trustmark Capital Accumulation Plan [Member] | BancTrust Financial Group, Inc. [Member]
Net periodic benefit cost [Abstract]
Recorded liability	2,700,000
Supplemental Retirement Plan [Member]
Net periodic benefit cost [Abstract]
Service cost	149,000	170,000
Interest cost	484,000	517,000
Amortization of prior service credits	63,000	62,000
Recognized net actuarial loss	259,000	215,000
Net periodic benefit cost	$ 955,000	$ 964,000