RETIREMENT PLAN AND POST-RETIREMENT BENEFITS, Components of Net Periodic Cost and Expected Benefit Payments (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Average percentage of net periodic costs capitalized [Abstract]
Average percentage of net periodic costs capitalized (in hundredths)	33.40%		34.00%
Pension Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	$ 18,427	$ 18,910	$ 18,837
Interest cost	40,676	42,872	44,145
Expected return on plan assets	48,767	44,275	37,159
Amortization of:
Prior service (credit)/cost	(2,952)	(1,794)	(1,794)
Actuarial (gain)/loss	(16,620)	(15,106)	(27,575)
Remeasurement adjustment
Total net benefit cost	24,004	30,819	51,604
Special termination benefits excluded from net periodic cost	286	0	300
Other Post-Retirement Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	2,611	2,466	2,421
Interest cost	8,360	8,736	10,072
Expected return on plan assets	6,386	6,223	6,048
Amortization of:
Prior service (credit)/cost	(3,947)	(3,890)	(1,466)
Actuarial (gain)/loss	(4,333)	(4,342)	(5,296)
Remeasurement adjustment			(336)
Total net benefit cost	4,971	5,431	10,611
Special termination benefits excluded from net periodic cost	100	0	2,800
NPC [Member]
Average percentage of net periodic costs capitalized [Abstract]
Average percentage of net periodic costs capitalized (in hundredths)	36.90%	37.00%	39.40%
NPC [Member] | Pension Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	9,781	9,567	9,572
Interest cost	19,521	20,092	21,079
Expected return on plan assets	24,677	21,447	17,847
Amortization of:
Prior service (credit)/cost	(1,879)	(1,733)	(1,733)
Actuarial (gain)/loss	(6,758)	(7,056)	(13,192)
Remeasurement adjustment
Total net benefit cost	9,504	13,535	24,263
NPC [Member] | Other Post-Retirement Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	1,454	1,413	1,325
Interest cost	2,459	2,474	2,437
Expected return on plan assets	2,360	2,270	2,067
Amortization of:
Prior service (credit)/cost	916	946	1,104
Actuarial (gain)/loss	(1,208)	(1,199)	(1,272)
Remeasurement adjustment			(57)
Total net benefit cost	3,677	3,762	4,128
SPPC [Member]
Average percentage of net periodic costs capitalized [Abstract]
Average percentage of net periodic costs capitalized (in hundredths)	31.70%	34.20%	36.40%
Expected federal subsidy [Abstract]
2012	0
2013	0
2014	0
2015	0
2016	0
2017-2021	0
SPPC [Member] | Pension Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	7,361	8,016
Interest cost	20,050	21,557
Expected return on plan assets	22,964	21,723
Amortization of:
Prior service (credit)/cost	(1,108)
Actuarial (gain)/loss	(9,647)	(7,876)
Remeasurement adjustment
Total net benefit cost	12,986	15,622
Expected cash flows for the plans, including trust accounts [Abstract]
2012	53,237
2013	57,364
2014	57,264
2015	56,548
2016	63,314
2017-2021	302,569
SPPC [Member] | Other Post-Retirement Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	1,086	977
Interest cost	5,830	6,187
Expected return on plan assets	3,905	3,844
Amortization of:
Prior service (credit)/cost	(4,878)	(4,851)
Actuarial (gain)/loss	(3,092)	(3,109)
Remeasurement adjustment
Total net benefit cost	1,225
Expected cash flows for the plans, including trust accounts [Abstract]
2012	9,569
2013	9,736
2014	9,984
2015	10,070
2016	10,173
2017-2021	$ 50,756