CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
OTHER INCOME (EXPENSE):
AFUDC-debt	$ 1,908	$ 2,835	$ 3,503	$ 9,045
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest [Abstract]
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Tax	(83)	(394)	(172)	(1,009)
Market value of risk management assets and liabilities	123	0	264	0
NPC [Member]
OTHER INCOME (EXPENSE):
AFUDC-debt	1,314	2,330	2,493	8,120
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest [Abstract]
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Tax	(38)	(108)	(70)	(513)
SPPC [Member]
OTHER INCOME (EXPENSE):
AFUDC-debt	594	505	1,010	925
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest [Abstract]
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Tax	$ (23)	$ (210)	$ (46)	$ (1,051)