Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Summary of Loan Type Related to Each Balance in Allowance for Loan Losses (Details) - USD ($) $ in Thousands		Sep. 30, 2017	Dec. 31, 2016
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment		$ 49,500	$ 27,100
Total LHFI	[1]	8,407,341	7,851,213
Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment		49,518	27,075
LHFI, Collectively Evaluated For Impairment		8,357,823	7,824,138
Total LHFI		8,407,341	7,851,213
Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI		950,144	831,437
Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment		1,531	2,441
LHFI, Collectively Evaluated For Impairment		948,613	828,996
Total LHFI		950,144	831,437
Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI		1,648,733	1,660,043
Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment		4,674	4,649
LHFI, Collectively Evaluated For Impairment		1,644,059	1,655,394
Total LHFI		1,648,733	1,660,043
Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI		2,172,885	2,034,176
Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment		15,384	6,219
LHFI, Collectively Evaluated For Impairment		2,157,501	2,027,957
Total LHFI		2,172,885	2,034,176
Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI		482,163	318,148
Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Collectively Evaluated For Impairment		482,163	318,148
Total LHFI		482,163	318,148
Commercial and Industrial Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI		1,568,588	1,528,434
Commercial and Industrial Loans [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment		23,500	13,669
LHFI, Collectively Evaluated For Impairment		1,545,088	1,514,765
Total LHFI		1,568,588	1,528,434
Consumer Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI		173,061	170,562
Consumer Loans [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment		1	2
LHFI, Collectively Evaluated For Impairment		173,060	170,560
Total LHFI		173,061	170,562
State and Other Political Subdivision Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI		936,614	917,515
State and Other Political Subdivision Loans [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Collectively Evaluated For Impairment		936,614	917,515
Total LHFI		936,614	917,515
Other Loans [Member]
Loans and Leases Receivable, Other Information [Abstract]
Total LHFI		475,153	390,898
Other Loans [Member] | Allowance for Loan Losses, LHFI [Member]
Loans and Leases Receivable, Other Information [Abstract]
LHFI, Individually Evaluated For Impairment		4,428	95
LHFI, Collectively Evaluated For Impairment		470,725	390,803
Total LHFI		$ 475,153	$ 390,898

[1] During the first quarter of 2017, Trustmark reclassified $36.7 million of acquired loans not accounted for under FASB ASC Topic 310-30 to LHFI due to the discount on these loans being fully amortized.