AVAILABLE-FOR-SALE SECURITIES (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 2,448,051,000	$ 1,074,350,000
Gross Unrealized Gains	65,884,000	25,321,000
Gross Unrealized Losses	417,000	3,609,000
Estimated Fair Value	2,513,518,000	1,096,062,000
Carrying value of FHLB stock	8,700,000	18,700,000
Required investment in FHLB stock	8,300,000	12,500,000
Gross gains recognized on available-for-sale securities	12,400,000	4,500,000	84,000
Gross losses recognized on available-for-sale securities	327,000	2,100,000	250,000
Carrying value of available-for-sale securities pledged	1,600,000,000
Equity securities, considered maturity (in years)	10Y
Available-for-sale securities, amortized cost, by contractual maturity [Abstract]
Maturing in one year or less	295,485,000
Maturing after one year through five years	1,582,442,000
Maturing after five years through ten years	113,105,000
Maturing after ten years	457,019,000
Total	2,448,051,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Maturing in one year or less	300,106,000
Maturing after one year through five years	1,615,207,000
Maturing after five years through ten years	117,764,000
Maturing after ten years	480,441,000
Total	2,513,518,000	1,096,062,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	55,670,000	229,547,000
Less Than 12 Months Unrealized Losses	168,000	3,377,000
12 Months or Longer Fair Value	12,965,000	4,837,000
12 Months or Longer Unrealized Losses	249,000	232,000
Total Fair Value Available-for-sale Securities	68,635,000	234,384,000
Total Unrealized Losses Available-for-sale Securities	417,000	3,609,000
Available-for-sale securities, weighted average yield, by contractual maturity [Abstract]
Maturing in one year or less (in hundredths)	3.67%
Maturing after one year through five years (in hundredths)	2.01%
Maturing after five years through ten years (in hundredths)	4.37%
Maturing after ten years (in hundredths)	5.70%
U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,471,920,000	416,005,000
Gross Unrealized Gains	29,347,000	17,153,000
Gross Unrealized Losses	24,000	0
Estimated Fair Value	1,501,243,000	433,158,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Total	1,501,243,000	433,158,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	34,850,000	0
Less Than 12 Months Unrealized Losses	24,000	0
12 Months or Longer Fair Value	0	0
12 Months or Longer Unrealized Losses	0	0
Total Fair Value Available-for-sale Securities	34,850,000	0
Total Unrealized Losses Available-for-sale Securities	24,000	0
Government Agency Issued Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	394,894,000	498,874,000
Gross Unrealized Gains	9,786,000	5,954,000
Gross Unrealized Losses	70,000	1,599,000
Estimated Fair Value	404,610,000	503,229,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Total	404,610,000	503,229,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	0	184,820,000
Less Than 12 Months Unrealized Losses	0	1,599,000
12 Months or Longer Fair Value	3,751,000	0
12 Months or Longer Unrealized Losses	70,000	0
Total Fair Value Available-for-sale Securities	3,751,000	184,820,000
Total Unrealized Losses Available-for-sale Securities	70,000	1,599,000
Government Agency Issued Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	31,161,000	29,582,000
Gross Unrealized Gains	3,438,000	676,000
Gross Unrealized Losses	0	264,000
Estimated Fair Value	34,599,000	29,994,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Total	34,599,000	29,994,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	0	7,843,000
Less Than 12 Months Unrealized Losses	0	177,000
12 Months or Longer Fair Value	0	3,996,000
12 Months or Longer Unrealized Losses	0	87,000
Total Fair Value Available-for-sale Securities	0	11,839,000
Total Unrealized Losses Available-for-sale Securities	0	264,000
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	541,138,000	110,946,000
Gross Unrealized Gains	22,705,000	965,000
Gross Unrealized Losses	323,000	1,746,000
Estimated Fair Value	563,520,000	110,165,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Total	563,520,000	110,165,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	20,820,000	36,884,000
Less Than 12 Months Unrealized Losses	144,000	1,601,000
12 Months or Longer Fair Value	9,214,000	841,000
12 Months or Longer Unrealized Losses	179,000	145,000
Total Fair Value Available-for-sale Securities	30,034,000	37,725,000
Total Unrealized Losses Available-for-sale Securities	323,000	1,746,000
Obligations of States and Political Subdivisions [Member] | State of Mississippi [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	276,800,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Total	276,800,000
Obligations of States and Political Subdivisions [Member] | State of Arkansas [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	143,100,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Total	143,100,000
Other Available for Sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,938,000	18,943,000
Gross Unrealized Gains	608,000	573,000
Gross Unrealized Losses	0	0
Estimated Fair Value	9,546,000	19,516,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Total	9,546,000	19,516,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	0	0
Less Than 12 Months Unrealized Losses	0	0
12 Months or Longer Fair Value	0	0
12 Months or Longer Unrealized Losses	0	0
Total Fair Value Available-for-sale Securities	0	0
Total Unrealized Losses Available-for-sale Securities	$ 0	$ 0