CONDENSED CONSOLIDATED BALANCE SHEETS (UNAUDITED) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
ASSETS
Cash and cash equivalents	$ 434,706	$ 468,532
Trade receivables, net	89,498us-gaap_AccountsReceivableNetCurrent	91,057us-gaap_AccountsReceivableNetCurrent
Other receivables	126,248us-gaap_OtherReceivablesGrossCurrent	127,037us-gaap_OtherReceivablesGrossCurrent
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
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
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
Stockholders' equity
Preferred stock, $0.01 par value per share: Authorized—20,000,000 shares; none issued	0us-gaap_PreferredStockValue	0us-gaap_PreferredStockValue
Common stock, $0.01 par value per share: Authorized—200,000,000 shares; issued 80,723,701 and 71,978,113 shares, respectively	807us-gaap_CommonStockValue	720us-gaap_CommonStockValue
Capital in excess of par value	1,678,710	1,668,407
Accumulated deficit	(1,249,694)	(1,169,498)
Accumulated other comprehensive loss	(232,674)	(218,048)
Total stockholders' equity	197,149us-gaap_StockholdersEquity	281,581us-gaap_StockholdersEquity
Total liabilities and stockholders' equity	$ 5,202,437	$ 5,331,364

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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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant