Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash, Cash Equivalents and Restricted Cash:
Cash and cash equivalents		$ 102.1	$ 5.1
Restricted cash		113.5	65.2
Total cash, cash equivalents and restricted cash shown in the Unaudited Condensed Statements of Consolidated Cash Flows	$ 107.1	215.6	$ 70.3	$ 417.6
Operating lease obligations [Abstract]
Minimum operating lease payment obligations		431.9
Scenario, Previously Reported [Member]
Cash, Cash Equivalents and Restricted Cash:
Decrease in restricted cash	$ 309.8
Initial Margin Requirement [Member]
Cash, Cash Equivalents and Restricted Cash:
Restricted cash		28.2
Variation Margin Requirement [Member]
Cash, Cash Equivalents and Restricted Cash:
Restricted cash		$ 85.3