Condensed Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Current assets
Cash and cash equivalents	$ 689,972	$ 734,231
Short term investments	249,518	248,954
Trade accounts receivable	198,314	211,506
Other receivables	15,435	20,511
Inventories	240,113	190,253
Prepaid royalties	12,841	11,118
Deferred income taxes	50,414	52,728
Coal derivative assets	14,777	13,257
Other current assets	59,605	60,193
Total current assets	1,530,989	1,542,751
Property, plant and equipment, net	6,371,335	6,453,458
Other assets
Prepaid royalties	63,622	66,806
Equity investments	225,030	235,842
Other noncurrent assets	123,964	130,866
Total other assets	412,616	433,514
Total assets	8,314,940	8,429,723
Current liabilities
Accounts payable	167,480	180,113
Accrued expenses and other current liabilities	321,503	302,396
Current maturities of debt	34,368	36,885
Total current liabilities	523,351	519,394
Long-term debt	5,117,982	5,123,485
Asset retirement obligations	404,844	398,896
Accrued pension benefits	14,436	16,260
Accrued postretirement benefits other than pension	34,453	32,668
Accrued workers’ compensation	98,683	94,291
Deferred income taxes	419,064	422,809
Other noncurrent liabilities	141,804	153,766
Total liabilities	6,754,617	6,761,569
Stockholders' equity
Common stock, $0.01 par value, authorized 260,000 shares, issued 214,179 shares and 213,791 shares at March 31, 2015 and December 31, 2014, respectively	2,145	2,141
Paid-in capital	3,050,216	3,048,460
Treasury stock, at cost, 1,517 shares at March 31, 2015 and December 31, 2014	(53,863)	(53,863)
Accumulated deficit	(1,445,020)	(1,331,825)
Accumulated other comprehensive income	6,845	3,241
Total stockholders’ equity	1,560,323	1,668,154
Total liabilities and stockholders’ equity	$ 8,314,940	$ 8,429,723