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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

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