RETIREMENT PLAN AND POST-RETIREMENT BENEFITS (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
NPC [Member] | Pension Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	$ 2,445	$ 2,392	$ 4,891	$ 4,784
Interest cost	4,880	5,023	9,760	10,046
Expected return on plan assets	(6,169)	(5,362)	(12,339)	(10,724)
Amortization of prior service cost	(470)	(433)	(939)	(866)
Amortization of net loss	1,690	1,764	3,379	3,528
Net periodic benefit cost	2,376	3,384	4,752	6,768
Average percentage of net periodic costs capitalized (in hundredths)			37.32%	35.71%
NPC [Member] | Other Postretirement Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	363	353	727	707
Interest cost	615	619	1,230	1,237
Expected return on plan assets	(590)	(567)	(1,180)	(1,135)
Amortization of prior service cost	229	236	458	473
Amortization of net loss	302	300	604	599
Net periodic benefit cost	919	941	1,839	1,881
SPPC [Member] | Pension Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	1,840	2,004	3,680	4,008
Interest cost	5,013	5,389	10,025	10,779
Expected return on plan assets	(5,741)	(5,431)	(11,482)	(10,862)
Amortization of prior service cost	(277)	(26)	(554)	(52)
Amortization of net loss	2,412	1,969	4,824	3,938
Net periodic benefit cost	3,247	3,905	6,493	7,811
Average percentage of net periodic costs capitalized (in hundredths)			30.22%	34.55%
SPPC [Member] | Other Postretirement Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	271	245	543	489
Interest cost	1,457	1,547	2,915	3,094
Expected return on plan assets	(976)	(961)	(1,952)	(1,922)
Amortization of prior service cost	(1,219)	(1,213)	(2,439)	(2,426)
Amortization of net loss	773	777	1,546	1,554
Net periodic benefit cost	306	395	613	789
Pension Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	4,607	4,727	9,213	9,455
Interest cost	10,169	10,718	20,338	21,436
Expected return on plan assets	(12,192)	(11,069)	(24,383)	(22,138)
Amortization of prior service cost	(738)	(448)	(1,476)	(897)
Amortization of net loss	4,155	3,777	8,310	7,553
Net periodic benefit cost	6,001	7,705	12,002	15,409
Average percentage of net periodic costs capitalized (in hundredths)			32.77%	33.58%
Other Postretirement Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	653	617	1,306	1,233
Interest cost	2,090	2,184	4,180	4,368
Expected return on plan assets	(1,596)	(1,556)	(3,193)	(3,111)
Amortization of prior service cost	(987)	(972)	(1,973)	(1,945)
Amortization of net loss	1,083	1,085	2,166	2,171
Net periodic benefit cost	$ 1,243	$ 1,358	$ 2,486	$ 2,716