FAIR VALUE MEASUREMENTS (Details) (USD $)	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2012 Contract For Difference [Member]	Jun. 30, 2012 Weather Insurance Contract [Member]	Jun. 30, 2012 Fair Value, Measurements, Recurring [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member]	Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Significant Other Observable Inputs (Level 2) [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Significant Other Observable Inputs (Level 2) [Member]	Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Significant Unobservable Inputs (Level 3) [Member]
Assets:
Contract for differences				$ 81,571,000	$ 83,942,000	$ 0	$ 0	$ 0	$ 0	$ 81,571,000	$ 83,942,000
Weather insurance contracts				1,000,000	3,512,000	0	0	0	0	1,000,000	3,512,000
Noncurrent investments available for sale				9,618,000	9,152,000	0	0	9,618,000	9,152,000	0	0
Deferred Compensation Plan				3,601,000	3,739,000	0	0	3,601,000	3,739,000	0	0
Supplemental retirement benefit trust life insurance policies				6,035,000	5,655,000	6,035,000	5,655,000	0	0	0	0
Total financial assets, fair value				101,825,000	106,000,000	6,035,000	5,655,000	13,219,000	12,891,000	82,571,000	87,454,000
Liabilities:
Contract for differences				259,895,000	268,035,000	0	0	0	0	259,895,000	268,035,000
Long-term debt				1,878,780,000	1,715,724,000	1,878,780,000	1,715,724,000	0	0	0	0
Total financial liabilities, fair value				2,138,675,000	1,983,759,000	1,878,780,000	1,715,724,000	0	0	259,895,000	268,035,000
Net fair value assets/(liabilities)				(2,036,850,000)	(1,877,759,000)	(1,872,745,000)	(1,710,069,000)	13,219,000	12,891,000	(177,324,000)	(180,581,000)
Unobservable input [Abstract]
Risk of non-performance, lower range (in hundredths)		1.09%
Risk of non-performance, upper range (in hundredths)		1.62%
Discount rate, lower range (in hundredths)		1.67%
Discount rate, upper range (in hundredths)		2.03%
Forward pricing, lower range		1.40
Forward pricing, upper range		9.83
Heating degree days, upper range (in days)			4,976D
Changes in fair value of assets and liabilities classified as level 3 [Roll Forward]
Net fair value assets/(liabilities), December 31, 2011	(180,581,000)
Unrealized gains and (losses), net [Abstract]
Included in earnings	3,488,000
Included in regulatory assets/(liabilities)	5,769,000
Settlements	(6,000,000)
Net fair value assets/(liabilities), June 30, 2012	(177,324,000)
Change in unrealized gains (losses), net relating to net derivative assets/(liabilities), still held as of June 30, 2012	5,769,000
Change in regulatory asset/liability balances [Roll Forward]
Net regulatory assets/(liabilities), December 31, 2011	184,093,000
Unrealized (gains) and losses, net	(5,769,000)
Net regulatory assets/(liabilities), June 30, 2012	$ 178,324,000