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Commitments, Guarantees and Contingencies - Environmental Matters (Details)
$ in Millions
 6 Months Ended 12 Months Ended
Jun. 30, 2018
USD ($)
MW
Dec. 31, 2017
USD ($)
Environmental Restoration Costs [Member]    
Environmental Matters [Line Items]    
Estimated Costs of Compliance (Accrued) $ 7.0 $ 8.0
NOV Consent Decree [Member]    
Environmental Matters [Line Items]    
Consent Decree Wind Energy Generation Addition (MW) | MW 200  
Ozone NAAQS [Member]    
Environmental Matters [Line Items]    
Estimated Costs of Compliance $ 0.0  
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