FAIR VALUE (Details) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Available-for-sale investment securities:
Fair value	$ 1,848,652,000	$ 2,121,394,000
Assets and Liabilities Measured on Nonrecurring Basis [Abstract]
Discount on appraisal value of foreclosed real estate, minimum (in hundredths)	11.00%
Discount on appraisal value of foreclosed real estate, maximum (in hundredths)	67.00%
Mortgage Servicing Rights, Valuation Allowance	200,000	200,000
Recurring [Member]
Available-for-sale investment securities:
U.S. Treasury and agency securities	877,272,000	1,079,257,000
Obligations of state and political subdivisions	625,995,000	662,892,000
Government agency mortgage-backed securities	244,899,000	269,951,000
Pooled trust preferred securities	51,251,000	49,600,000
Government agency collateralized mortgage obligations	23,936,000	33,935,000
Corporate debt securities	24,807,000	25,357,000
Marketable equity securities	492,000	402,000
Fair value	1,848,652,000	2,121,394,000
Forward sales commitments	(129,000)
Commitments to originate real estate loans for sale	268,000
Mortgages loans held for sale	945,000
Total	1,849,736,000
Recurring [Member] | Level 1 [Member]
Available-for-sale investment securities:
U.S. Treasury and agency securities	862,068,000	891,803,000
Obligations of state and political subdivisions	0	0
Government agency mortgage-backed securities	0	0
Pooled trust preferred securities	0	0
Government agency collateralized mortgage obligations	0	0
Corporate debt securities	0	0
Marketable equity securities	492,000	402,000
Fair value	862,560,000	892,205,000
Forward sales commitments	0
Commitments to originate real estate loans for sale	0
Mortgages loans held for sale	0
Total	862,560,000
Recurring [Member] | Level 2 [Member]
Available-for-sale investment securities:
U.S. Treasury and agency securities	15,204,000	187,454,000
Obligations of state and political subdivisions	625,995,000	662,892,000
Government agency mortgage-backed securities	244,899,000	269,951,000
Pooled trust preferred securities	0	0
Government agency collateralized mortgage obligations	23,936,000	33,935,000
Corporate debt securities	24,807,000	25,357,000
Marketable equity securities	0	0
Fair value	934,841,000	1,179,589,000
Forward sales commitments	(129,000)
Commitments to originate real estate loans for sale	0
Mortgages loans held for sale	945,000
Total	935,657,000
Mortgage loans held for sale, at principal value	900,000
Mortgage loans held for sale, Unrealized gains (losses)	43,000
Recurring [Member] | Level 3 [Member]
Available-for-sale investment securities:
U.S. Treasury and agency securities	0	0
Obligations of state and political subdivisions	0	0
Government agency mortgage-backed securities	0	0
Pooled trust preferred securities	51,251,000	49,600,000
Government agency collateralized mortgage obligations	0	0
Corporate debt securities	0	0
Marketable equity securities	0	0
Fair value	51,251,000	49,600,000
Forward sales commitments	0
Commitments to originate real estate loans for sale	268,000
Mortgages loans held for sale	0
Total	51,519,000
Nonrecurring [Member]
Assets and Liabilities Measured on Nonrecurring Basis [Abstract]
Impaired loan balance	1,746,000	1,186,000
Other real estate owned	5,218,000	4,788,000
Mortgage servicing rights	1,011,000	1,028,000
Total	7,975,000	7,002,000
Nonrecurring [Member] | Level 1 [Member]
Assets and Liabilities Measured on Nonrecurring Basis [Abstract]
Impaired loan balance	0	0
Other real estate owned	0	0
Mortgage servicing rights	0	0
Total	0	0
Nonrecurring [Member] | Level 2 [Member]
Assets and Liabilities Measured on Nonrecurring Basis [Abstract]
Impaired loan balance	0	0
Other real estate owned	0	0
Mortgage servicing rights	0	0
Total	0	0
Nonrecurring [Member] | Level 3 [Member]
Assets and Liabilities Measured on Nonrecurring Basis [Abstract]
Impaired loan balance	1,746,000	1,186,000
Other real estate owned	5,218,000	4,788,000
Mortgage servicing rights	1,011,000	1,028,000
Total	$ 7,975,000	$ 7,002,000