Energy Purchase Contracts (PUD Contracts Expenses) (Details) $ in Millions	12 Months Ended
Dec. 31, 2016 USD ($)
PUD Contracts Expenses [Abstract]
Long-term Contract for Purchase of Electric Power, Amount of Long-term Debt or Lease Obligation Outstanding	$ 65.2

Under long-term contracts with public utility districts, cooperatives or other organizations, a utility company receives a portion of the output of a production plant constructed and financed by the district or cooperative. The utility has only a nominal or no investment at all in the plant but pays a proportionate part of the plant's costs, including debt service.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.D)
-URI http://asc.fasb.org/extlink&oid=6499423&loc=d3e659339-123030

Name:	us-gaap_LongTermContractForPurchaseOfElectricPowerAmountOfLongTermDebtOrLeaseObligationOutstanding
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration