Commitments, Guarantees and Contingencies - Environmental Matters (Details) $ in Millions	3 Months Ended
	Mar. 31, 2016 USD ($) MW
Ozone NAAQS [Member]
Environmental Matters [Line Items]
Estimated Environmental Compliance Costs	$ 0.0
Minimum [Member] | Coal Combustion Residuals [Member]
Environmental Matters [Line Items]
Estimated Environmental Compliance Costs	65.0
Maximum [Member] | Clean Water Act - Aquatic Organisms [Member]
Environmental Matters [Line Items]
Estimated Environmental Compliance Costs	15.0
Maximum [Member] | Coal Combustion Residuals [Member]
Environmental Matters [Line Items]
Estimated Environmental Compliance Costs	$ 100.0
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	$ 20.0
NOV Consent Decree [Member] | Maximum [Member]
Environmental Matters [Line Items]
Estimated Capital Expenditures	$ 40.0