RETIREMENT PLAN AND POST-RETIREMENT BENEFITS, Components of Net Periodic Cost and Expected Benefit Payments (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Average percentage of net periodic costs capitalized [Abstract]
Average percentage of net periodic costs capitalized (in hundredths)	33.20%	32.20%
Plan participants' contributions	$ 900,000
Pension Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	4,406,000	4,607,000
Interest cost	10,228,000	10,169,000
Expected return on plan assets	(12,447,000)	(12,192,000)
Amortization of:
Prior service (credit)/cost	(724,000)	(738,000)
Amortization of (gain)/loss	3,473,000	4,155,000
Total net benefit cost	4,936,000	6,001,000
Other Post-Retirement Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	595,000	653,000
Interest cost	1,905,000	2,090,000
Expected return on plan assets	(1,563,000)	(1,596,000)
Amortization of:
Prior service (credit)/cost	(987,000)	(987,000)
Amortization of (gain)/loss	731,000	1,083,000
Total net benefit cost	681,000	1,243,000
NPC [Member]
Average percentage of net periodic costs capitalized [Abstract]
Average percentage of net periodic costs capitalized (in hundredths)	35.60%	37.20%
NPC [Member] | Pension Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	2,358,000	2,445,000
Interest cost	4,881,000	4,880,000
Expected return on plan assets	(6,237,000)	(6,169,000)
Amortization of:
Prior service (credit)/cost	(456,000)	(470,000)
Amortization of (gain)/loss	1,363,000	1,690,000
Total net benefit cost	1,909,000	2,376,000
NPC [Member] | Other Post-Retirement Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	350,000	363,000
Interest cost	602,000	615,000
Expected return on plan assets	(592,000)	(590,000)
Amortization of:
Prior service (credit)/cost	229,000	229,000
Amortization of (gain)/loss	221,000	302,000
Total net benefit cost	810,000	919,000
SPPC [Member]
Average percentage of net periodic costs capitalized [Abstract]
Average percentage of net periodic costs capitalized (in hundredths)	33.00%	29.30%
SPPC [Member] | Pension Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	1,695,000	1,840,000
Interest cost	5,043,000	5,013,000
Expected return on plan assets	(5,937,000)	(5,741,000)
Amortization of:
Prior service (credit)/cost	(277,000)	(277,000)
Amortization of (gain)/loss	2,026,000	2,412,000
Total net benefit cost	2,550,000	3,247,000
SPPC [Member] | Other Post-Retirement Benefits [Member]
Net periodic benefit cost [Abstract]
Service cost	227,000	271,000
Interest cost	1,283,000	1,457,000
Expected return on plan assets	(941,000)	(976,000)
Amortization of:
Prior service (credit)/cost	(1,220,000)	(1,219,000)
Amortization of (gain)/loss	504,000	773,000
Total net benefit cost	$ (147,000)	306,000