Commitments and Contingencies - Contingencies (Details) AUD in Millions, $ in Millions					9 Months Ended
	Nov. 17, 2016 USD ($)	Oct. 13, 2016 USD ($)	Oct. 11, 2016 USD ($)	Aug. 19, 2016 USD ($)	Sep. 30, 2017 AUD defendant site	Sep. 30, 2017 USD ($)	Mar. 16, 2017 USD ($)	Dec. 31, 2016 USD ($)
Monto Coal Pty Limited
Loss Contingency [Abstract]
Ownership Percentage In Subsidiaries						51.00%
Number of defendants | defendant					3
Gold Fields
Loss Contingency [Abstract]
Number of national priority list sites based on the Superfund Amendments and Reauthorization Act of 1986 at which Gold Fields or the former affiliate, has been named a potentially responsible party (PRP) | site					4
Undiscounted environmental clean-up liabilities, total								$ 62.8
Minimum | Monto Coal Pty Limited
Loss Contingency [Abstract]
Damages sought, value | AUD					AUD 15.6
Maximum | Monto Coal Pty Limited
Loss Contingency [Abstract]
Damages sought, value | AUD					AUD 1,800.0
Berenergy Corporation vs. the Company | Berenergy
Loss Contingency [Abstract]
Litigation settlement damages awarded to plaintiff		$ 0.9
Loss contingency accrual						$ 13.1
Bankruptcy unliquidated, general unsecured claim | Blue Tee
Loss Contingency [Abstract]
Bankruptcy claims, amount of claims filed				$ 65.6
Bankruptcy additional unliquidated, in an unknown amount | Blue Tee
Loss Contingency [Abstract]
Bankruptcy claims, amount of claims filed				150.0
Bankruptcy further contingent claims, at known and unknown sites | Blue Tee
Loss Contingency [Abstract]
Bankruptcy claims, amount of claims filed				$ 500.0
Bankruptcy unliquidated, secured and general unsecured | Gold Fields
Loss Contingency [Abstract]
Bankruptcy claims, amount of claims filed	$ 1,200.0		$ 2,700.0
Debtors vs. Blue Tee
Loss Contingency [Abstract]
Bankruptcy claims, amount of federal tax refunds to be foregone to settle claims							$ 11.0
Bankruptcy claims, amount of insurance settlement proceeds to be transferred to settle claims							12.0
Bankruptcy claims, amount to be paid directly to settle claims							$ 20.0