Valuation and Qualifying Accounts (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Advance royalty recoupment reserve [Member]
Valuation and Qualifying Accounts
Balance at Beginning of Period	$ 21.3		$ 19.9		$ 17.2
Charged to Costs and Expenses	2.9		1.8		1.9
Deductions	(9.0)	[1],[2]	(0.1)	[1]	(0.2)	[1]
Other	0.1	[3]	(0.3)	[3]	1.0	[3]
Balance at End of Period	15.3		21.3		19.9
Reserve for materials and supplies [Member]
Valuation and Qualifying Accounts
Balance at Beginning of Period	6.5		6.2		6.2
Charged to Costs and Expenses	13.7		3.7		0.9
Deductions	(4.2)	[1]	(3.4)	[1]	(0.9)	[1]
Other	0		0		0
Balance at End of Period	16.0		6.5		6.2
Allowance for doubtful accounts [Member]
Valuation and Qualifying Accounts
Balance at Beginning of Period	17.0		30.3		18.3
Charged to Costs and Expenses	(0.5)		(3.7)		26.7
Deductions	(0.7)	[1]	(0.4)	[1]	(6.9)	[1]
Other	(2.1)	[4]	(9.2)	[4]	(7.8)	[4]
Balance at End of Period	13.7		17.0		30.3
Valuation Allowance of Deferred Tax Assets [Member]
Valuation and Qualifying Accounts
Balance at Beginning of Period	79.8		65.0		87.2
Charged to Costs and Expenses	521.5		15.4		(28.7)
Deductions	(77.0)	[1],[5]	0	[1]	0	[1]
Other	957.5	[6]	(0.6)		6.5
Balance at End of Period	$ 1,481.8		$ 79.8		$ 65.0

[1] Reserves utilized, unless otherwise indicated.

[2] Deductions to advance royalty recoupment reserve represents the termination of federal and state leases.

[3] Balances transferred (to) from other accounts or reserves recorded as part of a property transaction or acquisition.

[4] Represents subsequent recovery of receivable amounts previously reserved.

[5] Deductions include write-off of loss carryforwards and reversal of related valuation allowances.

[6] Includes changes to valuation allowances primarily related to deferred tax assets acquired in business combinations and initial deferred tax assets resulting from the newly created minerals resource rent tax in Australia.