Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Additional Information (Details 2)		3 Months Ended		9 Months Ended
		Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) KeyRatio	Sep. 30, 2019 USD ($)	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
Exposure for commercial non accrual loans to be reviewed on individual basis		$ 500,000		$ 500,000
Exposure for commercial accrual loans deemed to be reviewed on individual basis		500,000		500,000
LHFS past due 90 days or more		121,300,000		$ 121,300,000		$ 41,600,000
Percentage of outstanding principal to be repurchased under GNMA optional repurchase program				100.00%
Financing Receivable [Abstract]
PCL	[1]	1,760,000	$ 3,039,000	$ 40,526,000	$ 7,136,000
Allowance for Credit Losses, ACL [Member]
Financing Receivable [Abstract]
PCL		1,760,000	$ 3,039,000	40,526,000	$ 7,136,000
Commercial and Industrial Loans [Member] | Allowance for Credit Losses, ACL [Member]
Financing Receivable [Abstract]
PCL		232,000		(3,700,000)
State and Other Political Subdivision Loans [Member] | Allowance for Credit Losses, ACL [Member]
Financing Receivable [Abstract]
PCL		(456,000)		(1,726,000)
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.