Acquisition of Macarthur Coal Limited (Details) (Macarthur [Member], USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012	Dec. 31, 2011
Macarthur [Member]
Business Acquisition [Line Items]
Business Acquisition, Purchase Price Allocation, Current Assets, Receivables	$ 115.4	$ 115.4	$ 106.6
Business Acquisition, Purchase Price Allocation, Current Assets, Inventory	56.6	56.6	67.1
Business combination, provisional information, initial accounting incomplete, adjustment, asset retirement obligation	3.7	3.7
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Accounts Receivable, Net		8.8
Business Acquisition, Purchase Price Allocation, Current Assets, Prepaid Expense and Other Assets	133.6	133.6	137.5
Business Acquisition, Purchase Price Allocation, Property, Plant and Equipment	3,715.6	3,715.6	3,457.0
Business Acquisition, Purchase Price Allocation, Other Noncurrent Assets	1,090.8	1,090.8	1,275.1
Business Acquisition, Purchase Price Allocation, Current Liabilities, Long-term Debt	(11.0)	(11.0)	(11.0)
Business Acquisition, Purchase Price Allocation, Current Liabilities	(159.9)	(159.9)	(133.8)
Business Acquisition, Purchase Price Allocation, Noncurrent Liabilities, Long-term Debt	(59.2)	(59.2)	(59.2)
Business Acquisition, Purchase Price, Asset Retirement Obligations	(54.5)	(54.5)	(39.3)
Business Acquisition, Purchase Price Allocation, Other Noncurrent Liabilities	(58.8)	(58.8)	(31.4)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Inventory		(10.5)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Current Assets		(3.9)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Property, Plant, Equipment and Mine Development, Net		258.6
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Investments and Other Assets		(184.3)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Current Maturities of Long Term Debt		0
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Accounts Payable and Accrued Expenses		(26.1)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Long Term Debt, Less Current Maturities		0
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Asset Retirement Obligations		(15.2)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Current Liabilities		(27.4)
Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Fair Value	(2,011.9)	(2,011.9)	(2,011.9)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Noncontrolling Interests		0
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Less Noncontrolling Interest	2,756.7	2,756.7	2,756.7
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Total		$ 0