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Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Cash, Cash Equivalents and Restricted Cash:
Cash and cash equivalents	$ 229.4	$ 1,059.9
Restricted cash	105.0	98.2
Total cash, cash equivalents and restricted cash shown in the Unaudited Condensed Statements of Consolidated Cash Flows	334.4	$ 1,158.1	$ 2,115.6	$ 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	0.2
Charged to other accounts	1.1
Deductions	(0.5)
Balance at end of period	$ 47.3