SECURITIES (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 2,512,847		$ 2,448,051
Gross Unrealized Gains	62,077		65,884
Gross Unrealized Losses	1,389		417
Estimated Fair Value	2,573,535	1,145,463	2,513,518
Gross gains recognized on available-for-sale securities	94,000	1,000
Gross losses recognized on available-for-sale securities	20,000	0
Equity securities, considered maturity (in years)	10Y
Available-for-sale securities, amortized cost, by contractual maturity [Abstract]
Maturing in one year or less	342,145
Maturing after one year through five years	1,631,097
Maturing after five years through ten years	85,957
Maturing after ten years	453,648
Total	2,512,847
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Maturing in one year or less	346,944
Maturing after one year through five years	1,658,411
Maturing after five years through ten years	89,456
Maturing after ten years	478,724
Total	2,573,535	1,145,463	2,513,518
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	188,449		55,670
Less Than 12 Months Unrealized Losses	1,231		168
12 Months or Longer Fair Value	3,916		12,965
12 Months or Longer Unrealized Losses	158		249
Total Fair Value Available-for-sale Securities	192,365		68,635
Total Unrealized Losses Available-for-sale Securities	1,389		417
Available-for-sale securities, weighted average yield, by contractual maturity [Abstract]
Maturing in one year or less (in hundredths)	3.11%
Maturing after one year through five years (in hundredths)	1.86%
Maturing after five years through ten years (in hundredths)	4.55%
Maturing after ten years (in hundredths)	5.68%
U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,552,991		1,471,920
Gross Unrealized Gains	25,975		29,347
Gross Unrealized Losses	525		24
Estimated Fair Value	1,578,441		1,501,243
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Total	1,578,441		1,501,243
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	120,223		34,850
Less Than 12 Months Unrealized Losses	525		24
12 Months or Longer Fair Value	0		0
12 Months or Longer Unrealized Losses	0		0
Total Fair Value Available-for-sale Securities	120,223		34,850
Total Unrealized Losses Available-for-sale Securities	525		24
Government Agency Issued Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	377,767		394,894
Gross Unrealized Gains	7,910		9,786
Gross Unrealized Losses	531		70
Estimated Fair Value	385,146		404,610
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Total	385,146		404,610
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	45,471		0
Less Than 12 Months Unrealized Losses	445		0
12 Months or Longer Fair Value	3,599		3,751
12 Months or Longer Unrealized Losses	86		70
Total Fair Value Available-for-sale Securities	49,070		3,751
Total Unrealized Losses Available-for-sale Securities	531		70
Government Agency Issued Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	28,876		31,161
Gross Unrealized Gains	2,780		3,438
Gross Unrealized Losses	9		0
Estimated Fair Value	31,647		34,599
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Total	31,647		34,599
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	1,023		0
Less Than 12 Months Unrealized Losses	9		0
12 Months or Longer Fair Value	0		0
12 Months or Longer Unrealized Losses	0		0
Total Fair Value Available-for-sale Securities	1,023		0
Total Unrealized Losses Available-for-sale Securities	9		0
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	544,267		541,138
Gross Unrealized Gains	24,699		22,705
Gross Unrealized Losses	324		323
Estimated Fair Value	568,642		563,520
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Total	568,642		563,520
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	21,732		20,820
Less Than 12 Months Unrealized Losses	252		144
12 Months or Longer Fair Value	317		9,214
12 Months or Longer Unrealized Losses	72		179
Total Fair Value Available-for-sale Securities	22,049		30,034
Total Unrealized Losses Available-for-sale Securities	324		323
Other Available for Sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,946		8,938
Gross Unrealized Gains	713		608
Gross Unrealized Losses	0		0
Estimated Fair Value	9,659		9,546
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Total	9,659		9,546
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