Commitments and Contingencies (Details) (USD $)		12 Months Ended									105 Months Ended	129 Months Ended		12 Months Ended
	Dec. 31, 2011	Dec. 31, 2011 Capital Expenditures [Member]	Dec. 31, 2011 Manufactured Gas Plant Sites [Member]	Dec. 31, 2011 Coal combustion product landfill sites [Member]	Dec. 31, 2010 Coal combustion product landfill sites [Member]	Dec. 31, 2009 Coal combustion product landfill sites [Member]	Dec. 31, 2011 Bechtel, contractor of the Oak Creek expansion [Member]	Dec. 31, 2011 Bechtel's first claim [Member]	Dec. 31, 2011 Bechtel's second claim [Member]	Dec. 31, 2011 EPA - Consent Decree [Member]	Dec. 31, 2011 EPA - Consent Decree [Member]	Dec. 31, 2013 EPA - Consent Decree [Member]	Dec. 08, 2011 Valley Power Plant [Member] T	Dec. 31, 2011 Cash Balance Pension Plan [Member]
Commitment and Contingencies (Textuals) [Abstract]
Estimated total capital expenditures commitment amount		$ 740,200,000
Reserves related to future remediation costs			37,500,000
Expense related to environmental remediation				200,000	400,000	300,000					1,000,000,000
Reserves related to landfill sites				0
Loss Contingency, Damages Sought, Value							517,500,000	445,500,000	72,000,000
Payments for legal settlements									67,000,000
Loss contingency, settlement agreement, terms							The remaining milestone payments are tied to final acceptance of the units. In addition, Bechtel received 120 days of schedule relief for OC 1 and 60 days for OC 2. Therefore, the guaranteed in-service date of September 29, 2009 for OC 1 was extended to January 27, 2010, and the guaranteed in-service date of September 29, 2010 for OC 2 was extended to November 28, 2010. Bechtel subsequently received an additional 21 days of schedule relief for OC 2 as part of a change order signed concurrent with the turnover of OC 2. Therefore, the total schedule relief granted to Bechtel was 120 days for OC 1 and 81 days for OC 2.
Future minimum payments for the next five years and thereafter for our operating lease contracts
2012	16,300,000
2013	6,500,000
2014	3,900,000
2015	4,000,000
2016	3,700,000
Thereafter	29,000,000
Total	63,400,000
Commitments and Contingencies (Textuals) [Abstract]
Estimated future detailed site investigation and remediation costs, minimum			21,000,000
Estimated Future detailed site investigation and remediation costs, maximum			65,000,000
Period for future costs of detailed site investigation and remediation costs			over the next ten years
Expense related to environmental remediation				200,000	400,000	300,000					1,000,000,000
Number of coal units retired										4	4
Estimated Future Environmental Remediation Expense												$ 1,100,000,000
Responsibility for amounts paid under the settlement agreement in percentage							85.00%
Period to recover remediation cost of manufactured gas plant sites			5
Period of Total cost related to installation of air quality controls and retirement of coal units under consent decree										10
Loss Contingency, Settlement Agreement, Consideration														In November 2011, we entered into a settlement agreement with the plaintiffs for $45.0 million, and the court promptly issued an order preliminarily approving the settlement. As part of the settlement agreement, we agreed to class certification for all similarly situated plaintiffs.
Loss Contingency Range Of Possible Loss, Per Share, Maximum														$ 0.04
Target Annual Sulfur Dioxide Emissions Threshold													4,500
Target Reduction in Sulfur Dioxide Emissions, As a Percent													65.00%