Revenue - Additional Information (Details) $ in Millions	3 Months Ended
Mar. 31, 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/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 270
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=84173487&loc=SL51747714-108306

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 13
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=82913847&loc=SL49130556-203045

Name:	us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:gYearItemType
Balance Type:	na
Period Type:	duration