Asset Retirement Obligations (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Apr. 01, 2017	Dec. 31, 2017	Dec. 31, 2018
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Surety bonds and bank guarantees outstanding to secure reclamation obligations or activities		$ 1,136.8	$ 1,317.0
Letters of credit in support of reclamation obligations or activities		188.5	142.3
Successor
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of period			691.1
Liabilities incurred or acquired		0.0	16.3
Liabilities settled or disposed		(65.2)	(57.8)
Accretion expense		32.6	48.5
Revisions to estimates		59.5	52.1
Balance at end of period		691.1
Less: Current portion (included in “Accounts payable and accrued expenses”)		34.1	63.8
Noncurrent obligation (included in “Asset retirement obligations”)		657.0	686.4
Balance at end of period — active locations		612.9	671.8
Balance at end of period — closed or inactive locations		78.2	78.4
Surety bonds and bank guarantees outstanding to secure reclamation obligations or activities			$ 1,589.8
Cash collateral in support of reclamation obligations or activities		$ 205.2
Successor | LOM 3 Years or Less
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Credit adjusted risk free interest rates		7.61%
Successor | LOM Greater than 20 Years
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Credit adjusted risk free interest rates		11.54%
Predecessor
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of period	$ 758.8	$ 664.2
Liabilities incurred or acquired	0.0
Liabilities settled or disposed	(2.7)
Accretion expense	12.5
Revisions to estimates	(104.4)
Balance at end of period	664.2
Less: Current portion (included in “Accounts payable and accrued expenses”)	31.1
Noncurrent obligation (included in “Asset retirement obligations”)	633.1
Balance at end of period — active locations	540.1
Balance at end of period — closed or inactive locations	$ 124.1