RETIREMENT PLAN AND POST-RETIREMENT BENEFITS (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Pension And Other Postretirement Benefit Contributions			$ 10,000,000
Net periodic benefit cost [Abstract]
Average percentage of net periodic costs capitalized (in hundredths)			33.26%	33.85%
NPC [Member]
Net periodic benefit cost [Abstract]
Average percentage of net periodic costs capitalized (in hundredths)			37.18%	36.34%
NPC [Member] | Pension Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	2,445,000	2,392,000	7,336,000	7,175,000
Interest cost	4,880,000	5,023,000	14,640,000	15,069,000
Expected return on plan assets	(6,169,000)	(5,362,000)	(18,508,000)	(16,085,000)
Amortization of prior service cost	(470,000)	(433,000)	(1,409,000)	(1,300,000)
Amortization of net loss	1,690,000	1,764,000	5,069,000	5,292,000
Net periodic benefit cost	2,376,000	3,384,000	7,128,000	10,151,000
NPC [Member] | Other Postretirement Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	363,000	353,000	1,090,000	1,060,000
Interest cost	615,000	619,000	1,844,000	1,856,000
Expected return on plan assets	(590,000)	(567,000)	(1,769,000)	(1,702,000)
Amortization of prior service cost	229,000	236,000	687,000	709,000
Amortization of net loss	302,000	300,000	906,000	899,000
Net periodic benefit cost	919,000	941,000	2,758,000	2,822,000
SPPC [Member]
Net periodic benefit cost [Abstract]
Average percentage of net periodic costs capitalized (in hundredths)			31.22%	34.57%
SPPC [Member] | Pension Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	1,840,000	2,004,000	5,520,000	6,012,000
Interest cost	5,013,000	5,389,000	15,038,000	16,167,000
Expected return on plan assets	(5,741,000)	(5,431,000)	(17,223,000)	(16,292,000)
Amortization of prior service cost	(277,000)	(26,000)	(831,000)	(78,000)
Amortization of net loss	2,412,000	1,969,000	7,235,000	5,907,000
Net periodic benefit cost	3,247,000	3,905,000	9,739,000	11,716,000
SPPC [Member] | Other Postretirement Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	271,000	245,000	815,000	733,000
Interest cost	1,457,000	1,547,000	4,372,000	4,640,000
Expected return on plan assets	(976,000)	(961,000)	(2,929,000)	(2,883,000)
Amortization of prior service cost	(1,219,000)	(1,213,000)	(3,658,000)	(3,638,000)
Amortization of net loss	773,000	777,000	2,319,000	2,332,000
Net periodic benefit cost	306,000	395,000	919,000	1,184,000
Pension Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	4,607,000	4,727,000	13,820,000	14,182,000
Interest cost	10,169,000	10,718,000	30,507,000	32,154,000
Expected return on plan assets	(12,192,000)	(11,069,000)	(36,575,000)	(33,206,000)
Amortization of prior service cost	(738,000)	(448,000)	(2,214,000)	(1,345,000)
Amortization of net loss	4,155,000	3,777,000	12,465,000	11,329,000
Net periodic benefit cost	6,001,000	7,705,000	18,003,000	23,114,000
Other Postretirement Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	653,000	617,000	1,958,000	1,850,000
Interest cost	2,090,000	2,184,000	6,270,000	6,551,000
Expected return on plan assets	(1,596,000)	(1,556,000)	(4,789,000)	(4,667,000)
Amortization of prior service cost	(987,000)	(972,000)	(2,961,000)	(2,917,000)
Amortization of net loss	1,083,000	1,085,000	3,250,000	3,256,000
Net periodic benefit cost	$ 1,243,000	$ 1,358,000	$ 3,728,000	$ 4,073,000