Commitments and Contingencies (Details 2) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended			12 Months Ended				12 Months Ended		12 Months Ended
	Sep. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2014 USD ($) T	Mar. 18, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2013 Gulf Power Company Litigation [Member] USD ($)	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	Dec. 31, 2014 Oklahoma Lead Litigation [Member] USD ($) sites	Dec. 31, 2013 Oklahoma Lead Litigation [Member] USD ($)	Dec. 31, 2014 Monto Coal Pty Limited [Member] AUD	Dec. 31, 2014 Wilkie Creek [Member] AUD	Dec. 31, 2014 Prarie State Energy Campus [Member] mW
Loss Contingency [Abstract]
Ownership Percentage In Subsidiaries												51.00%
Damages sought												1,193.2	113.1
Class B Shareholders Claim for Dividends Payable and Related Interest			50.1
Damages awarded by the arbitrator to Eagle on some of Eagles 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	(10.6)	(15.6)	15.6
Tons Shipped Subject To Contract Pricing Arbitration			4,800,000
Pricing rebate provided in settlement of contract pricing arbitration			68.7
Alleged shortage of past and future coal shipments under a coal supply agreement, tons									5,000,000
Litigation Settlement Damages Awarded to Plaintiff								20.6
Litigation Settlement, 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 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										69.4	63.8
Undiscounted environmental clean-up liabilities, current			$ 19.4		$ 18.3					$ 19.4	$ 18.3
Capacity Of New Electricity Generation Project														1,600
Undivided Interest Percent Of New Electricity Generation Project														5.06%