Accounting Pronouncements Accounting Pronouncements (Details) - Accounting Standards Update 2017-07 $ in Millions	6 Months Ended
Jun. 30, 2017 USD ($)
Operating and Maintenance Expense
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net periodic benefit cost (credit)	$ (1.4)
Other Income
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net periodic benefit cost (credit)	$ 1.4

Amount of net periodic benefit cost (credit) for defined benefit plan.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration