SECURITIES (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 2,547,697,000		$ 2,365,962,000
Gross Unrealized Gains	60,278,000		68,644,000
Gross Unrealized Losses	799,000		574,000
Estimated Fair Value	2,607,176,000	2,573,535,000	2,434,032,000	[1]
Gross gains recognized on available-for-sale securities	34,000	94,000
Gross losses recognized on available-for-sale securities	15,000	20,000
Equity securities, considered maturity	10 years
Available-for-sale securities, amortized cost, by contractual maturity [Abstract]
Maturing in one year or less	486,274,000
Maturing after one year through five years	1,151,485,000
Maturing after five years through ten years	403,630,000
Maturing after ten years	506,308,000
Total	2,547,697,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Maturing in one year or less	490,208,000
Maturing after one year through five years	1,170,308,000
Maturing after five years through ten years	417,612,000
Maturing after ten years	529,048,000
Estimated Fair Value	2,607,176,000	2,573,535,000	2,434,032,000	[1]
Available-for-sale securities, weighted average yield, by contractual maturity [Abstract]
Maturing in one year or less (in hundredths)	1.65%
Maturing after one year through five years (in hundredths)	1.42%
Maturing after five years through ten years (in hundredths)	3.59%
Maturing after ten years (in hundredths)	4.20%
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	180,386,000		138,820,000
Less Than 12 Months Unrealized Losses	754,000		417,000
12 Months or Longer Fair Value	3,315,000		3,152,000
12 Months or Longer Unrealized Losses	45,000		157,000
Total Fair Value Available-for-sale Securities	183,701,000		141,972,000
Total Unrealized Losses Available-for-sale Securities	799,000		574,000
U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,498,886,000		1,380,979,000
Gross Unrealized Gains	18,841,000		21,081,000
Gross Unrealized Losses	2,000		64,000
Estimated Fair Value	1,517,725,000		1,401,996,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Estimated Fair Value	1,517,725,000		1,401,996,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	47,457,000		47,395,000
Less Than 12 Months Unrealized Losses	2,000		64,000
12 Months or Longer Fair Value	0		0
12 Months or Longer Unrealized Losses	0		0
Total Fair Value Available-for-sale Securities	47,457,000		47,395,000
Total Unrealized Losses Available-for-sale Securities	2,000		64,000
Government Agency Issued Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	327,586,000		358,677,000
Gross Unrealized Gains	7,230,000		8,457,000
Gross Unrealized Losses	266,000		259,000
Estimated Fair Value	334,550,000		366,875,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Estimated Fair Value	334,550,000		366,875,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	38,110,000		55,939,000
Less Than 12 Months Unrealized Losses	235,000		145,000
12 Months or Longer Fair Value	2,746,000		2,839,000
12 Months or Longer Unrealized Losses	31,000		114,000
Total Fair Value Available-for-sale Securities	40,856,000		58,778,000
Total Unrealized Losses Available-for-sale Securities	266,000		259,000
Government Agency Issued Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	192,912,000		87,314,000
Gross Unrealized Gains	3,974,000		4,266,000
Gross Unrealized Losses	427,000		135,000
Estimated Fair Value	196,459,000		91,445,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Estimated Fair Value	196,459,000		91,445,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	77,829,000		26,239,000
Less Than 12 Months Unrealized Losses	427,000		135,000
12 Months or Longer Fair Value	0		0
12 Months or Longer Unrealized Losses	0		0
Total Fair Value Available-for-sale Securities	77,829,000		26,239,000
Total Unrealized Losses Available-for-sale Securities	427,000		135,000
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	521,255,000		531,940,000
Gross Unrealized Gains	29,318,000		34,049,000
Gross Unrealized Losses	98,000		116,000
Estimated Fair Value	550,475,000		565,873,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Estimated Fair Value	550,475,000		565,873,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	10,319,000		9,247,000
Less Than 12 Months Unrealized Losses	84,000		73,000
12 Months or Longer Fair Value	569,000		313,000
12 Months or Longer Unrealized Losses	14,000		43,000
Total Fair Value Available-for-sale Securities	10,888,000		9,560,000
Total Unrealized Losses Available-for-sale Securities	98,000		116,000
Other Available for Sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,058,000		7,052,000
Gross Unrealized Gains	915,000		791,000
Gross Unrealized Losses	6,000		0
Estimated Fair Value	7,967,000		7,843,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Estimated Fair Value	7,967,000		7,843,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	6,671,000
Less Than 12 Months Unrealized Losses	6,000
12 Months or Longer Fair Value	0
12 Months or Longer Unrealized Losses	0
Total Fair Value Available-for-sale Securities	6,671,000
Total Unrealized Losses Available-for-sale Securities	$ 6,000

[1] Derived from audited financial statements.