Commitments and Contingent Liabilities, Variable Interest Entities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011 MW	Dec. 31, 2010 MW
Independent Power Producing Entities [Member]
Purchased Power Agreements [Abstract]
Generating capacity (in MW)	3,773	4,101
Low-Income Housing Limited Partnerships [Member]
Amounts reflected in Consolidated Balance Sheets [Abstract]
Current assets	4,034	3,794
Property, plant and equipment, net	90,914	97,602
Other noncurrent assets	8,053	8,236
Total assets	103,001	109,632
Current liabilities	12,297	11,884
Mortgages and other long-term debt payable	48,863	53,195
Other noncurrent liabilities	8,278	8,333
Total liabilities	69,438	73,412