FAIR VALUE MEASUREMENTS (Details) (USD $)	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013 Contract For Difference [Member]	Sep. 30, 2013 Fair Value, Measurements, Recurring [Member]	Dec. 31, 2012 Fair Value, Measurements, Recurring [Member]	Sep. 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]	Sep. 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]	Sep. 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			$ 54,822,000	$ 79,838,000	$ 0	$ 0	$ 0	$ 0	$ 54,822,000	$ 79,838,000
Noncurrent investments available for sale			10,725,000	9,902,000	10,725,000	9,902,000	0	0	0	0
Deferred Compensation Plan			3,604,000	3,745,000	3,604,000	3,745,000	0	0	0	0
Supplemental retirement benefit trust life insurance policies			7,317,000	6,438,000	0	0	7,317,000	6,438,000	0	0
Total financial assets, fair value			76,468,000	99,923,000	14,329,000	13,647,000	7,317,000	6,438,000	54,822,000	79,838,000
Liabilities:
Derivative liabilities			202,577,000	255,443,000	0	0	0	0	202,577,000	255,443,000
Long-term debt			1,757,983,000	1,886,440,000	0	0	1,757,983,000	1,886,440,000	0	0
Total financial liabilities, fair value			1,960,560,000	2,141,883,000	0	0	1,757,983,000	1,886,440,000	202,577,000	255,443,000
Net fair value assets/(liabilities)			(1,884,092,000)	(2,041,960,000)	14,329,000	13,647,000	(1,750,666,000)	(1,880,002,000)	(147,755,000)	(175,605,000)
Unobservable input [Abstract]
Risk of non-performance, lower range (in hundredths)		0.33%
Risk of non-performance, upper range (in hundredths)		0.92%
Discount rate, lower range (in hundredths)		2.02%
Discount rate, upper range (in hundredths)		2.83%
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)	28,850,000
Settlements	(1,000,000)
Net fair value assets/(liabilities), Ending balance	(147,555,000)
Change in unrealized gains (losses), net relating to net fair value assets/(liabilities), still held as of September 30, 2013	28,850,000
Change in regulatory asset/liability balances [Roll Forward]
Net regulatory assets/(liabilities), Beginning balance	176,605,000
Unrealized (gains) and losses, net	(28,850,000)
Net regulatory assets/(liabilities), Ending balance	$ 147,755,000