Summary of Significant Accounting Policies - Recoverable Energy Costs (Details)	12 Months Ended
Dec. 31, 2017
Minimum
Public Utilities, General Disclosures [Line Items]
Recovery or refund of energy costs, deferral period	1 month
Maximum
Public Utilities, General Disclosures [Line Items]
Recovery or refund of energy costs, deferral period	2 months

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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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration