SECURITIES (Details) (USD $)	6 Months Ended	9 Months Ended
	Jun. 30, 2011	Sep. 30, 2012	Sep. 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
Fair value of held-to-maturity securities reclassified to available-for-sale	1,700,000,000
Increase in other comprehensive income due to transfers to available-for-sale securities	19,700,000
Amortized Cost		2,407,910,000		2,448,051,000
Gross Unrealized Gains		76,137,000		65,884,000
Gross Unrealized Losses		441,000		417,000
Estimated Fair Value		2,483,606,000	2,481,555,000	2,513,518,000	[1]
Gross gains recognized on available-for-sale securities		329,000	12,400,000
Gross losses recognized on available-for-sale securities		39,000	327,000
Equity securities, considered maturity		10 years
Available-for-sale securities, amortized cost, by contractual maturity [Abstract]
Maturing in one year or less		483,074,000
Maturing after one year through five years		1,351,410,000
Maturing after five years through ten years		139,123,000
Maturing after ten years		434,303,000
Total		2,407,910,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Maturing in one year or less		487,503,000
Maturing after one year through five years		1,382,562,000
Maturing after five years through ten years		143,655,000
Maturing after ten years		469,886,000
Total		2,483,606,000	2,481,555,000	2,513,518,000	[1]
Available-for-sale securities, weighted average yield, by contractual maturity [Abstract]
Maturing in one year or less (in hundredths)		2.06%
Maturing after one year through five years (in hundredths)		1.75%
Maturing after five years through ten years (in hundredths)		3.39%
Maturing after ten years (in hundredths)		5.62%
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value		76,302,000		55,670,000
Less Than 12 Months Unrealized Losses		276,000		168,000
12 Months or Longer Fair Value		3,432,000		12,965,000
12 Months or Longer Unrealized Losses		165,000		249,000
Total Fair Value Available-for-sale Securities		79,734,000		68,635,000
Total Unrealized Losses Available-for-sale Securities		441,000		417,000
U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		1,448,179,000		1,471,920,000
Gross Unrealized Gains		24,612,000		29,347,000
Gross Unrealized Losses		44,000		24,000
Estimated Fair Value		1,472,747,000		1,501,243,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Total		1,472,747,000		1,501,243,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value		47,417,000		34,850,000
Less Than 12 Months Unrealized Losses		44,000		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		47,417,000		34,850,000
Total Unrealized Losses Available-for-sale Securities		44,000		24,000
Government Agency Issued Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		327,757,000		394,894,000
Gross Unrealized Gains		10,647,000		9,786,000
Gross Unrealized Losses		174,000		70,000
Estimated Fair Value		338,230,000		404,610,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Total		338,230,000		404,610,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value		6,484,000		0
Less Than 12 Months Unrealized Losses		68,000		0
12 Months or Longer Fair Value		3,118,000		3,751,000
12 Months or Longer Unrealized Losses		106,000		70,000
Total Fair Value Available-for-sale Securities		9,602,000		3,751,000
Total Unrealized Losses Available-for-sale Securities		174,000		70,000
Government Agency Issued Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		86,455,000		31,161,000
Gross Unrealized Gains		3,986,000		3,438,000
Gross Unrealized Losses		135,000		0
Estimated Fair Value		90,306,000		34,599,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Total		90,306,000		34,599,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value		20,065,000		0
Less Than 12 Months Unrealized Losses		135,000		0
12 Months or Longer Fair Value		0		0
12 Months or Longer Unrealized Losses		0		0
Total Fair Value Available-for-sale Securities		20,065,000		0
Total Unrealized Losses Available-for-sale Securities		135,000		0
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		538,473,000		541,138,000
Gross Unrealized Gains		36,174,000		22,705,000
Gross Unrealized Losses		88,000		323,000
Estimated Fair Value		574,559,000		563,520,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Total		574,559,000		563,520,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value		2,336,000		20,820,000
Less Than 12 Months Unrealized Losses		29,000		144,000
12 Months or Longer Fair Value		314,000		9,214,000
12 Months or Longer Unrealized Losses		59,000		179,000
Total Fair Value Available-for-sale Securities		2,650,000		30,034,000
Total Unrealized Losses Available-for-sale Securities		88,000		323,000
Other Available for Sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		7,046,000		8,938,000
Gross Unrealized Gains		718,000		608,000
Gross Unrealized Losses		0		0
Estimated Fair Value		7,764,000		9,546,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Total		7,764,000		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.