Commitments and Contingencies (Details 2) In Millions, unless otherwise specified	1 Months Ended	9 Months Ended			9 Months Ended							9 Months Ended		12 Months Ended			48 Months Ended
	Jan. 31, 2014 USD ($)	Sep. 30, 2014 USD ($) T	Mar. 18, 2014 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2014 Prarie State Energy Campus [Member] mW	Sep. 30, 2014 Monto Coal Pty Limited [Member] AUD	Sep. 30, 2014 Wilkie Creek [Member] AUD	Sep. 30, 2013 Gulf Power Company Litigation [Member] USD ($)	Jan. 19, 2012 Gulf Power Company Litigation [Member] USD ($)	Sep. 30, 2011 Gulf Power Company Litigation [Member] USD ($)	Dec. 31, 2007 Gulf Power Company Litigation [Member] T	Sep. 30, 2014 Oklahoma Lead Litigation [Member] USD ($) sites	Dec. 31, 2013 Oklahoma Lead Litigation [Member] USD ($)	Dec. 31, 2017 Scenario, Forecast [Member] USD ($)	Dec. 31, 2016 Scenario, Forecast [Member] USD ($)	Dec. 31, 2015 Scenario, Forecast [Member] USD ($)	Dec. 31, 2017 Scenario, Forecast [Member] USD ($)
Loss Contingency [Abstract]
Ownership percentage in subsidiaries						51.00%
Damages sought						1,193.2	113.1
Class B shareholders claim for dividends payable		42.5
Interest on Class B shareholders claim		10.0
Damages awarded by the arbitrator to Eagle on some of Eagle's claims			23.4
Estimated loss associated with Eagle Mining arbitration				7.8
Additional charge recorded as a result of the damages awarded to Eagle in arbitration		15.6
Tons shipped subject to contract pricing arbitration		3,600,000
Amount disputed pursuant to a contract pricing arbitration proceeding		35.00%
Damages sought for alleged past and future tonnage shortfalls											5,000,000
Litigation settlement damages awarded to plaintiff										20.6
Prejudgement interest									6.9
Charge recorded for settlement damages								20.6
Charge recorded for prejudgement interest								6.9
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)												5
Number of additional national priority list sites in which CERCLA claims were asserted												13
Total number of national priority list sites												18
Reduced number of national priority list sites due to completion of work, transfer or regulatory inactivity												8
Undiscounted environmental clean-up liabilities, total												70.3	63.8
Undiscounted environmental clean-up liabilities, current												29.6	18.3
Funding of the newly established VEBA																	310
Initial payment based on the construct of the negotiated settlement	90
Payment to Patriot based on the construct of the negotiated settlement	70
Payment to the VEBA based on the construct of the negotiated settlement	$ 20													$ 70	$ 75	$ 75
Capacity of new electricity generation project					1,600
Undivided interest percent of new electricity generation project					5.06%