FAIR VALUE (Details) (USD $)			3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended					3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Pooled Trust Preferred Securities [Member]	Dec. 31, 2012 Pooled Trust Preferred Securities [Member]	Mar. 31, 2013 Impaired Loans [Member]	Dec. 31, 2012 Impaired Loans [Member]	Mar. 31, 2013 Other Real Estate Owned [Member]	Dec. 31, 2012 Other Real Estate Owned [Member]	Mar. 31, 2013 Mortgage Servicing Rights [Member]	Dec. 31, 2012 Mortgage Servicing Rights [Member]	Mar. 31, 2013 Carrying Value [Member]	Dec. 31, 2012 Carrying Value [Member]	Mar. 31, 2013 Fair Value [Member]	Dec. 31, 2012 Fair Value [Member]	Mar. 31, 2013 Pooled Trust Preferred Securities [Member]		Mar. 31, 2012 Pooled Trust Preferred Securities [Member]		Mar. 31, 2013 Recurring [Member]	Dec. 31, 2012 Recurring [Member]	Mar. 31, 2013 Recurring [Member] Level 1 [Member]	Dec. 31, 2012 Recurring [Member] Level 1 [Member]	Mar. 31, 2013 Recurring [Member] Level 2 [Member]	Dec. 31, 2012 Recurring [Member] Level 2 [Member]	Mar. 31, 2013 Recurring [Member] Level 3 [Member]	Dec. 31, 2012 Recurring [Member] Level 3 [Member]	Mar. 31, 2013 Nonrecurring [Member]	Dec. 31, 2012 Nonrecurring [Member]	Mar. 31, 2013 Nonrecurring [Member] Level 1 [Member]	Dec. 31, 2012 Nonrecurring [Member] Level 1 [Member]	Mar. 31, 2013 Nonrecurring [Member] Level 2 [Member]	Dec. 31, 2012 Nonrecurring [Member] Level 2 [Member]	Mar. 31, 2013 Nonrecurring [Member] Level 3 [Member]	Dec. 31, 2012 Nonrecurring [Member] Level 3 [Member]
Available-for-sale investment securities:
U.S. Treasury and agency securities																			$ 769,746,000	$ 1,079,257,000	$ 677,768,000	$ 891,803,000	$ 91,978,000	$ 187,454,000	$ 0	$ 0
Obligations of state and political subdivisions																			646,693,000	662,892,000	0	0	646,693,000	662,892,000	0	0
Government agency mortgage-backed securities																			253,345,000	269,951,000	0	0	253,345,000	269,951,000	0	0
Pooled trust preferred securities																			47,486,000	49,600,000	0	0	0	0	47,486,000	49,600,000
Government agency collateralized mortgage obligations																			30,636,000	33,935,000	0	0	30,636,000	33,935,000	0	0
Corporate debt securities																			25,288,000	25,357,000	0	0	25,288,000	25,357,000	0	0
Marketable equity securities																			440,000	402,000	440,000	402,000	0	0	0	0
Estimated Fair Value	1,773,634,000	2,121,394,000																	1,773,634,000	2,121,394,000	678,208,000	892,205,000	1,047,940,000	1,179,589,000	47,486,000	49,600,000
Assets and Liabilities Measured on Nonrecurring Basis [Abstract]
Impaired loans	1,600,000																										1,186,000	1,186,000	0	0	0	0	1,186,000	1,186,000
Other real estate owned																											6,838,000	4,788,000	0	0	0	0	6,838,000	4,788,000
Mortgage servicing rights																											899,000	1,028,000	0	0	0	0	899,000	1,028,000
Total			47,486,000	49,600,000	1,186,000	1,186,000	6,838,000	4,788,000	899,000	1,028,000																	8,923,000	7,002,000	0	0	0	0	8,923,000	7,002,000
Discount on appraisal value of foreclosed real estate, minimum (in hundredths)	11.00%
Discount on appraisal value of foreclosed real estate, maximum (in hundredths)	65.00%
Impaired Loans Valuation Allowance	400,000	400,000
Level 3 Assets Measured at Fair Value on a Recurring Basis [Abstract]
Beginning balance															49,600,000		43,846,000
Total gains (losses) included in earnings															103,000	[1]	47,000	[1]
Total gains included in other comprehensive income															1,892,000	[2]	4,567,000	[2]
Principal reductions															(4,109,000)		(1,075,000)
Ending balance															47,486,000		47,385,000
Financial assets [Abstract]
Net loans											3,861,601,000	3,865,576,000	3,862,036,000	3,881,354,000
Financial liabilities [Abstract]
Deposits											5,774,824,000	5,628,039,000	5,783,231,000	5,635,320,000
Borrowings											361,422,000	728,061,000	404,561,000	820,377,000
Subordinated debt held by unconsolidated subsidiary trusts											102,079,000	102,073,000	103,238,000	97,899,000
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fair value of assets classified as Level 3			$ 47,486,000	$ 49,600,000	$ 1,186,000	$ 1,186,000	$ 6,838,000	$ 4,788,000	$ 899,000	$ 1,028,000																	$ 8,923,000	$ 7,002,000	$ 0	$ 0	$ 0	$ 0	$ 8,923,000	$ 7,002,000
Pooled Trust Preferred Securities, Valuation Techniques			Consensus pricing	Consensus pricing
Impaired Loans, Valuation Techniques					Fair Value of Collateral	Fair value of collateral
Other Real Estate Owned, Valuation Techniques							Fair Value of Collateral	Fair value of collateral
Mortgage Servicing Rights, Valuation Techniques									Discounted cash flow	Discounted cash flow
Significant Unobservable Inputs, Inputs Description			Weighting of offered quote	Weighting of offered quotes	Estimated cost of disposal/market adjustment	Estimated cost of disposal/market adjustment	Estimated cost of disposal/market adjustment	Estimated cost of disposal/market adjustment	Discount rate/Adequate compensation	Discount rate/Adequate compensation
Significant Unobservable Input Range, Minimum (in hundredths)			75.50%	65.30%
Significant Unobservable Input Range, Maximum (in hundredths)			84.40%	85.10%
Significant Unobservable Input Range, Weighted Average (in hundredths)			80.20%	78.40%
Estimated cost of disposal/market adjustment Range Minimum (in hundredths)					20.00%	25.00%	11.00%	11.00%
Estimated cost of disposal/market adjustment Range Maximum (in hundredths)					50.00%	50.00%	65.00%	60.20%
Estimated cost of disposal/market adjustment Weighted Average (in hundredths)					23.00%	27.50%	25.00%	19.90%
Constant Prepayment Input Range Minimum (in hundredths)									20.10%	1.10%
Constant Prepayment Input Range Maximum (in hundredths)									41.30%	39.60%
Constant Prepayment Input Range Weighted Average (in hundredths)									37.60%	34.40%
Weighted average discount rate Minimum (in hundredths)									2.62%	2.50%
Weighted average discount rate Maximum (in hundredths)									3.46%	3.30%
Weighted average discount rate Weighted Average (in hundredths)									3.25%	3.10%
Adequate Compensation Input Range									$7/loan	$7/loan

[1] Amounts included in earnings associated with the pooled trust preferred securities relate to accretion of related discount and are reported in interest and dividends on taxable investments.

[2] Amounts included in other comprehensive income associated with the pooled trust preferred securities are relate to changes in unrealized loss and are reported as a component of unrealized gains on securities in the Statement of Comprehensive Income.