Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Current assets
Cash and cash equivalents	$ 1,133,128	$ 784,622
Restricted cash		3,453
Short term investments	248,724	234,305
Trade accounts receivable	190,723	247,539
Other receivables	24,116	84,541
Inventories	287,409	365,424
Prepaid royalties	8,350	11,416
Deferred income taxes	67,381	67,360
Coal derivative assets	22,836	22,975
Other current assets	46,972	92,469
Total current assets	2,029,639	1,914,104
Property, plant and equipment, net	6,778,225	7,337,098
Other assets
Prepaid royalties	85,001	87,773
Goodwill	265,423	265,423
Equity investments	223,554	242,215
Other noncurrent assets	149,613	160,164
Total other assets	723,591	755,575
Total assets	9,531,455	10,006,777
Current liabilities
Accounts payable	207,273	224,418
Coal derivative liabilities	421	1,737
Accrued expenses and other current liabilities	349,033	318,018
Current maturities of debt	23,050	32,896
Total current liabilities	579,777	577,069
Long-term debt	5,074,384	5,085,879
Asset retirement obligations	411,121	409,705
Accrued pension benefits	68,539	67,630
Accrued postretirement benefits other than pension	44,279	45,086
Accrued workers’ compensation	82,014	81,629
Deferred income taxes	484,130	664,182
Other noncurrent liabilities	205,557	221,030
Total liabilities	6,949,801	7,152,210
Stockholders' equity
Common stock, $0.01 par value, authorized 260,000 shares, issued 213,792 and 213,759 shares at September 30, 2013 and December 31, 2012	2,141	2,141
Paid-in capital	3,035,732	3,026,823
Treasury stock, at cost	(53,848)	(53,848)
Accumulated deficit	(393,765)	(104,042)
Accumulated other comprehensive loss	(8,606)	(16,507)
Total stockholders’ equity	2,581,654	2,854,567
Total liabilities and stockholders’ equity	$ 9,531,455	$ 10,006,777