Equity Method Investments - Schedule of Equity Method Investments (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 Investments	$ 24.6	$ 56.9
Loss (income) from equity affiliates	60.1	(3.4)	$ (68.1)
Equity method investment and financing receivables related to Middlemount
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	24.6	56.3
Loss (income) from equity affiliates	60.1	(9.0)	(69.3)
Other equity method investments
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	0.0	0.6
Loss (income) from equity affiliates	$ 0.0	$ 5.6	$ 1.2