Amount of cash and cash equivalents restricted as to withdrawal or usage, classified as current. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Name:	us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Cash, Cash Equivalents and Restricted Cash:
Cash and cash equivalents	$ 99.3	$ 344.8
Restricted cash	8.2	65.3
Total cash, cash equivalents and restricted cash shown in the Unaudited Condensed Statements of Consolidated Cash Flows	107.5	$ 410.1	$ 215.6	$ 70.3
Initial Margin Requirement [Member]
Cash, Cash Equivalents and Restricted Cash:
Restricted cash	34.3
Variation Margin Requirement [Member]
Cash, Cash Equivalents and Restricted Cash:
Restricted cash	$ (26.1)