Condensed Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Current assets
Cash and cash equivalents	$ 865,761	$ 911,099
Short term investments	248,572	248,414
Trade accounts receivable	230,002	198,020
Other receivables	44,810	31,553
Inventories	224,806	264,161
Prepaid royalties	6,896	8,083
Deferred income taxes	48,869	49,144
Coal derivative assets	12,316	14,851
Other current assets	55,296	56,746
Total current assets	1,737,328	1,782,071
Property, plant and equipment, net	6,616,144	6,734,286
Other assets
Prepaid royalties	87,552	87,577
Equity investments	223,235	221,456
Other noncurrent assets	158,925	164,803
Total other assets	469,712	473,836
Total assets	8,823,184	8,990,193
Current liabilities
Accounts payable	160,361	176,142
Accrued expenses and other current liabilities	328,561	278,587
Current maturities of debt	29,950	33,493
Total current liabilities	518,872	488,222
Long-term debt	5,112,995	5,118,002
Asset retirement obligations	390,408	402,713
Accrued pension benefits	10,484	7,111
Accrued postretirement benefits other than pension	37,995	39,255
Accrued workers’ compensation	75,817	78,062
Deferred income taxes	368,057	413,546
Other noncurrent liabilities	181,866	190,033
Total liabilities	6,696,494	6,736,944
Stockholders' equity
Common stock, $0.01 par value, authorized 260,000 shares, issued 213,792 shares at both March 31, 2014 and December 31, 2013.	2,141	2,141
Paid-in capital	3,040,946	3,038,613
Treasury stock, at cost	(53,848)	(53,848)
Accumulated deficit	(897,611)	(771,349)
Accumulated other comprehensive income	35,062	37,692
Total stockholders’ equity	2,126,690	2,253,249
Total liabilities and stockholders’ equity	$ 8,823,184	$ 8,990,193