Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Cash, Cash Equivalents and Restricted Cash:
Cash and cash equivalents	$ 57.9		$ 5.1
Restricted cash	283.6		65.2
Total cash, cash equivalents and restricted cash shown in the Unaudited Condensed Statements of Consolidated Cash Flows	341.5	$ 64.0	$ 70.3	$ 417.6
Operating lease obligations [Abstract]
Minimum operating lease payment obligations	430.0
Scenario, Previously Reported [Member]
Cash, Cash Equivalents and Restricted Cash:
Decrease in restricted cash		$ 319.1
Initial Margin Requirement [Member]
Cash, Cash Equivalents and Restricted Cash:
Restricted cash	51.4
Variation Margin Requirement [Member]
Cash, Cash Equivalents and Restricted Cash:
Restricted cash	$ 232.2
Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New accounting pronouncement or change in accounting principle, description of financial statement line items	Based on current estimates, we expect that the total of ROU assets we would recognize under ASC 842 will account for less than 1% of total consolidated assets. Likewise, the corresponding lease liabilities would account for less than 1% of total consolidated liabilities.