Long-term Obligations - Long-term Obligations (Details) - USD ($) $ in Thousands		6 Months Ended
		Jun. 30, 2015		Dec. 31, 2014
Debt Instruments [Abstract]
FHLB advances	[1]	$ 572,254		$ 600,052
Long-term debt	[2]	60,311		60,311
Total long-term obligations		$ 632,565		660,363
Federal Home Loan Bank Advances
Advances from Federal Home Loan Banks [Abstract]
Weighted average cost (in hundredths)		1.31%
Maturity date range, earliest		Jul. 25, 2016
Maturity date range, latest		Jul. 03, 2028
Southside Statutory Trust III
Debt Instrument [Line Items]
Maturity date		2033
Debt Instruments [Abstract]
Long-term debt	[2]	$ 20,619	[3]	20,619
Long-term Debt, Other Disclosures [Abstract]
Adjusted rate of debt		3.22175%
Date of interest rate adjustment		Sep. 29, 2015
Debt instrument, description of variable rate basis		three-month LIBOR plus 294 basis points
Southside Statutory Trust IV
Debt Instrument [Line Items]
Maturity date		2037
Debt Instruments [Abstract]
Long-term debt	[2]	$ 23,196	[4]	23,196
Long-term Debt, Other Disclosures [Abstract]
Adjusted rate of debt		1.57815%
Date of interest rate adjustment		Jul. 29, 2015
Debt instrument, description of variable rate basis		three-month LIBOR plus 130 basis points
Southside Statutory Trust V
Debt Instrument [Line Items]
Maturity date		2037
Debt Instruments [Abstract]
Long-term debt	[2]	$ 12,887	[5]	12,887
Long-term Debt, Other Disclosures [Abstract]
Adjusted rate of debt		2.53585%
Date of interest rate adjustment		Sep. 14, 2015
Debt instrument, description of variable rate basis		three-month LIBOR plus 225 basis points
Magnolia Trust Company I
Debt Instrument [Line Items]
Maturity date		2035
Debt Instruments [Abstract]
Long-term debt	[2]	$ 3,609	[6]	$ 3,609
Long-term Debt, Other Disclosures [Abstract]
Adjusted rate of debt		2.082%
Date of interest rate adjustment		Aug. 23, 2015
Debt instrument, description of variable rate basis		three-month LIBOR plus 180 basis points
LIBOR | Southside Statutory Trust III
Long-term Debt, Other Disclosures [Abstract]
Basis spread on variable rate		2.94%
LIBOR | Southside Statutory Trust IV
Long-term Debt, Other Disclosures [Abstract]
Basis spread on variable rate		1.30%
LIBOR | Southside Statutory Trust V
Long-term Debt, Other Disclosures [Abstract]
Basis spread on variable rate		2.25%
LIBOR | Magnolia Trust Company I
Long-term Debt, Other Disclosures [Abstract]
Basis spread on variable rate		1.80%

[1] At June 30, 2015, the weighted average cost of these advances was 1.31%. Long-term FHLB Advances have maturities ranging from July 2016 through July 2028.

[2] This long-term debt consists of trust preferred securities that qualify under the risk-based capital guidelines as Tier 1 capital, subject to certain limitations.

[3] This debt carries an adjustable rate of 3.22175% through September 29, 2015 and adjusts quarterly thereafter at a rate equal to three-month LIBOR plus 294 basis points.

[4] This debt carries an adjustable rate of 1.57815% through July 29, 2015 and adjusts quarterly thereafter at a rate equal to three-month LIBOR plus 130 basis points.

[5] This debt carries an adjustable rate of 2.53585% through September 14, 2015 and adjusts quarterly thereafter at a rate equal to three-month LIBOR plus 225 basis points.

[6] This debt carries an adjustable rate of 2.082% through August 23, 2015 and adjusts quarterly thereafter at a rate equal to three-month LIBOR plus 180 basis points.