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Investments Equity Method Investments (Details) - USD ($) $ in Millions	3 Months Ended									9 Months Ended	12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Apr. 01, 2017	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	$ 56.9				$ 45.9						$ 56.9	$ 45.9
Income from equity affiliates	(10.9)	$ (20.7)	$ (9.7)			$ (17.2)	$ (25.2)	$ (22.0)	$ (15.0)	$ (49.0)	(3.4)	(68.1)
Depreciation, Depletion and Amortization									119.9	521.6	601.0	679.0
Interest Expense	36.8	$ 35.4	$ 36.0	$ 35.8	36.5	$ 38.2	$ 38.3	$ 36.3	32.9	119.7	144.0	149.3
Income Tax Expense (Benefit)									(263.8)	(161.0)	46.0	18.4
Equity method investment and financing receivables related to Middlemount
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	56.3				45.0						56.3	45.0
Income from equity affiliates									(17.4)	(48.6)	(9.0)	(69.3)
Other equity method investments
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	$ 0.6				$ 0.9						0.6	0.9
Income from equity affiliates									$ 2.4	$ (0.4)	$ 5.6	$ 1.2