Operations and Significant Accounting Policies - Revenue - Nature of Revenue Streams (Details)	9 Months Ended
	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