Operations and Significant Accounting Policies - Revision of Prior Balance Sheet (Details) $ in Millions	12 Months Ended
Dec. 31, 2016 USD ($)
Regulatory Assets [Member]
Revision of Prior Balance Sheet [Line Items]
Immaterial Revision	$ 29.5
Deferred Income Taxes [Member]
Revision of Prior Balance Sheet [Line Items]
Immaterial Revision	$ 29.5

Amount of each individual error being corrected in the correcting adjustment.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SAB Topic 1.N)
-URI http://asc.fasb.org/extlink&oid=26874127&loc=d3e30840-122693

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration