Carrying value as of the balance sheet obligations incurred and payable, pertaining to costs that are statuary in nature, are incurred on contractual obligations, or accumulate over time and for which invoices have not yet been received or will not be rendered, and of current obligations not separately disclosed in the balance sheet due to materiality considerations, which are expected to be paid within one year.

No definition available.

Supplemental Consolidating Financial Information (Schedule of Consolidating Balance Sheets) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 01, 2012	Dec. 31, 2011
Assets
Cash and cash equivalents	$ 734,231	$ 911,099	$ 784,622		$ 138,149
Short term investments	248,954	248,414
Receivables, Net, Current	232,017us-gaap_ReceivablesNetCurrent	229,573us-gaap_ReceivablesNetCurrent
Inventory, Net	190,253us-gaap_InventoryNet	264,161us-gaap_InventoryNet
Prepaid Expense and Other Assets, Current	137,296	128,824
Assets, Current	1,542,751us-gaap_AssetsCurrent	1,782,071us-gaap_AssetsCurrent
Property, Plant and Equipment, Net	6,453,458us-gaap_PropertyPlantAndEquipmentNet	6,734,286us-gaap_PropertyPlantAndEquipmentNet
Prepaid Expense and Other Assets, Noncurrent		473,836
Assets, Noncurrent	433,514us-gaap_AssetsNoncurrent	473,836us-gaap_AssetsNoncurrent
Total assets	8,429,723us-gaap_Assets	8,990,193us-gaap_Assets	10,006,777us-gaap_Assets
Liabilities and Stockholders’ Equity
Accounts payable	180,113us-gaap_AccountsPayableCurrent	176,142us-gaap_AccountsPayableCurrent
Accrued Expenses Derivatives And Other Liabilities Current		278,587
Accrued expenses and other current liabilities	302,396	278,587
Debt, Current	36,885us-gaap_DebtCurrent	33,493us-gaap_DebtCurrent
Liabilities, Current	519,394us-gaap_LiabilitiesCurrent	488,222us-gaap_LiabilitiesCurrent
Long-term Debt, Excluding Current Maturities	5,123,485us-gaap_LongTermDebtNoncurrent	5,118,002us-gaap_LongTermDebtNoncurrent
Mine Reclamation and Closing Liability, Noncurrent	398,896	402,713
Defined Benefit Pension Plan, Liabilities, Noncurrent	16,260	7,111
Other Postretirement Defined Benefit Plan, Liabilities, Noncurrent	32,668	39,255
Accrued workers’ compensation	94,291	78,062
Deferred income taxes	422,809	413,546
Other noncurrent liabilities	153,766us-gaap_OtherLiabilitiesNoncurrent	190,033us-gaap_OtherLiabilitiesNoncurrent
Liabilities	6,761,569us-gaap_Liabilities	6,736,944us-gaap_Liabilities
Stockholders' Equity Attributable to Parent	1,668,154us-gaap_StockholdersEquity	2,253,249us-gaap_StockholdersEquity	2,854,567us-gaap_StockholdersEquity	3,578,040us-gaap_StockholdersEquity
Liabilities and Equity	8,429,723	8,990,193
Parent/Issuer [Member]
Assets
Cash and cash equivalents	572,185	799,333	671,313		61,375
Short term investments	248,954	248,414
Receivables, Net, Current	9,656	14,177
Inventory, Net	0
Prepaid Expense and Other Assets, Current	89,211	84,401
Assets, Current	920,006	1,146,325
Property, Plant and Equipment, Net	10,470	24,851
Investment in subsidiaries	7,464,221	7,741,589
Due from Affiliate, Noncurrent	0
Notes Receivable, Related Parties	675,000	675,000
Prepaid Expense and Other Assets, Noncurrent	131,884	162,287
Assets, Noncurrent		8,578,876
Total assets	9,201,581	9,750,052
Liabilities and Stockholders’ Equity
Accounts payable	23,394	17,781
Accrued Expenses Derivatives And Other Liabilities Current		53,779
Accrued expenses and other current liabilities	85,899
Debt, Current	27,625	28,882
Liabilities, Current	136,918	100,442
Long-term Debt, Excluding Current Maturities	5,084,839	5,099,833
Intercompany payables	1,817,755	1,772,624
Notes Payable, Related Parties	0
Mine Reclamation and Closing Liability, Noncurrent	981	1,095
Defined Benefit Pension Plan, Liabilities, Noncurrent	5,967	7,797
Other Postretirement Defined Benefit Plan, Liabilities, Noncurrent	4,430	12,079
Accrued workers’ compensation	9,172	21,546
Deferred income taxes	422,809	413,546
Other noncurrent liabilities	50,919	67,841
Liabilities	7,533,790	7,496,803
Stockholders' Equity Attributable to Parent	1,667,791	2,253,249
Liabilities and Equity	9,201,581	9,750,052
Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	150,358	100,418	100,468		75,425
Short term investments	0
Receivables, Net, Current	15,933	23,018
Inventory, Net	190,253	264,161
Prepaid Expense and Other Assets, Current	41,455	43,617
Assets, Current	397,999	431,214
Property, Plant and Equipment, Net	6,442,623	6,709,398
Investment in subsidiaries	0
Due from Affiliate, Noncurrent	2,021,110	1,953,719
Notes Receivable, Related Parties	0
Prepaid Expense and Other Assets, Noncurrent	300,058	311,463
Assets, Noncurrent		2,265,182
Total assets	9,161,790	9,405,794
Liabilities and Stockholders’ Equity
Accounts payable	156,664	158,224
Accrued Expenses Derivatives And Other Liabilities Current		228,664
Accrued expenses and other current liabilities	220,017
Debt, Current	9,260	4,611
Liabilities, Current	385,941	391,499
Long-term Debt, Excluding Current Maturities	38,646	18,169
Notes Payable		675,000
Notes Payable, Related Parties	675,000
Mine Reclamation and Closing Liability, Noncurrent	397,915	401,618
Defined Benefit Pension Plan, Liabilities, Noncurrent	10,293	(686)
Other Postretirement Defined Benefit Plan, Liabilities, Noncurrent	28,238	27,176
Accrued workers’ compensation	85,119	56,516
Other noncurrent liabilities	102,461	121,794
Liabilities	1,723,613	1,691,086
Stockholders' Equity Attributable to Parent	7,438,177	7,714,708
Liabilities and Equity	9,161,790	9,405,794
Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	11,688	11,348	12,841		1,349
Short term investments	0
Receivables, Net, Current	211,043	197,015
Inventory, Net	0
Prepaid Expense and Other Assets, Current	6,630	806
Assets, Current	229,361	209,169
Property, Plant and Equipment, Net	2	37
Investment in subsidiaries	0
Due from Affiliate, Noncurrent	0
Notes Receivable, Related Parties	0
Prepaid Expense and Other Assets, Noncurrent	1,572	86
Assets, Noncurrent		86
Total assets	230,935	209,292
Liabilities and Stockholders’ Equity
Accounts payable	55	137
Accrued Expenses Derivatives And Other Liabilities Current		781
Accrued expenses and other current liabilities	1,095
Debt, Current	0
Liabilities, Current	1,150	918
Long-term Debt, Excluding Current Maturities	0
Intercompany payables	203,355	181,095
Notes Payable, Related Parties	0
Mine Reclamation and Closing Liability, Noncurrent	0
Defined Benefit Pension Plan, Liabilities, Noncurrent	0
Other Postretirement Defined Benefit Plan, Liabilities, Noncurrent	0
Accrued workers’ compensation	0
Deferred income taxes	0
Other noncurrent liabilities	386	398
Liabilities	204,891	182,411
Stockholders' Equity Attributable to Parent	26,044	26,881
Liabilities and Equity	230,935	209,292
Eliminations [Member]
Assets
Cash and cash equivalents	0
Short term investments	0
Receivables, Net, Current	(4,615)	(4,637)
Inventory, Net	0
Prepaid Expense and Other Assets, Current	0
Assets, Current	(4,615)	(4,637)
Property, Plant and Equipment, Net	363
Investment in subsidiaries	(7,464,221)	(7,741,589)
Due from Affiliate, Noncurrent	(2,021,110)	(1,953,719)
Notes Receivable, Related Parties	(675,000)	(675,000)
Prepaid Expense and Other Assets, Noncurrent	0
Assets, Noncurrent		(10,370,308)
Total assets	(10,164,583)	(10,374,945)
Liabilities and Stockholders’ Equity
Accounts payable	0
Accrued Expenses Derivatives And Other Liabilities Current		(4,637)
Accrued expenses and other current liabilities	(4,615)
Debt, Current	0
Liabilities, Current	(4,615)	(4,637)
Long-term Debt, Excluding Current Maturities	0
Intercompany payables	(2,021,110)	(1,953,719)
Notes Payable		(675,000)
Notes Payable, Related Parties	(675,000)
Mine Reclamation and Closing Liability, Noncurrent	0
Defined Benefit Pension Plan, Liabilities, Noncurrent	0
Other Postretirement Defined Benefit Plan, Liabilities, Noncurrent	0
Accrued workers’ compensation	0
Deferred income taxes	0
Other noncurrent liabilities	0
Liabilities	(2,700,725)	(2,633,356)
Stockholders' Equity Attributable to Parent	(7,463,858)	(7,741,589)
Liabilities and Equity	$ (10,164,583)	$ (10,374,945)