Acquisition of Macarthur Coal Limited Details Textuals (Details) (USD $)	3 Months Ended		9 Months Ended						9 Months Ended		2 Months Ended	3 Months Ended		9 Months Ended	12 Months Ended						2 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Additional Paid-in Capital [Member]	Sep. 30, 2012 MCGH [Member]	Feb. 03, 2012 MCGH [Member]	Feb. 03, 2012 MCGH [Member] Term Loan Facility Three [Member]	Sep. 30, 2012 MCGH [Member] Additional Paid-in Capital [Member]	Dec. 21, 2011 ArcelorMittal Mining Australasia B.V. [Member]	Dec. 20, 2011 Macarthur [Member]	Dec. 31, 2012 Macarthur [Member]	Sep. 30, 2012 Macarthur [Member]	Sep. 30, 2012 Macarthur [Member]	Dec. 31, 2015 Macarthur [Member]	Dec. 31, 2014 Macarthur [Member]	Dec. 31, 2013 Macarthur [Member]	Dec. 21, 2011 Macarthur [Member]	Oct. 26, 2011 Macarthur [Member]	Oct. 23, 2011 Macarthur [Member]	Dec. 20, 2011 Macarthur [Member] PEAMCoal [Member]
Business Acquisition [Line Items]
Ownership percentage of the Company in the newly formed company																		100.00%		60.00%
Ownership percentage of the joint owner in the newly formed company										40.00%										40.00%
Payments to Acquire Businesses, Net of Cash Acquired											$ 2,800,000,000										$ 4,800,000,000
Cash Acquired from Acquisition																					261,200,000
Business Acquisition, Other Acquisitions										2,000,000,000
Business Combination, provisional information, initial accounting incomplete, adjustment, depreciation, depletion and amortization													4,700,000	12,200,000
Business Combinations, Provisional information, Initial accounting incomplete, adjustment, operating costs and expenses														10,100,000
Business Acquisition Purchase Price Allocation: Take or Pay Liability													47,400,000	47,400,000					58,500,000
Business Combinations Provisional Information Initial Accounting Incomplete Adjustment Expenses Related to Take or Pay Arrangements													3,800,000	11,100,000
Take Or Pay Commitments Due Within The Next Year												3,700,000
Take or Pay Commitments Due Within Next Two Years																	18,100,000
Take or Pay Commitments Due Within Next Three Year																15,600,000
Take Or Pay Commitments Due Within Next Four Years															10,000,000
Revenues	2,058,800,000	1,980,600,000	6,060,600,000	5,666,100,000									182,500,000	479,500,000
Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest	172,200,000	330,400,000	605,900,000	1,030,100,000									(4,400,000)	98,700,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Investments and Other Assets								384,600,000
Equity Interest, Converted From Receivable, Per Court Ruling							90.00%
business combinations, recognized identifiable assets acquired and liabilities assumed, noncontrolling interest							53,400,000
Acquisition of MCG Coal Holdings Pty Ltd noncontrolling interests			(49,800,000)	0	3,600,000	49,800,000			3,600,000
Net Income (Loss) Attributable to Parent	$ 42,900,000	$ 274,100,000	$ 420,300,000	$ 735,400,000