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Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Cash, Cash Equivalents and Restricted Cash:
Cash and cash equivalents	$ 404.5	$ 1,059.9
Restricted cash	206.5	98.2
Total cash, cash equivalents and restricted cash shown in the Unaudited Condensed Statements of Consolidated Cash Flows	611.0	$ 1,158.1	$ 1,436.6	$ 410.0
Allowance for Credit Losses [Member]
Movement in valuation allowances and reserves [Roll Forward]
Balance at beginning of period	46.5
Charged to costs and expenses	2.3
Charged to other accounts	4.4
Deductions	(3.2)
Balance at end of period	$ 50.0