Income Taxes (Details) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Non-Current Regulatory Liabilities	$ 1,296.1	$ 1,283.4
Interim Rate Revenue Reserved	32.8
Income taxes refundable to customers, net [Member]
Non-Current Regulatory Liabilities	1,028.0
Income taxes recoverable from customers, net
Non-Current Regulatory Liabilities	$ 944.2	$ 955.5

The amount for the individual regulatory noncurrent liability as itemized in a table of regulatory noncurrent liabilities as of the end of the period.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.24)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_RegulatoryLiabilityNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant