CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
OTHER INCOME (EXPENSE):
AFUDC-debt	$ (8,718)	$ (23,355)	$ (20,229)
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest [Abstract]
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Tax	202	217	72
Market value of risk management assets and liabilities	369
NPC [Member]
OTHER INCOME (EXPENSE):
AFUDC-debt	(6,770)	(21,443)	(17,184)
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest [Abstract]
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Tax	129	205	(96)
SPPC [Member]
OTHER INCOME (EXPENSE):
AFUDC-debt	(1,948)	(1,912)	(3,044)
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest [Abstract]
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Tax	$ (645)	$ 116	$ 48