Commitments, Guarantees and Contingencies - Environmental Matters (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) MW
Boswell Unit 4 [Member]
Environmental Matters [Line Items]
Total Project Costs	$ 220
Minimum [Member] | Coal Combustion Residuals [Member]
Environmental Matters [Line Items]
Estimated Costs of Compliance	80
Maximum [Member] | Clean Water Act - Aquatic Organisms [Member]
Environmental Matters [Line Items]
Estimated Costs of Compliance	15
Maximum [Member] | Coal Combustion Residuals [Member]
Environmental Matters [Line Items]
Estimated Costs of Compliance	$ 100
NOV Consent Decree [Member]
Environmental Matters [Line Items]
Additional Wind Energy 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