Commitments, Guarantees and Contingencies - Environmental Matters (Details) - Jun. 30, 2015 $ in Millions	USD ($) MW
Boswell Unit 4 [Member]
Environmental Matters [Line Items]
Estimated Capital Expenditures	$ 260
Capital Expenditures to Date	184
Maximum [Member] | Clean Water Act - Aquatic Organisms [Member]
Environmental Matters [Line Items]
Estimated Environmental Compliance Costs	15
Maximum [Member] | Coal Combustion Residuals [Member]
Environmental Matters [Line Items]
Estimated Environmental Compliance Costs	$ 100
NOV Consent Decree [Member]
Environmental Matters [Line Items]
Additional Wind Generating Capacity (MW) | MW	200
NOV Consent Decree [Member] | Minimum [Member]
Environmental Matters [Line Items]
Estimated Capital Expenditures	$ 20
NOV Consent Decree [Member] | Maximum [Member]
Environmental Matters [Line Items]
Estimated Capital Expenditures	$ 40