Revenue: Revenue Not in Scope of ASC 606 (Details)	9 Months Ended
Sep. 30, 2018
Sale Agreement Between Colorado IPP and Affiliate Colorado Electric
Disaggregation of Revenue [Line Items]
Long-term Purchase Commitment, Period	20 years

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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Balance Type:	na
Period Type:	duration