Contingencies - Summary of Changes in ACL on Off-balance Sheet Credit Exposures (Details) - USD ($) $ in Thousands		3 Months Ended	9 Months Ended
		Sep. 30, 2020	Sep. 30, 2020
Commitments And Contingencies Disclosure [Abstract]
Balance at beginning of period		$ 42,663	$ 0	[1]
FASB ASU 2016-13 adoption adjustment		0	29,638
Credit loss expense related to off-balance sheet credit exposures	[1]	(3,004)	10,021
Balance at end of period	[1]	$ 39,659	$ 39,659

[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.