Income Taxes (Details Textuals) (USD $) In Millions, unless otherwise specified	12 Months Ended			3 Months Ended				6 Months Ended	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Jun. 30, 2014	Dec. 31, 2015
Income Taxes (Textuals) [Abstract]
Alternative minimum tax credits	$ 290.3			$ 290.3
Foreign tax credits	147.9			147.9
General business credits	89.9			89.9
State net operating loss carryforwards	41.0			41.0
Foreign loss carryforwards included in Company's tax credits and loss carryforwards	1,103.3			1,103.3
Valuation allowance reserve for U.S. capital losses, state NOLs, foreign NOLs and certain foreign deferred tax assets	(1,169.0)	(656.4)		(1,169.0)
Minerals resource rent tax, net of federal tax	16.1	(87.4)	77.2
MRRT market value allowance deferred tax asset	993.2			993.2
Valuation allowance, minerals resource rent tax	0	(977.7)		0
Change in unrecognized tax benefit	99.4
Net unrecognized tax benefits	40.9	140.5		40.9
Accrued interest related to unrecognized tax benefits included in income tax provision	(8.0)	(36.0)	21.2
Accrued interest related to uncertain tax positions	3.4	11.4		3.4
Valuation allowance minerals resource rent tax	977.7			977.7
Undistributed Earnings of Foreign Subsidiaries	0			0
Australia Deferred Tax Assets [Member]
Income Taxes (Textuals) [Abstract]
Valuation Allowance, Deferred Tax Asset, Change in Amount	289.3			90.4	80.6	75.7	42.6
US Deferred Tax Assets [Member]
Income Taxes (Textuals) [Abstract]
Capital loss	51.1			51.1
Valuation Allowance, Deferred Tax Asset, Change in Amount	351.2			280.1
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability	280.1
Other Comprehensive Income (Loss), Tax	71.1
Australian Minerals and Resource Rent Tax [Member]
Income Taxes (Textuals) [Abstract]
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability					70.1			54.0
Scenario, Forecast [Member]
Income Taxes (Textuals) [Abstract]
Tax adjustments, settlements, and unusual provisions									$ 28.6