Commitments, Guarantees and Contingencies - Environmental Matters (Details) $ in Millions	6 Months Ended
	Jun. 30, 2017 USD ($) MW
NOV Consent Decree [Member]
Environmental Matters [Line Items]
Additional Wind Generating Capacity (MW) | MW	200
Ozone NAAQS [Member]
Environmental Matters [Line Items]
Estimated Costs of Compliance	$ 0.0
SO2 and NO2 NAAQS [Member]
Environmental Matters [Line Items]
Estimated Costs of Compliance	0.0
Clean Water Act - Aquatic Organisms [Member] | Maximum [Member]
Environmental Matters [Line Items]
Estimated Costs of Compliance	15.0
Coal Combustion Residuals [Member] | Minimum [Member]
Environmental Matters [Line Items]
Estimated Costs of Compliance	65.0
Coal Combustion Residuals [Member] | Maximum [Member]
Environmental Matters [Line Items]
Estimated Costs of Compliance	$ 100.0