Defined Benefit and Other Postretirement Benefits - Plan Assets Measured at Fair Value (Details) - Trustmark Capital Accumulation Plan [Member] - Trustmark Corporation Pension Plan for Certain Employees of Acquired Financial Institutions [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Asset target allocations [Abstract]
Fair value of plan assets	$ 4,596	$ 54,734	$ 74,137
Level 1 [Member]
Asset target allocations [Abstract]
Fair value of plan assets	4,596	54,734
Level 2 [Member]
Asset target allocations [Abstract]
Fair value of plan assets	0	0
Level 3 [Member]
Asset target allocations [Abstract]
Fair value of plan assets	0	0
Money Market Funds [Member]
Asset target allocations [Abstract]
Fair value of plan assets	269	6,859
Money Market Funds [Member] | Level 1 [Member]
Asset target allocations [Abstract]
Fair value of plan assets	269	6,859
Money Market Funds [Member] | Level 2 [Member]
Asset target allocations [Abstract]
Fair value of plan assets	0	0
Money Market Funds [Member] | Level 3 [Member]
Asset target allocations [Abstract]
Fair value of plan assets	0	0
Equity Securities [Member]
Asset target allocations [Abstract]
Fair value of plan assets	2,199
Equity Securities [Member] | Level 1 [Member]
Asset target allocations [Abstract]
Fair value of plan assets	2,199
Equity Securities [Member] | Level 2 [Member]
Asset target allocations [Abstract]
Fair value of plan assets	0
Equity Securities [Member] | Level 3 [Member]
Asset target allocations [Abstract]
Fair value of plan assets	0
Fixed Income [Member]
Asset target allocations [Abstract]
Fair value of plan assets	1,462
Fixed Income [Member] | Level 1 [Member]
Asset target allocations [Abstract]
Fair value of plan assets	1,462
Fixed Income [Member] | Level 2 [Member]
Asset target allocations [Abstract]
Fair value of plan assets	0
Fixed Income [Member] | Level 3 [Member]
Asset target allocations [Abstract]
Fair value of plan assets	0
International [Member]
Asset target allocations [Abstract]
Fair value of plan assets	666
International [Member] | Level 1 [Member]
Asset target allocations [Abstract]
Fair value of plan assets	666
International [Member] | Level 2 [Member]
Asset target allocations [Abstract]
Fair value of plan assets	0
International [Member] | Level 3 [Member]
Asset target allocations [Abstract]
Fair value of plan assets	$ 0
U.S. Treasuries [Member]
Asset target allocations [Abstract]
Fair value of plan assets		47,875
U.S. Treasuries [Member] | Level 1 [Member]
Asset target allocations [Abstract]
Fair value of plan assets		47,875
U.S. Treasuries [Member] | Level 2 [Member]
Asset target allocations [Abstract]
Fair value of plan assets		0
U.S. Treasuries [Member] | Level 3 [Member]
Asset target allocations [Abstract]
Fair value of plan assets		$ 0