Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015 Segment	Mar. 31, 2014
Segment Reporting Information [Line Items]
Number of segments in which the business operates	3us-gaap_NumberOfReportableSegments
Net interest income	$ 97,392us-gaap_InterestIncomeExpenseNet	$ 94,904us-gaap_InterestIncomeExpenseNet
Provision for loan losses, net	2,132	(742)
Noninterest income	42,363us-gaap_NoninterestIncome	44,078us-gaap_NoninterestIncome
Noninterest expense	99,216us-gaap_NoninterestExpense	101,618us-gaap_NoninterestExpense
Income before income taxes	38,407	38,106
Income taxes	9,259us-gaap_IncomeTaxExpenseBenefit	9,103us-gaap_IncomeTaxExpenseBenefit
Net Income	29,148us-gaap_NetIncomeLoss	29,003us-gaap_NetIncomeLoss
Selected Financial Information [Abstract]
Average assets	12,125,303trmk_AverageAssetsForPeriod	11,918,816trmk_AverageAssetsForPeriod
Depreciation and amortization	9,080	8,756
General Banking [Member] \| Reportable Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	97,260	94,686
Provision for loan losses, net	2,132	(742)
Noninterest income	25,740	27,883
Noninterest expense	85,517	88,587
Income before income taxes	35,351	34,724
Income taxes	8,084	7,913
Net Income	27,267	26,811
Selected Financial Information [Abstract]
Average assets	12,053,721	11,851,429
Depreciation and amortization	8,875	8,481
Wealth Management [Member] \| Reportable Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	55	148
Noninterest income	8,007	8,097
Noninterest expense	6,770	6,434
Income before income taxes	1,292	1,811
Income taxes	513	601
Net Income	779	1,210
Selected Financial Information [Abstract]
Average assets	3,263	1,945
Depreciation and amortization	46	45
Insurance [Member] \| Reportable Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	77	70
Noninterest income	8,616	8,098
Noninterest expense	6,929	6,597
Income before income taxes	1,764	1,571
Income taxes	662	589
Net Income	1,102	982
Selected Financial Information [Abstract]
Average assets	68,319	65,442
Depreciation and amortization	$ 159	$ 230

Amount of average assets attributed to the reportable segment for the reporting period.

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