Asset Retirement Obligations (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Sep. 30, 2016
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Total asset retirement obligation		$ 758.8	$ 758.8	$ 712.1	$ 752.5
Current portion		41.0	41.0	25.5
Liabilities incurred or acquired			0.0	1.3
Liabilities settled or disposed			(41.5)	(53.3)
Accretion expense			45.7	42.7
Revision to estimates			42.5	(31.1)
Noncurrent obligation		717.8	717.8	686.6
Balance at end of year — active locations		651.1	651.1	656.8
Balance at end of year — closed or inactive locations		107.7	107.7	55.3
Net gain on disposal of assets	$ (13.7)	21.4	(23.2)	$ (45.0)	$ (41.4)
Credit adjusted, risk-free interest rates				50.83%	6.82%
Surety bonds and bank guarantees outstanding to secure reclamation obligations or activities		374.3	374.3	$ 609.4
Amount of reclamation self-bonding in certain states in which the company qualifies		1,094.2	1,094.2	1,430.8
Letters of credit in support of reclamation obligations or activities		80.0	80.0	$ 126.6		$ 15.7
Cash collateral in support of reclamation obligations or activities		$ 233.2	$ 233.2
U.S. Obligations [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Credit adjusted, risk-free interest rates		13.45%	13.45%
Australian Operations [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Credit adjusted, risk-free interest rates		4.92%	4.92%