Acquisition of Macarthur Coal Limited (Details) (USD $)	3 Months Ended						2 Months Ended	3 Months Ended		12 Months Ended							2 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Feb. 23, 2012	Feb. 03, 2012	Feb. 03, 2012 MCGH Loan Facility [Member]	Dec. 21, 2011 ArcelorMittal Mining Australasia B.V. [Member]	Dec. 20, 2011 Macarthur [Member]	Mar. 31, 2012 Macarthur [Member]	Mar. 31, 2011 Macarthur [Member]	Dec. 31, 2014 Macarthur [Member]	Dec. 31, 2013 Macarthur [Member]	Dec. 31, 2012 Macarthur [Member]	Dec. 31, 2011 Macarthur [Member]	Dec. 21, 2011 Macarthur [Member]	Oct. 26, 2011 Macarthur [Member]	Oct. 23, 2011 Macarthur [Member]	Dec. 20, 2011 Macarthur [Member] PEAMCoal [Member]	Oct. 23, 2011 Coppabella, Moorvale, and Codrilla Mines [Member]
Business Acquisition [Line Items]
business combinations, recognized identifiable assets acquired and liabilities assumed, noncontrolling interest			$ 53,400,000
Business Combinations, Provisional information, Initial accounting incomplete, adjustment, operating costs and expenses								10,100,000
Business Combination, provisional information, initial accounting incomplete, adjustment, depreciation, depletion and amortization								10,600,000
Take or Pay Liability								30,200,000							33,300,000
Expenses Related To Take Or Pay Arrangements								3,100,000
Take Or Pay Commitments Due Within The Next Year										1,700,000	17,100,000	11,400,000
Revenue, Net								145,200,000
Business Combination, Acquired Receivables, Fair Value								114,300,000					106,600,000
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Accounts Receivable, Net								7,700,000
Business Acquisition, Pro Forma Revenue									1,807,800,000
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%
Business Acquisition, Other Acquisitions						2,000,000,000
Payments to Acquire Businesses, Net of Cash Acquired							2,800,000,000										4,800,000,000
Cash Acquired from Acquisition																	261,200,000
Percentage of Undivided Interests Acquired																		73.30%
Companys purchase price share of acquisition								2,756,700,000					2,756,700,000
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Total								0
Other Current Assets								133,600,000					137,500,000
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Current Assets								(3,900,000)
Investments and Other Assets					384,600,000			1,196,400,000					1,275,100,000
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Investments and Other Assets								(78,700,000)
Equity Interest, Converted From Receivable, Per Court Ruling				90.00%
Business Acquisition, Pro Forma Net Income (Loss)									113,100,000
Business Acquisition, Pro Forma Earnings Per Share, Basic									$ 0.35
Business Acquisition, Pro Forma Earnings Per Share, Diluted									$ 0.35
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Inventory								57,000,000					67,100,000
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Inventory								(10,100,000)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment and Mine Development, Net								3,569,500,000					3,457,000,000
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Property, Plant, Equipment and Mine Development, Net								112,500,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Maturities of Long-term Debt								(11,000,000)					(11,000,000)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Current Maturities of Long Term Debt								0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Accounts Payable and Accrued Expenses								(158,800,000)					(133,800,000)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Accounts Payable and Accrued Expenses								25,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Long-term Debt, Less Current Maturities								(59,200,000)					(59,200,000)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Long Term Debt, Less Current Maturities								0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Asset Retirement Obligations								(39,300,000)					(39,300,000)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Asset Retirement Obligations								0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Liabilities								(33,900,000)					(31,400,000)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Current Liabilities								2,500,000
Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Fair Value								(2,011,900,000)					(2,011,900,000)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Noncontrolling Interests								0
Net Income (Loss) Attributable to Parent	$ 172,700,000	$ 176,500,000						$ 6,500,000