Valuation and Qualifying Accounts (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Advance royalty recoupment reserve [Member]
Valuation and Qualifying Accounts
Balance at Beginning of Period		$ 7.6		$ 9.7		$ 15.3
Charged to Costs and Expenses		0.0		(0.2)		0.1
Deductions	[1],[2]	(0.9)		(1.9)		(5.7)
Other		1.6	[3]	0.0		0.0	[4]
Balance at End of Period		8.3		7.6		9.7
Reserve for materials and supplies [Member]
Valuation and Qualifying Accounts
Balance at Beginning of Period		4.6		7.4		16.0
Charged to Costs and Expenses		0.4		(0.1)		1.7
Deductions		(0.3)		(2.7)	[2]	(10.3)	[2]
Other		0.0		0.0		0.0
Balance at End of Period		4.7		4.6		7.4
Allowance for doubtful accounts [Member]
Valuation and Qualifying Accounts
Balance at Beginning of Period		5.8		7.4		13.7
Charged to Costs and Expenses		8.0		1.5		4.3
Deductions		(7.2)		(1.4)	[2]	(10.1)	[2]
Other		0.0	[3]	(1.7)	[5]	(0.5)	[5]
Balance at End of Period		6.6		5.8		7.4
Tax valuation allowances [Member]
Valuation and Qualifying Accounts
Balance at Beginning of Period		1,169.0		1,634.1		1,481.8
Charged to Costs and Expenses		462.0		569.4		(29.4)
Deductions	[2]	0.0		0.0		0.0	[6]
Other		(183.7)	[7]	(1,034.5)	[4]	181.7	[6]
Balance at End of Period		$ 1,447.3		$ 1,169.0		$ 1,634.1

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

[2] Reserves utilized, unless otherwise indicated.

[3] Balances transferred from other accounts.

[4] Includes the write-off of valuation allowance against deferred tax assets related to the Australian Minerals and Resource Rent Tax (MRRT) due to the repeal of that legislation in 2014, along with an increase in valuation allowance during the period reflected directly in "Accumulated other comprehensive loss" and the impact of the 2014 decrease in Australian dollar exchange rates.

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

[6] Related to the MRRT, as offset by the impact of the 2013 decrease in Australian dollar exchange rates.

[7] Includes a decrease in valuation allowance during the period reflected directly in "Accumulated other comprehensive loss" and the impact of the 2015 decrease in Australian dollar exchange rates.