AVAILABLE-FOR-SALE SECURITIES (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 2,124,567,000	$ 2,460,608,000
Gross Unrealized Gains	38,496,000	27,987,000
Gross Unrealized Losses	6,136,000	21,606,000
Estimated Fair Value	2,156,927,000	2,466,989,000
Carrying value of FHLB stock	6,800,000
Required investment in FHLB stock	6,600,000
Gross gains recognized on available-for-sale securities	49,000	72,000	480,000
Gross losses recognized on available-for-sale securities	12,000	26,000	38,000
Carrying value of available-for-sale securities pledged	1,700,000,000
Available-for-sale securities, amortized cost, by contractual maturity [Abstract]
Maturing in one year or less	305,125,000
Maturing after one year through five years	1,036,760,000
Maturing after five years through ten years	53,547,000
Maturing after ten years	282,768,000
Mortgage-backed securities	446,367,000
Total	2,124,567,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Maturing in one year or less	306,995,000
Maturing after one year through five years	1,038,685,000
Maturing after five years through ten years	55,447,000
Maturing after ten years	306,002,000
Mortgage-backed securities	449,798,000
Estimated Fair Value	2,156,927,000	2,466,989,000
Available-for-sale securities, weighted average yield, by contractual maturity [Abstract]
Maturing in one year or less (in hundredths)	1.44%
Maturing after one year through five years (in hundredths)	1.30%
Maturing after five years through ten years (in hundredths)	4.90%
Maturing after ten years (in hundredths)	6.00%
Mortgage-backed securities (in hundredths)	2.07%
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	259,030,000	913,301,000
Less Than 12 Months Unrealized Losses	557,000	19,684,000
12 Months or Longer Fair Value	522,344,000	35,295,000
12 Months or Longer Unrealized Losses	5,579,000	1,922,000
Total Fair Value	781,374,000	948,596,000
Total Unrealized Losses	6,136,000	21,606,000
Mississippi [Member]
Schedule of Available-for-sale Securities [Line Items]
Carrying value of available-for-sale securities pledged	238,100,000
Arkansas [Member]
Schedule of Available-for-sale Securities [Line Items]
Carrying value of available-for-sale securities pledged	98,400,000
U.S. Agency Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,213,310,000	1,455,417,000
Gross Unrealized Gains	4,093,000	9,065,000
Gross Unrealized Losses	2,349,000	6,133,000
Estimated Fair Value	1,215,054,000	1,458,349,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Estimated Fair Value	1,215,054,000	1,458,349,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	237,891,000	533,326,000
Less Than 12 Months Unrealized Losses	471,000	6,133,000
12 Months or Longer Fair Value	283,643,000
12 Months or Longer Unrealized Losses	1,878,000
Total Fair Value	521,534,000	533,326,000
Total Unrealized Losses	2,349,000	6,133,000
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	204,918,000	249,682,000
Gross Unrealized Gains	4,751,000	3,118,000
Gross Unrealized Losses	439,000	2,566,000
Estimated Fair Value	209,230,000	250,234,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Estimated Fair Value	209,230,000	250,234,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value		106,179,000
Less Than 12 Months Unrealized Losses		2,418,000
12 Months or Longer Fair Value	24,565,000	4,407,000
12 Months or Longer Unrealized Losses	439,000	148,000
Total Fair Value	24,565,000	110,586,000
Total Unrealized Losses	439,000	2,566,000
Government Agency Issued Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	241,449,000	239,313,000
Gross Unrealized Gains	2,319,000	1,773,000
Gross Unrealized Losses	3,200,000	10,174,000
Estimated Fair Value	240,568,000	230,912,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Estimated Fair Value	240,568,000	230,912,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	3,822,000	176,253,000
Less Than 12 Months Unrealized Losses	24,000	8,578,000
12 Months or Longer Fair Value	203,520,000	27,225,000
12 Months or Longer Unrealized Losses	3,176,000	1,596,000
Total Fair Value	207,342,000	203,478,000
Total Unrealized Losses	3,200,000	10,174,000
US States and Political Subdivisions Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	458,026,000	509,255,000
Gross Unrealized Gains	25,986,000	12,883,000
Gross Unrealized Losses	148,000	2,733,000
Estimated Fair Value	483,864,000	519,405,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Estimated Fair Value	483,864,000	519,405,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	17,317,000	97,543,000
Less Than 12 Months Unrealized Losses	62,000	2,555,000
12 Months or Longer Fair Value	10,616,000	3,663,000
12 Months or Longer Unrealized Losses	86,000	178,000
Total Fair Value	27,933,000	101,206,000
Total Unrealized Losses	148,000	2,733,000
Other Available for Sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,864,000	6,941,000
Gross Unrealized Gains	1,347,000	1,148,000
Estimated Fair Value	8,211,000	8,089,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Estimated Fair Value	$ 8,211,000	$ 8,089,000