FAIR VALUE (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Available-for-sale investment securities:
Total available-for-sale investment securities	$ 2,472,925,000	$ 2,186,163,000
Assets and Liabilities Measured on Nonrecurring Basis [Abstract]
Discount on appraisal value of foreclosed real estate, minimum (in hundredths)	10.00%
Discount on appraisal value of foreclosed real estate, maximum (in hundredths)	78.00%
Valuation allowance	0	50,000
Mortgage Servicing Rights [Member]
Assets and Liabilities Measured on Nonrecurring Basis [Abstract]
Fair value of MSRs at end of period	1,600,000
Valuation allowance	0
Carrying value before valuation allowance at end of period	1,100,000
Recurring [Member]
Available-for-sale investment securities:
U.S. Treasury and agency securities	1,517,733,000	1,212,147,000
Obligations of state and political subdivisions	671,903,000	668,982,000
Government agency mortgage-backed securities	237,728,000	254,978,000
Corporate debt securities	27,091,000	27,587,000
Government agency collateralized mortgage obligations	18,025,000	22,048,000
Marketable equity securities	445,000	421,000
Total available-for-sale investment securities	2,472,925,000	2,186,163,000
Mortgages loans held for sale	1,042,000	728,000
Commitments to originate real estate loans for sale	185,000	44,000
Forward sales commitments	(43,000)	27,000
Total	2,474,109,000	2,186,962,000
Recurring [Member] | Level 1 [Member]
Available-for-sale investment securities:
U.S. Treasury and agency securities	1,496,667,000	1,182,261,000
Obligations of state and political subdivisions	0	0
Government agency mortgage-backed securities	0	0
Corporate debt securities	0	0
Government agency collateralized mortgage obligations	0	0
Marketable equity securities	445,000	421,000
Total available-for-sale investment securities	1,497,112,000	1,182,682,000
Mortgages loans held for sale	0	0
Commitments to originate real estate loans for sale	0	0
Forward sales commitments	0	0
Total	1,497,112,000	1,182,682,000
Recurring [Member] | Level 2 [Member]
Available-for-sale investment securities:
U.S. Treasury and agency securities	21,066,000	29,886,000
Obligations of state and political subdivisions	671,903,000	668,982,000
Government agency mortgage-backed securities	237,728,000	254,978,000
Corporate debt securities	27,091,000	27,587,000
Government agency collateralized mortgage obligations	18,025,000	22,048,000
Marketable equity securities	0	0
Total available-for-sale investment securities	975,813,000	1,003,481,000
Mortgages loans held for sale	1,042,000	728,000
Commitments to originate real estate loans for sale	0	0
Forward sales commitments	(43,000)	27,000
Total	976,812,000	1,004,236,000
Mortgage loans held for sale, at principal value	1,000,000
Mortgage loans held for sale, gross unrealized gain (Loss)	70,000
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
Corporate debt securities	0	0
Government agency collateralized mortgage obligations	0	0
Marketable equity securities	0	0
Total available-for-sale investment securities	0	0
Mortgages loans held for sale	0	0
Commitments to originate real estate loans for sale	185,000	44,000
Forward sales commitments	0	0
Total	185,000	44,000
Nonrecurring [Member]
Assets and Liabilities Measured on Nonrecurring Basis [Abstract]
Impaired loans	0	600,000
Other real estate owned	1,855,000	5,060,000
Total	1,855,000	5,660,000
Nonrecurring [Member] | Level 1 [Member]
Assets and Liabilities Measured on Nonrecurring Basis [Abstract]
Impaired loans	0	0
Other real estate owned	0	0
Total	0	0
Nonrecurring [Member] | Level 2 [Member]
Assets and Liabilities Measured on Nonrecurring Basis [Abstract]
Impaired loans	0	600,000
Other real estate owned	0	0
Total	0	600,000
Nonrecurring [Member] | Level 3 [Member]
Assets and Liabilities Measured on Nonrecurring Basis [Abstract]
Impaired loans	0	0
Other real estate owned	1,855,000	5,060,000
Total	$ 1,855,000	$ 5,060,000