Defined Benefit and Other Postretirement Benefits - Plan Assets Measured at Fair Value (Details) - Trustmark Capital Accumulation Plan [Member] - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Asset target allocations [Abstract]
Fair value of plan assets	$ 54,734	$ 74,137	$ 86,287
Level 1 [Member]
Asset target allocations [Abstract]
Fair value of plan assets	54,734	74,137
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	6,859	2,678
Money Market Funds [Member] | Level 1 [Member]
Asset target allocations [Abstract]
Fair value of plan assets	6,859	2,678
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
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
Fixed Income Mutual Funds [Member]
Asset target allocations [Abstract]
Fair value of plan assets		21,472
Fixed Income Mutual Funds [Member] | Level 1 [Member]
Asset target allocations [Abstract]
Fair value of plan assets		21,472
Fixed Income Mutual Funds [Member] | Level 2 [Member]
Asset target allocations [Abstract]
Fair value of plan assets		0
Fixed Income Mutual Funds [Member] | Level 3 [Member]
Asset target allocations [Abstract]
Fair value of plan assets		0
Equity Mutual Funds [Member]
Asset target allocations [Abstract]
Fair value of plan assets		11,922
Equity Mutual Funds [Member] | Level 1 [Member]
Asset target allocations [Abstract]
Fair value of plan assets		11,922
Equity Mutual Funds [Member] | Level 2 [Member]
Asset target allocations [Abstract]
Fair value of plan assets		0
Equity Mutual Funds [Member] | Level 3 [Member]
Asset target allocations [Abstract]
Fair value of plan assets		0
Equity Securities [Member]
Asset target allocations [Abstract]
Fair value of plan assets		38,065
Equity Securities [Member] | Level 1 [Member]
Asset target allocations [Abstract]
Fair value of plan assets		38,065
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