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Summary of Significant Accounting Policies - Utility Capital Assets (Details)
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Public Utility, Property, Plant and Equipment [Line Items]    
Utility capital assets, composite depreciation rate 2.80% 3.10%
Utility capital assets, generation remaining useful life 26 years 27 years
Utility capital assets, other remaining useful life 14 years 8 years
Minimum    
Public Utility, Property, Plant and Equipment [Line Items]    
Utility capital assets, straight line depreciation rate 0.90% 1.30%
Utility capital assets, generation useful life 5 years 5 years
Utility capital assets, other useful life 3 years 3 years
Maximum    
Public Utility, Property, Plant and Equipment [Line Items]    
Utility capital assets, straight line depreciation rate 34.60% 43.20%
Utility capital assets, generation useful life 85 years 85 years
Utility capital assets, other useful life 70 years 70 years
Electric Distribution    
Public Utility, Property, Plant and Equipment [Line Items]    
Utility capital assets, distribution remaining useful life 32 years 30 years
Electric Distribution | Minimum    
Public Utility, Property, Plant and Equipment [Line Items]    
Utility capital assets, distribution useful life 5 years 5 years
Electric Distribution | Maximum    
Public Utility, Property, Plant and Equipment [Line Items]    
Utility capital assets, distribution useful life 80 years 80 years
Gas Distribution    
Public Utility, Property, Plant and Equipment [Line Items]    
Utility capital assets, distribution remaining useful life 33 years 33 years
Gas Distribution | Minimum    
Public Utility, Property, Plant and Equipment [Line Items]    
Utility capital assets, distribution useful life 7 years 4 years
Gas Distribution | Maximum    
Public Utility, Property, Plant and Equipment [Line Items]    
Utility capital assets, distribution useful life 95 years 95 years
Electric Transmission    
Public Utility, Property, Plant and Equipment [Line Items]    
Utility capital assets, transmission remaining useful life 41 years 29 years
Electric Transmission | Minimum    
Public Utility, Property, Plant and Equipment [Line Items]    
Utility capital assets, transmission useful life 20 years 20 years
Electric Transmission | Maximum    
Public Utility, Property, Plant and Equipment [Line Items]    
Utility capital assets, transmission useful life 80 years 80 years
Gas Transmission    
Public Utility, Property, Plant and Equipment [Line Items]    
Utility capital assets, transmission remaining useful life 34 years 36 years
Gas Transmission | Minimum    
Public Utility, Property, Plant and Equipment [Line Items]    
Utility capital assets, transmission useful life 7 years 7 years
Gas Transmission | Maximum    
Public Utility, Property, Plant and Equipment [Line Items]    
Utility capital assets, transmission useful life 80 years 80 years