FAIR VALUE MEASUREMENTS (Details) (USD $)	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013 Contract For Difference [Member]	Jun. 30, 2013 Fair Value, Measurements, Recurring [Member]	Dec. 31, 2012 Fair Value, Measurements, Recurring [Member]	Jun. 30, 2013 Fair Value, Measurements, Recurring [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Dec. 31, 2012 Fair Value, Measurements, Recurring [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]	Jun. 30, 2013 Fair Value, Measurements, Recurring [Member] Significant Other Observable Inputs (Level 2) [Member]	Dec. 31, 2012 Fair Value, Measurements, Recurring [Member] Significant Other Observable Inputs (Level 2) [Member]	Jun. 30, 2013 Fair Value, Measurements, Recurring [Member] Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2012 Fair Value, Measurements, Recurring [Member] Significant Unobservable Inputs (Level 3) [Member]
Assets:
Derivative assets			$ 58,700,000	$ 79,838,000	$ 0	$ 0	$ 0	$ 0	$ 58,700,000	$ 79,838,000
Noncurrent investments available for sale			10,322,000	9,902,000	10,322,000	9,902,000	0	0	0	0
Deferred Compensation Plan			3,516,000	3,745,000	3,516,000	3,745,000	0	0	0	0
Supplemental retirement benefit trust life insurance policies			6,953,000	6,438,000	0	0	6,953,000	6,438,000	0	0
Total financial assets, fair value			79,491,000	99,923,000	13,838,000	13,647,000	6,953,000	6,438,000	58,700,000	79,838,000
Liabilities:
Derivative liabilities			212,042,000	255,443,000	0	0	0	0	212,042,000	255,443,000
Long-term debt			1,790,907,000	1,886,440,000	0	0	1,790,907,000	1,886,440,000	0	0
Total financial liabilities, fair value			2,002,949,000	2,141,883,000	0	0	1,790,907,000	1,886,440,000	212,042,000	255,443,000
Net fair value assets/(liabilities)			(1,923,458,000)	(2,041,960,000)	13,838,000	13,647,000	(1,783,954,000)	(1,880,002,000)	(153,342,000)	(175,605,000)
Unobservable input [Abstract]
Risk of non-performance, lower range (in hundredths)		0.00%
Risk of non-performance, upper range (in hundredths)		1.03%
Discount rate, lower range (in hundredths)		1.96%
Discount rate, upper range (in hundredths)		2.70%
Forward pricing, lower range (in dollars per MW)		1.40
Forward pricing, upper range (in dollars per MW)		9.83
Changes in fair value of assets and liabilities classified as level 3 [Roll Forward]
Net fair value assets/(liabilities), Beginning balance	(175,605,000)
Unrealized gains and (losses), net Included in regulatory assets/(liabilities)	23,263,000
Settlements	(1,000,000)
Net fair value assets/(liabilities), Ending balance	(153,342,000)
Change in unrealized gains (losses), net relating to net fair value assets/(liabilities), still held as of June 30, 2013	23,263,000
Change in regulatory asset/liability balances [Roll Forward]
Net regulatory assets/(liabilities), Beginning balance	176,605,000
Unrealized (gains) and losses, net	(23,263,000)
Net regulatory assets/(liabilities), Ending balance	$ 153,342,000