Revenue - Additional Information (Details) $ in Millions	Jun. 30, 2018 USD ($)
Regulated Operating Revenue [Abstract]
Remaining performance obligations, transaction price	$ 140
Remaining performance obligations, expected year of revenue recognition	2033

Year in which remaining performance obligation is expected to be recognized, in CCYY format.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 13
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130556-203045

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Namespace Prefix:	us-gaap_
Data Type:	us-types:gYearListItemType
Balance Type:	na
Period Type:	instant