RETIREMENT PLAN AND POST-RETIREMENT BENEFITS, Components of Net Periodic Cost and Expected Benefit Payments (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Average percentage of net periodic costs capitalized [Abstract]
Average percentage of net periodic costs capitalized (in hundredths)			34.60%	32.80%
Company contributions to Pension and Other Postretirement benefits			$ 1,500,000
Pension Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	4,406,000	4,607,000	8,813,000	9,213,000
Interest cost	10,228,000	10,169,000	20,456,000	20,338,000
Expected return on plan assets	(12,447,000)	(12,192,000)	(24,894,000)	(24,383,000)
Amortization of:
Prior service (credit)/cost	(724,000)	(738,000)	(1,448,000)	(1,476,000)
Actuarial (gain)/loss	3,473,000	4,155,000	6,945,000	8,310,000
Total net benefit cost	4,936,000	6,001,000	9,872,000	12,002,000
Other Post-Retirement Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	595,000	653,000	1,191,000	1,306,000
Interest cost	1,905,000	2,090,000	3,810,000	4,180,000
Expected return on plan assets	(1,563,000)	(1,596,000)	(3,126,000)	(3,193,000)
Amortization of:
Prior service (credit)/cost	(987,000)	(987,000)	(1,974,000)	(1,973,000)
Actuarial (gain)/loss	731,000	1,083,000	1,462,000	2,166,000
Total net benefit cost	681,000	1,243,000	1,363,000	2,486,000
NPC [Member]
Average percentage of net periodic costs capitalized [Abstract]
Average percentage of net periodic costs capitalized (in hundredths)			36.60%	37.30%
NPC [Member] | Pension Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	2,358,000	2,445,000	4,715,000	4,891,000
Interest cost	4,881,000	4,880,000	9,762,000	9,760,000
Expected return on plan assets	(6,237,000)	(6,169,000)	(12,474,000)	(12,339,000)
Amortization of:
Prior service (credit)/cost	(456,000)	(470,000)	(912,000)	(939,000)
Actuarial (gain)/loss	1,363,000	1,690,000	2,726,000	3,379,000
Total net benefit cost	1,909,000	2,376,000	3,817,000	4,752,000
NPC [Member] | Other Post-Retirement Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	350,000	363,000	700,000	727,000
Interest cost	602,000	615,000	1,205,000	1,230,000
Expected return on plan assets	(592,000)	(590,000)	(1,183,000)	(1,180,000)
Amortization of:
Prior service (credit)/cost	229,000	229,000	458,000	458,000
Actuarial (gain)/loss	221,000	302,000	441,000	604,000
Total net benefit cost	810,000	919,000	1,621,000	1,839,000
SPPC [Member]
Average percentage of net periodic costs capitalized [Abstract]
Average percentage of net periodic costs capitalized (in hundredths)			34.50%	30.20%
SPPC [Member] | Pension Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	1,695,000	1,840,000	3,391,000	3,680,000
Interest cost	5,043,000	5,013,000	10,086,000	10,025,000
Expected return on plan assets	(5,937,000)	(5,741,000)	(11,875,000)	(11,482,000)
Amortization of:
Prior service (credit)/cost	(277,000)	(277,000)	(554,000)	(554,000)
Actuarial (gain)/loss	2,026,000	2,412,000	4,052,000	4,824,000
Total net benefit cost	2,550,000	3,247,000	5,100,000	6,493,000
SPPC [Member] | Other Post-Retirement Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	227,000	271,000	455,000	543,000
Interest cost	1,283,000	1,457,000	2,566,000	2,915,000
Expected return on plan assets	(941,000)	(976,000)	(1,882,000)	(1,952,000)
Amortization of:
Prior service (credit)/cost	(1,220,000)	(1,219,000)	(2,439,000)	(2,439,000)
Actuarial (gain)/loss	504,000	773,000	1,007,000	1,546,000
Total net benefit cost	$ (147,000)	$ 306,000	$ (293,000)	$ 613,000