Defined Benefit and Other Postretirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Trustmark Capital Accumulation Plan [Member] \| Trustmark Corporation Pension Plan for Certain Employees of Acquired Financial Institutions [Member]
Net periodic benefit cost [Abstract]
Service cost	$ 254	$ 211	$ 277
Interest cost	241	361	332
Expected return on plan assets	(154)	(202)	(227)
Recognized net loss due to lump sum settlements	119	312	161
Recognized net actuarial loss	326	373	571
Net periodic benefit cost	786	1,055	1,114
Other changes in plan assets and benefit obligation recognized in other comprehensive income (loss), before taxes:
Net (gain) loss - Total recognized in other comprehensive income (loss)	671	(277)	(1,017)
Total recognized in net periodic benefit cost and other comprehensive income (loss)	$ 1,457	$ 778	$ 97
Weighted-average assumptions as of end of year [Abstract]
Discount rate for benefit obligation (in hundredths)	1.95%	2.84%	3.97%
Discount rate for net periodic benefit cost (in hundredths)	2.84%	3.97%	3.32%
Expected long-term return on plan assets (in hundredths)	5.00%	5.00%	5.00%
Supplemental Retirement Plan [Member]
Net periodic benefit cost [Abstract]
Service cost	$ 77	$ 109	$ 116
Interest cost	1,576	2,044	1,865
Amortization of prior service credits	150	250	250
Recognized net actuarial loss	957	627	884
Net periodic benefit cost	2,760	3,030	3,115
Other changes in plan assets and benefit obligation recognized in other comprehensive income (loss), before taxes:
Net loss (gain)	3,211	4,872	(4,111)
Amortization of prior service cost	(150)	(250)	(250)
Net (gain) loss - Total recognized in other comprehensive income (loss)	3,061	4,622	(4,361)
Total recognized in net periodic benefit cost and other comprehensive income (loss)	$ 5,821	$ 7,652	$ (1,246)
Weighted-average assumptions as of end of year [Abstract]
Discount rate for benefit obligation (in hundredths)	1.95%	2.84%	3.97%
Discount rate for net periodic benefit cost (in hundredths)	2.84%	3.97%	3.32%

Defined benefit plan recognized in net periodic benefit cost and other comprehensive income (loss).

No definition available.

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Namespace Prefix:	trmk_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of gain (loss), recognized immediately as component of net periodic benefit (cost) credit, for change in value of benefit obligation or plan assets from experience different from that assumed, change in actuarial assumption, or consequence of temporarily deviating from substantive plan. Includes, but is not limited to, amount in excess of ten percent of greater of fair value of plan assets or benefit obligation.