CONDENSED CONSOLIDATED BALANCE SHEETS (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
ASSETS
Cash and cash equivalents	$ 235,791	$ 116,601
Receivables, net	315,296	256,967
Inventories	302,293	306,018
Deferred income taxes	55,629	58,526
Prepaid expenses	47,441	53,776
Other current assets	23,285	23,928
Total current assets	979,735	815,816
Mineral interests, net of accumulated depletion of $199.4 million and $179.6 million, respectively	2,927,591	2,965,557
Property, plant and equipment, net of accumulated depreciation of $845.2 million and $796.7 million, respectively	1,691,637	1,732,131
Deferred income taxes	185,387	160,422
Other long-term assets	99,092	94,494
Total assets	5,883,442	5,768,420
LIABILITIES AND STOCKHOLDERS' EQUITY
Current debt	17,398	18,793
Accounts payable	133,040	114,913
Accrued expenses	192,913	184,875
Accumulated postretirement benefits obligation	29,637	29,200
Other current liabilities	207,328	206,473
Total current liabilities	580,316	554,254
Long-term debt	2,593,584	2,397,372
Deferred income taxes	883,659	921,687
Accumulated postretirement benefits obligation	636,129	633,264
Other long-term liabilities	244,084	251,272
Total liabilities	4,937,772	4,757,849
Commitments and contingencies (Note 8)
Stockholders' equity:
Common stock, $0.01 par value per share: Authorized-200,000,000 shares, Issued-62,563,004 and 62,521,300 shares, respectively	626	625
Capital in excess of par value	1,615,407	1,628,244
Accumulated deficit	(389,313)	(347,448)
Accumulated other comprehensive income (loss):
Pension and other postretirement benefit plans, net of tax	(261,383)	(266,042)
Foreign currency translation adjustment	(17,127)	(1,502)
Unrealized loss on hedges, net of tax	(3,481)	(4,203)
Unrealized investment gain, net of tax	941	897
Total stockholders' equity	945,670	1,010,571
Total liabilities and stockholders' equity	$ 5,883,442	$ 5,768,420