Commitments, Guarantees and Contingencies - Environmental Matters (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) MW
NOV Consent Decree [Member]
Environmental Matters [Line Items]
Additional Wind Energy 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
Minimum [Member] | Coal Combustion Residuals [Member]
Environmental Matters [Line Items]
Estimated Costs of Compliance	65.0
Maximum [Member] | Clean Water Act - Aquatic Organisms [Member]
Environmental Matters [Line Items]
Estimated Costs of Compliance	15.0
Maximum [Member] | Coal Combustion Residuals [Member]
Environmental Matters [Line Items]
Estimated Costs of Compliance	$ 100.0