Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Additional Information (Details 2)		3 Months Ended		6 Months Ended
		Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) KeyRatio	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Financing Receivable, Recorded Investment, Aging [Abstract]
Number of days used as baseline in evaluating collateral documentation exceptions for loan policy				90 days
Number of key quality ratios | KeyRatio				6
LHFS past due 90 days or more		$ 56,300,000		$ 56,300,000		$ 41,600,000
Percentage of outstanding principal to be repurchased under GNMA optional repurchase program				100.00%
Minimum exposure for commercial accruing substandard loans to be reviewed on an individual basis		1,000,000.0		$ 1,000,000.0
Financing Receivable [Abstract]
PCL	[1]	18,185,000	$ 2,486,000	38,766,000	$ 4,097,000
Allowance for Credit Losses, ACL [Member]
Financing Receivable [Abstract]
PCL		18,185,000	$ 2,486,000	38,766,000	$ 4,097,000
Commercial and Industrial Loans [Member] | Allowance for Credit Losses, ACL [Member]
Financing Receivable [Abstract]
PCL		(1,908,000)		(3,932,000)
State and Other Political Subdivision Loans [Member] | Allowance for Credit Losses, ACL [Member]
Financing Receivable [Abstract]
PCL		(27,000)		(1,270,000)
Other Commercial Loans [Member] | Allowance for Credit Losses, ACL [Member]
Financing Receivable [Abstract]
PCL		307,000		(843,000)
Minimum [Member]
Financing Receivable [Abstract]
Credit amount used as baseline in evaluating loan policy		$ 100,000		$ 100,000

[1] Effective January 1, 2020, Trustmark adopted FASB ASU 2016-13 using the modified retrospective approach.  Therefore, prior period balances are presented under legacy GAAP and may not be comparable to current period presentation.