SECURITIES (Details) (USD $)	3 Months Ended	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of held-to-maturity securities reclassified to available-for-sale	$ 1,600,000,000		$ 1,600,000,000
Fair value of held-to-maturity securities reclassified to available-for-sale	1,700,000,000		1,700,000,000
Increase in other comprehensive income due to transfers to available-for-sale securities	19,700,000
Amortized Cost		2,393,916,000		2,448,051,000
Gross Unrealized Gains		69,229,000		65,884,000
Gross Unrealized Losses		314,000		417,000
Estimated Fair Value	2,560,824,000	2,462,831,000	2,560,824,000	2,513,518,000	[1]
Gross gains recognized on available-for-sale securities		271,000	10,300,000
Gross losses recognized on available-for-sale securities		20,000	260,000
Equity securities, considered maturity		10 years
Available-for-sale securities, amortized cost, by contractual maturity [Abstract]
Maturing in one year or less		360,188,000
Maturing after one year through five years		1,496,505,000
Maturing after five years through ten years		87,604,000
Maturing after ten years		449,619,000
Total		2,393,916,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Maturing in one year or less		364,523,000
Maturing after one year through five years		1,525,453,000
Maturing after five years through ten years		92,402,000
Maturing after ten years		480,453,000
Total	2,560,824,000	2,462,831,000	2,560,824,000	2,513,518,000	[1]
Available-for-sale securities, weighted average yield, by contractual maturity [Abstract]
Maturing in one year or less (in hundredths)		2.38%
Maturing after one year through five years (in hundredths)		1.80%
Maturing after five years through ten years (in hundredths)		4.44%
Maturing after ten years (in hundredths)		5.64%
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value		95,346,000		55,670,000
Less Than 12 Months Unrealized Losses		156,000		168,000
12 Months or Longer Fair Value		3,675,000		12,965,000
12 Months or Longer Unrealized Losses		158,000		249,000
Total Fair Value Available-for-sale Securities		99,021,000		68,635,000
Total Unrealized Losses Available-for-sale Securities		314,000		417,000
U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		1,456,274,000		1,471,920,000
Gross Unrealized Gains		24,822,000		29,347,000
Gross Unrealized Losses		36,000		24,000
Estimated Fair Value		1,481,060,000		1,501,243,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Total		1,481,060,000		1,501,243,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value		65,845,000		34,850,000
Less Than 12 Months Unrealized Losses		36,000		24,000
12 Months or Longer Fair Value		0		0
12 Months or Longer Unrealized Losses		0		0
Total Fair Value Available-for-sale Securities		65,845,000		34,850,000
Total Unrealized Losses Available-for-sale Securities		36,000		24,000
Government Agency Issued Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		352,093,000		394,894,000
Gross Unrealized Gains		8,525,000		9,786,000
Gross Unrealized Losses		129,000		70,000
Estimated Fair Value		360,489,000		404,610,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Total		360,489,000		404,610,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value		20,519,000		0
Less Than 12 Months Unrealized Losses		33,000		0
12 Months or Longer Fair Value		3,364,000		3,751,000
12 Months or Longer Unrealized Losses		96,000		70,000
Total Fair Value Available-for-sale Securities		23,883,000		3,751,000
Total Unrealized Losses Available-for-sale Securities		129,000		70,000
Government Agency Issued Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		31,538,000		31,161,000
Gross Unrealized Gains		4,357,000		3,438,000
Gross Unrealized Losses		0		0
Estimated Fair Value		35,895,000		34,599,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Total		35,895,000		34,599,000
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
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		546,972,000		541,138,000
Gross Unrealized Gains		30,806,000		22,705,000
Gross Unrealized Losses		149,000		323,000
Estimated Fair Value		577,629,000		563,520,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Total		577,629,000		563,520,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value		8,982,000		20,820,000
Less Than 12 Months Unrealized Losses		87,000		144,000
12 Months or Longer Fair Value		311,000		9,214,000
12 Months or Longer Unrealized Losses		62,000		179,000
Total Fair Value Available-for-sale Securities		9,293,000		30,034,000
Total Unrealized Losses Available-for-sale Securities		149,000		323,000
Other Available for Sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		7,039,000		8,938,000
Gross Unrealized Gains		719,000		608,000
Gross Unrealized Losses		0		0
Estimated Fair Value		7,758,000		9,546,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Total		7,758,000		9,546,000
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

[1] Derived from audited financial statements.