CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS' EQUITY (Parenthetical) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
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 Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Tax	129	205	(96)
SPPC [Member]
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Tax	$ (645)	$ 116	$ 48

Tax effects of the increase (decrease) to accumulated comprehensive income during the period related to benefit plans. While for technical reasons this element has no balance attribute, the default assumption is a credit balance consistent with its label.

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Total tax effect of the change in accumulated gains and losses from derivative instruments designated and qualifying as the effective portion of cash flow hedges. The change includes an entity's share of an equity investee's increase or decrease in deferred hedging gains or losses. While for technical reasons this element has no balance attribute, the default assumption is a credit balance consistent with its label.

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