Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Current assets
Cash and cash equivalents	$ 730,119	$ 784,622
Restricted cash	2,290	3,453
Short term investments	248,414	234,305
Trade accounts receivable	263,294	247,539
Other receivables	81,750	84,541
Inventories	368,240	365,424
Prepaid royalties	13,105	11,416
Deferred income taxes	67,337	67,360
Coal derivative assets	20,856	22,975
Other	88,977	92,469
Total current assets	1,884,382	1,914,104
Property, plant and equipment, net	7,272,541	7,337,098
Other assets
Prepaid royalties	91,691	87,773
Goodwill	265,423	265,423
Equity investments	246,807	242,215
Other	159,300	160,164
Total other assets	763,221	755,575
Total assets	9,920,144	10,006,777
Current liabilities
Accounts payable	229,269	224,418
Coal derivative liabilities	643	1,737
Accrued expenses and other current liabilities	352,040	318,018
Current maturities of debt	28,306	32,896
Total current liabilities	610,258	577,069
Long-term debt	5,082,205	5,085,879
Asset retirement obligations	410,975	409,705
Accrued pension benefits	69,342	67,630
Accrued postretirement benefits other than pension	46,413	45,086
Accrued workers' compensation	81,039	81,629
Deferred income taxes	610,195	664,182
Other noncurrent liabilities	227,363	221,030
Total liabilities	7,137,790	7,152,210
Stockholders' Equity
Common stock	2,141	2,141
Paid-in capital	3,029,536	3,026,823
Treasury stock, at cost	(53,848)	(53,848)
Retained earnings (accumulated deficit)	(180,459)	(104,042)
Accumulated other comprehensive loss	(15,016)	(16,507)
Total stockholders' equity	2,782,354	2,854,567
Total liabilities and stockholders' equity	$ 9,920,144	$ 10,006,777