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Operations and Significant Accounting Policies - Revenue - Nature of Revenue Streams (Details) | 9 Months Ended
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Sep. 30, 2018 Customers MW
Regulated Operations [Member] | Municipal [Member] | Municipal Customers [Member] | Wholesale Electric [Member] | Long-term Contract with Customer [Member] |
Disaggregation of Revenue [Line Items] |
Number of Customers | 16
Length of Notice Required to Terminate Contract | three-year
Regulated Operations [Member] | Industrial [Member] | Industrial Customers [Member] | Retail Electric [Member] |
Disaggregation of Revenue [Line Items] |
Percentage of Total Regulated Utility kWh Sales | 50.00%
Regulated Operations [Member] | Industrial [Member] | Industrial Customers [Member] | Retail Electric [Member] | Long-term Contract with Customer [Member] |
Disaggregation of Revenue [Line Items] |
Length of Notice Required to Terminate Contract | four-year
Number of Large Power Customer Contracts | 9
Large Power Customer Contract Serving Requirement | MW | 10
Regulated Operations [Member] | Other Power Suppliers [Member] | Other Power Supplier Customers [Member] | Sale of Energy under PSA [Member] | Long-term Contract with Customer [Member] |
Disaggregation of Revenue [Line Items] |
Number of Customers | 2
Regulated Operations [Member] | Alternative Programs [Member] |
Disaggregation of Revenue [Line Items] |
Revenue Collection Period Following the Annual Period it is Recognized | 24 months
U.S. Water Services [Member] | Contract [Member] | Chemicals, Consumable Equipment and Services [Member] |
Disaggregation of Revenue [Line Items] |
Length of Notice Required to Terminate Contract | 30-day

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