Acquired Loans - Changes in the Carrying Value of Acquired Loans (Details) - USD ($) $ in Thousands		9 Months Ended			12 Months Ended
		Sep. 30, 2018		Sep. 30, 2017	Dec. 31, 2017	Apr. 07, 2017
Carrying value of acquired loans [Abstract]
Carrying value, net, beginning		$ 257,438
Additions						$ 7,900
Accretion to interest income		7,870		$ 11,021
Carrying value, net, ending		130,901			$ 257,438
Acquired Not ASC 310-30 [Member]
Carrying value of acquired loans [Abstract]
Carrying value, net, beginning	[1]	77,868		42,743	42,743
Transfers	[1],[2]	(43,922)			(36,719)
Additions	[1],[3]				109,548
Accretion to interest income	[1]	782			1,578
Payments received, net	[1]	(16,096)			(39,208)
Other	[1],[4]	74			(74)
Change in allowance for loan losses, acquired loans	[1]	0			0
Carrying value, net, ending	[1]	18,706			77,868
Acquired Impaired [Member]
Carrying value of acquired loans [Abstract]
Carrying value, net, beginning		179,570		$ 218,107	218,107
Transfers		(26,497)	[5]		0
Additions	[3]				7,899
Accretion to interest income		7,870			14,924
Payments received, net		(50,940)			(68,317)
Other	[4]	(173)			(361)
Change in allowance for loan losses, acquired loans		2,365			7,318
Carrying value, net, ending		$ 112,195			$ 179,570

[1] “Acquired Not ASC 310-30” loans consist of loans that are not in scope for FASB ASC Topic 310-30.

[2] “Acquired Not ASC 310-30” loans transferred to LHFI due to the discount on these loans being fully amortized.

[3] Loans acquired in the Reliance merger on April 7, 2017.

[4] Includes miscellaneous timing adjustments as well as acquired loan terminations through foreclosure, charge-off and other terminations.

[5] During the second quarter of 2018, Trustmark transferred the remaining loans acquired in the Bay Bank merger on March 16, 2012 from acquired impaired loans to LHFI. During the third quarter of 2018, Trustmark transferred the remaining loans acquired in the Heritage acquisition on April 15, 2011 and the Reliance merger on April 7, 2017 from acquired impaired loans to LHFI.