Summary of Significant Accounting Policies accounting policies (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Public Utilities, Inventory [Line Items]
Public Utilities, Inventory	$ 34.3	$ 33.9
Allowance for Doubtful Accounts Receivable, Current	5.5	5.6
Public Utilities, Inventory, Fuel [Member]
Public Utilities, Inventory [Line Items]
Public Utilities, Inventory	8.5	8.2
Supplies [Member]
Public Utilities, Inventory [Line Items]
Public Utilities, Inventory	$ 25.8	$ 25.7

A valuation allowance for trade and other receivables due to an Entity within one year (or the normal operating cycle, whichever is longer) that are expected to be uncollectible.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.4)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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