Income Taxes Additional Information (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Deferred Tax Liability Not Recognized [Line Items]
Income Tax Reconciliation, Nondeductible Expense, Depletion	$ 111,104	$ 53,932
Uncertain tax positions	22,770	25,570
Uncertain tax positions that, if recognized, would impact effective tax rate	2,071	3,891
Accrued interest liability relating to uncertain tax positions	5,851	7,095
Interest expense included in accrued interest liability	1,020	1,722
Effective Income Tax Rate Reconciliation, Other Adjustments		$ 5,875