Financial Instruments and Other Guarantees Textuals (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	Apr. 03, 2017
Guarantee Obligations [Line Items]
Surety Bonds Outstanding	$ 1,609.2										$ 1,609.2
Asset retirement obligations	752.3				$ 750.2					$ 691.1	752.3	$ 750.2
Surety bonds and bank guarantees outstanding to secure reclamation obligations or activities	1,401.7				1,317.0						1,401.7	1,317.0
Letters of Credit Outstanding for Reclamation	106.1										106.1
Letters of credit outstanding, amount	200.5										200.5
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
Provision for North Goonyella equipment loss	58.5			$ 24.7	$ 17.1	$ 49.3			0.0	0.0	83.2	66.4
Obligations
Guarantee Obligations [Line Items]
Restricted cash collateral	0.0									323.1	0.0
Accounts Receivable Securitization Program, April 3, 2020
Guarantee Obligations [Line Items]
Exit facility, maximum borrowing capacity													$ 250.0
Line of Credit Facility, Remaining Borrowing Capacity	45.0										45.0
Secured Debt | Accounts Receivable Securitization Program, April 3, 2020
Guarantee Obligations [Line Items]
Long-term debt	0.0										0.0
Letters of credit outstanding, amount	132.7										132.7
Cash collateral required	0.0										0.0
Interest expense									$ 2.0	$ 5.3	5.5	5.2
Obligations
Guarantee Obligations [Line Items]
Restricted cash collateral	$ 0.0										0.0
Australian Mining
Guarantee Obligations [Line Items]
Provision for North Goonyella equipment loss											$ 0.3	$ 50.4