AVAILABLE-FOR-SALE SECURITIES (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 2,059,445	$ 2,124,567
Gross Unrealized Gains	28,855	38,496
Gross Unrealized Losses	5,971	6,136
Estimated Fair Value	2,082,329	2,156,927
Carrying value of FHLB stock	18,000
Required investment in FHLB stock	9,100
Gross gains recognized on available-for-sale securities	136	49	$ 72
Gross losses recognized on available-for-sale securities		12	$ 26
Carrying value of available-for-sale securities pledged	1,600,000
Available-for-sale securities, amortized cost, by contractual maturity [Abstract]
Maturing in one year or less	312,736
Maturing after one year through five years	1,036,108
Maturing after five years through ten years	49,860
Maturing after ten years	260,438
Mortgage-backed securities	400,303
Total	2,059,445
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Maturing in one year or less	313,092
Maturing after one year through five years	1,035,443
Maturing after five years through ten years	52,335
Maturing after ten years	280,226
Mortgage-backed securities	401,233
Estimated Fair Value	$ 2,082,329	2,156,927
Available-for-sale securities, weighted average yield, by contractual maturity [Abstract]
Maturing in one year or less (in hundredths)	1.34%
Maturing after one year through five years (in hundredths)	1.30%
Maturing after five years through ten years (in hundredths)	5.40%
Maturing after ten years (in hundredths)	5.81%
Mortgage-backed securities (in hundredths)	2.11%
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	$ 1,004,003	259,030
Less Than 12 Months Unrealized Losses	5,403	557
12 Months or Longer Fair Value	34,022	522,344
12 Months or Longer Unrealized Losses	568	5,579
Total Fair Value	1,038,025	781,374
Total Unrealized Losses	5,971	6,136
Mississippi [Member]
Schedule of Available-for-sale Securities [Line Items]
Carrying value of available-for-sale securities pledged	206,100
Arkansas [Member]
Schedule of Available-for-sale Securities [Line Items]
Carrying value of available-for-sale securities pledged	80,100
U.S. Agency Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,246,261	1,213,310
Gross Unrealized Gains	826	4,093
Gross Unrealized Losses	2,447	2,349
Estimated Fair Value	1,244,640	1,215,054
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Estimated Fair Value	1,244,640	1,215,054
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	762,568	237,891
Less Than 12 Months Unrealized Losses	2,447	471
12 Months or Longer Fair Value		283,643
12 Months or Longer Unrealized Losses		1,878
Total Fair Value	762,568	521,534
Total Unrealized Losses	2,447	2,349
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	138,759	204,918
Gross Unrealized Gains	1,957	4,751
Gross Unrealized Losses	176	439
Estimated Fair Value	140,540	209,230
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Estimated Fair Value	140,540	209,230
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	34,238
Less Than 12 Months Unrealized Losses	176
12 Months or Longer Fair Value		24,565
12 Months or Longer Unrealized Losses		439
Total Fair Value	34,238	24,565
Total Unrealized Losses	176	439
Government Agency Issued Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	261,544	241,449
Gross Unrealized Gains	2,414	2,319
Gross Unrealized Losses	3,265	3,200
Estimated Fair Value	260,693	240,568
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Estimated Fair Value	260,693	240,568
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	193,621	3,822
Less Than 12 Months Unrealized Losses	2,710	24
12 Months or Longer Fair Value	31,166	203,520
12 Months or Longer Unrealized Losses	555	3,176
Total Fair Value	224,787	207,342
Total Unrealized Losses	3,265	3,200
US States and Political Subdivisions Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	394,769	458,026
Gross Unrealized Gains	22,813	25,986
Gross Unrealized Losses	83	148
Estimated Fair Value	417,499	483,864
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Estimated Fair Value	417,499	483,864
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	13,576	17,317
Less Than 12 Months Unrealized Losses	70	62
12 Months or Longer Fair Value	2,856	10,616
12 Months or Longer Unrealized Losses	13	86
Total Fair Value	16,432	27,933
Total Unrealized Losses	83	148
Other Available for Sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	18,112	6,864
Gross Unrealized Gains	845	1,347
Estimated Fair Value	18,957	8,211
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Estimated Fair Value	$ 18,957	$ 8,211