Supplemental Guarantor and Non-Guarantor Financial Information (Details 2) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
ASSETS
Cash and cash equivalents	$ 293,134	$ 116,601	$ 129,922	$ 128,430
Receivables, net	305,354	256,967
Inventories	320,681	306,018
Deferred income taxes	58,073	58,526
Prepaid expenses	47,097	53,776
Other current assets	20,778	23,928
Total current assets	1,045,117	815,816
Mineral interests, net	2,917,902	2,965,557
Property, plant and equipment, net	1,628,441	1,732,131
Deferred income taxes	148,311	160,422
Other long-term assets	100,741	94,494
Total assets	5,840,512	5,768,420
LIABILITIES AND STOCKHOLDERS' EQUITY
Current debt	12,179	18,793
Accounts payable	97,693	114,913
Accrued expenses	154,470	184,875
Accumulated postretirement benefits obligation	30,511	29,200
Other current liabilities	214,300	206,473
Total current liabilities	509,153	554,254
Long-term debt	2,770,381	2,397,372
Accumulated postretirement benefits obligation	641,337	633,264
Deferred income taxes	854,408	921,687
Other long-term liabilities	236,555	251,272
Total liabilities	5,011,834	4,757,849
Total stockholders' equity	828,678	1,010,571
Total liabilities and stockholders' equity	5,840,512	5,768,420
Parent (Issuer)
ASSETS
Cash and cash equivalents	261,733	83,833	111,406	99,086
Receivables, net	120,361	64,106
Intercompany receivables		721,293
Intercompany loans receivable	55,583	118,079
Deferred income taxes	39,876	39,375
Prepaid expenses	2,931	1,869
Other current assets	17,867	17,559
Total current assets	498,351	1,046,114
Property, plant and equipment, net	6,753	8,448
Deferred income taxes	40,252	52,363
Investment in subsidiaries	4,613,231	3,530,094
Other long-term assets	74,318	71,622
Total assets	5,232,905	4,708,641
LIABILITIES AND STOCKHOLDERS' EQUITY
Accounts payable	3,446	5,128
Accrued expenses	53,472	27,197
Intercompany payables	274,189
Intercompany loans payable	1,096,126	1,074,879
Accumulated postretirement benefits obligation	161	131
Other current liabilities	168,993	157,044
Total current liabilities	1,596,387	1,264,379
Long-term debt	2,762,553	2,381,255
Accumulated postretirement benefits obligation	400	452
Other long-term liabilities	44,887	51,984
Total liabilities	4,404,227	3,698,070
Total stockholders' equity	828,678	1,010,571
Total liabilities and stockholders' equity	5,232,905	4,708,641
Guarantor Subsidiaries
ASSETS
Cash and cash equivalents		61	121	79
Receivables, net	109,229	113,182
Intercompany receivables	216,751
Intercompany loans receivable	1,096,126	1,074,879
Inventories	164,493	131,893
Deferred income taxes	17,687	17,687
Prepaid expenses	39,792	45,327
Other current assets	631	1,109
Total current assets	1,644,709	1,384,138
Mineral interests, net	7,687	18,475
Property, plant and equipment, net	759,471	790,900
Deferred income taxes	112,560	112,560
Other long-term assets	10,331	9,375
Total assets	2,534,758	2,315,448
LIABILITIES AND STOCKHOLDERS' EQUITY
Current debt	3,945	10,196
Accounts payable	72,706	78,260
Accrued expenses	44,752	83,155
Intercompany payables		567,360
Accumulated postretirement benefits obligation	30,350	29,069
Other current liabilities	23,202	24,389
Total current liabilities	174,955	792,429
Long-term debt		1,784
Accumulated postretirement benefits obligation	640,937	632,812
Other long-term liabilities	123,184	128,593
Total liabilities	939,076	1,555,618
Total stockholders' equity	1,595,682	759,830
Total liabilities and stockholders' equity	2,534,758	2,315,448
Non-Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	31,401	32,707	18,395	29,265
Receivables, net	75,764	79,679
Intercompany receivables	57,438
Inventories	156,188	174,125
Deferred income taxes	510	1,464
Prepaid expenses	4,374	6,580
Other current assets	2,280	5,260
Total current assets	327,955	299,815
Mineral interests, net	2,910,215	2,947,082
Property, plant and equipment, net	862,217	932,783
Deferred income taxes	(4,501)	(4,501)
Other long-term assets	16,092	13,497
Total assets	4,111,978	4,188,676
LIABILITIES AND STOCKHOLDERS' EQUITY
Current debt	8,234	8,597
Accounts payable	21,541	31,525
Accrued expenses	56,246	74,523
Intercompany payables		153,933
Intercompany loans payable	55,583	118,079
Other current liabilities	22,105	25,040
Total current liabilities	163,709	411,697
Long-term debt	7,828	14,333
Deferred income taxes	854,408	921,687
Other long-term liabilities	68,484	70,695
Total liabilities	1,094,429	1,418,412
Total stockholders' equity	3,017,549	2,770,264
Total liabilities and stockholders' equity	4,111,978	4,188,676
Eliminations
ASSETS
Intercompany receivables	(274,189)	(721,293)
Intercompany loans receivable	(1,151,709)	(1,192,958)
Total current assets	(1,425,898)	(1,914,251)
Investment in subsidiaries	(4,613,231)	(3,530,094)
Total assets	(6,039,129)	(5,444,345)
LIABILITIES AND STOCKHOLDERS' EQUITY
Intercompany payables	(274,189)	(721,293)
Intercompany loans payable	(1,151,709)	(1,192,958)
Total current liabilities	(1,425,898)	(1,914,251)
Total liabilities	(1,425,898)	(1,914,251)
Total stockholders' equity	(4,613,231)	(3,530,094)
Total liabilities and stockholders' equity	$ (6,039,129)	$ (5,444,345)