Investment in Unconsolidated Affiliate Summarized Income Statement of Equity Method Investment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Summarized Financial Information, Operating Income	$ 465.0	$ 569.0	$ 648.0
Operating revenues	2,122.3	2,231.6	2,270.3
Net income (loss)	(173.7)	433.6	425.5
Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Operating revenues	2,463.0	2,960.0	3,431.0
Cost of Revenue	965.0	1,279.0	1,819.0
Net income (loss)	$ 52.0	$ 360.0	$ 485.3