Supplemental Guarantor and Non-Guarantor Financial Information (Balance Sheet) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
ASSETS
Cash and cash equivalents	$ 434,706	$ 468,532	$ 404,715	$ 260,818
Trade receivables, net	89,498us-gaap_AccountsReceivableNetCurrent	91,057us-gaap_AccountsReceivableNetCurrent
Other receivables	126,248us-gaap_OtherReceivablesGrossCurrent	127,037us-gaap_OtherReceivablesGrossCurrent
Intercompany receivables	0us-gaap_DueFromRelatedPartiesCurrent	0us-gaap_DueFromRelatedPartiesCurrent
Inventories	172,335us-gaap_InventoryNet	201,598us-gaap_InventoryNet
Deferred income taxes	16,817us-gaap_DeferredTaxAssetsNetCurrent	16,819us-gaap_DeferredTaxAssetsNetCurrent
Prepaid expenses	40,161us-gaap_PrepaidExpenseCurrent	46,190us-gaap_PrepaidExpenseCurrent
Other current assets	8,966us-gaap_OtherAssetsCurrent	9,285us-gaap_OtherAssetsCurrent
Total current assets	888,731us-gaap_AssetsCurrent	960,518us-gaap_AssetsCurrent
Mineral interests, net	2,819,740us-gaap_MineralPropertiesNet	2,836,801us-gaap_MineralPropertiesNet
Property, plant and equipment, net	1,424,047us-gaap_PropertyPlantAndEquipmentNet	1,466,297us-gaap_PropertyPlantAndEquipmentNet
Investment in subsidiaries	0us-gaap_EquityMethodInvestments	0us-gaap_EquityMethodInvestments
Other long-term assets	69,919us-gaap_OtherAssetsNoncurrent	67,748us-gaap_OtherAssetsNoncurrent
Total assets	5,202,437us-gaap_Assets	5,331,364us-gaap_Assets
LIABILITIES AND STOCKHOLDERS' EQUITY
Accounts payable	47,547us-gaap_AccountsPayableCurrent	38,980us-gaap_AccountsPayableCurrent
Accrued expenses	155,645us-gaap_AccruedLiabilitiesCurrent	125,318us-gaap_AccruedLiabilitiesCurrent
Intercompany payables	0us-gaap_DueToRelatedPartiesCurrent	0us-gaap_DueToRelatedPartiesCurrent
Pension and other postretirement benefits obligation	29,410	29,032
Other current liabilities	213,597us-gaap_OtherLiabilitiesCurrent	215,952us-gaap_OtherLiabilitiesCurrent
Current debt	3,015,274us-gaap_DebtCurrent	12,327us-gaap_DebtCurrent
Total current liabilities	3,461,473us-gaap_LiabilitiesCurrent	421,609us-gaap_LiabilitiesCurrent
Long-term debt	0	3,068,878
Pension and other postretirement benefits obligation	643,453	641,231
Deferred income taxes	704,704	730,685
Other long-term liabilities	195,658us-gaap_OtherLiabilitiesNoncurrent	187,380us-gaap_OtherLiabilitiesNoncurrent
Total liabilities	5,005,288us-gaap_Liabilities	5,049,783us-gaap_Liabilities
Total stockholders' equity	197,149us-gaap_StockholdersEquity	281,581us-gaap_StockholdersEquity
Total liabilities and stockholders' equity	5,202,437	5,331,364
Reportable legal entities \| Parent (Issuer)
ASSETS
Cash and cash equivalents	394,156	421,533	379,521	234,150
Trade receivables, net	0	0
Other receivables	123,094	123,659
Intercompany receivables	0	0
Inventories	0	0
Deferred income taxes	15,984	15,986
Prepaid expenses	5,162	3,424
Other current assets	171	106
Total current assets	538,567	564,708
Mineral interests, net	0	0
Property, plant and equipment, net	6,978	7,558
Investment in subsidiaries	3,160,580	3,233,399
Other long-term assets	43,085	43,420
Total assets	3,749,210	3,849,085
LIABILITIES AND STOCKHOLDERS' EQUITY
Accounts payable	6,643	2,296
Accrued expenses	85,059	43,088
Intercompany payables	210,925	221,070
Pension and other postretirement benefits obligation	473	95
Other current liabilities	170,696	168,444
Current debt	3,001,037	0
Total current liabilities	3,474,833	434,993
Long-term debt		3,063,121
Pension and other postretirement benefits obligation	10,516	10,502
Deferred income taxes	26,206	23,766
Other long-term liabilities	40,506	35,122
Total liabilities	3,552,061	3,567,504
Total stockholders' equity	197,149	281,581
Total liabilities and stockholders' equity	3,749,210	3,849,085
Reportable legal entities \| Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	281	1,117	1,327	1,620
Trade receivables, net	81,359	88,959
Other receivables	1,984	2,193
Intercompany receivables	205,976	206,118
Inventories	110,545	110,882
Deferred income taxes	0	0
Prepaid expenses	33,427	40,044
Other current assets	6,675	7,029
Total current assets	440,247	456,342
Mineral interests, net	133,458	135,377
Property, plant and equipment, net	686,071	726,478
Investment in subsidiaries	6,528	6,459
Other long-term assets	13,445	17,857
Total assets	1,279,749	1,342,513
LIABILITIES AND STOCKHOLDERS' EQUITY
Accounts payable	35,921	30,697
Accrued expenses	57,215	60,762
Intercompany payables	0	0
Pension and other postretirement benefits obligation	28,937	28,937
Other current liabilities	26,640	27,172
Current debt	11,090	7,209
Total current liabilities	159,803	154,777
Long-term debt		5,704
Pension and other postretirement benefits obligation	632,937	630,729
Deferred income taxes	0	0
Other long-term liabilities	98,928	96,599
Total liabilities	891,668	887,809
Total stockholders' equity	388,081	454,704
Total liabilities and stockholders' equity	1,279,749	1,342,513
Reportable legal entities \| Non-Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	40,269	45,882	23,867	25,048
Trade receivables, net	8,139	2,098
Other receivables	1,170	1,185
Intercompany receivables	4,949	14,952
Inventories	61,790	90,716
Deferred income taxes	833	833
Prepaid expenses	1,572	2,722
Other current assets	2,120	2,150
Total current assets	120,842	160,538
Mineral interests, net	2,686,282	2,701,424
Property, plant and equipment, net	730,998	732,261
Investment in subsidiaries	0	0
Other long-term assets	13,389	6,471
Total assets	3,551,511	3,600,694
LIABILITIES AND STOCKHOLDERS' EQUITY
Accounts payable	4,983	5,987
Accrued expenses	13,371	21,468
Intercompany payables	0	0
Pension and other postretirement benefits obligation	0	0
Other current liabilities	16,261	20,336
Current debt	3,147	5,118
Total current liabilities	37,762	52,909
Long-term debt		53
Pension and other postretirement benefits obligation	0	0
Deferred income taxes	678,498	706,919
Other long-term liabilities	56,224	55,659
Total liabilities	772,484	815,540
Total stockholders' equity	2,779,027	2,785,154
Total liabilities and stockholders' equity	3,551,511	3,600,694
Eliminations
ASSETS
Cash and cash equivalents	0	0	0	0
Trade receivables, net	0	0
Other receivables	0
Intercompany receivables	(210,925)	(221,070)
Inventories	0	0
Deferred income taxes	0	0
Prepaid expenses	0	0
Other current assets	0	0
Total current assets	(210,925)	(221,070)
Mineral interests, net	0	0
Property, plant and equipment, net	0	0
Investment in subsidiaries	(3,167,108)	(3,239,858)
Other long-term assets	0	0
Total assets	(3,378,033)	(3,460,928)
LIABILITIES AND STOCKHOLDERS' EQUITY
Accounts payable	0	0
Accrued expenses	0	0
Intercompany payables	(210,925)	(221,070)
Pension and other postretirement benefits obligation	0	0
Other current liabilities	0	0
Current debt	0	0
Total current liabilities	(210,925)	(221,070)
Long-term debt		0
Pension and other postretirement benefits obligation	0	0
Deferred income taxes	0	0
Other long-term liabilities	0	0
Total liabilities	(210,925)	(221,070)
Total stockholders' equity	(3,167,108)	(3,239,858)
Total liabilities and stockholders' equity	$ (3,378,033)	$ (3,460,928)

Carrying value as of the balance sheet date of liabilities incurred (and for which invoices have typically been received) and payable to vendors for goods and services received that are used in an entity's business. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

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