Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Current assets
Cash and cash equivalents	$ 644,205	$ 784,622
Restricted cash	1,085	3,453
Short term investments	248,464	234,305
Trade accounts receivable	264,692	247,539
Other receivables	71,846	84,541
Inventories	356,653	365,424
Prepaid royalties	8,970	11,416
Deferred income taxes	67,350	67,360
Coal derivative assets	31,261	22,975
Other	60,811	92,469
Total current assets	1,755,337	1,914,104
Property, plant and equipment, net	7,231,221	7,337,098
Other assets:
Prepaid royalties	91,130	87,773
Goodwill	265,423	265,423
Equity investments	232,758	242,215
Other	154,860	160,164
Total other assets	744,171	755,575
Total assets	9,730,729	10,006,777
Current liabilities
Accounts payable	232,433	224,418
Coal derivative liabilities	542	1,737
Accrued expenses and other current liabilities	307,089	318,018
Current maturities of debt	23,842	32,896
Total current liabilities	563,906	577,069
Long-term debt	5,078,634	5,085,879
Asset retirement obligations	414,860	409,705
Accrued pension benefits	66,903	67,630
Accrued postretirement benefits other than pension	46,358	45,086
Accrued workers’ compensation	79,004	81,629
Deferred income taxes	565,047	664,182
Other noncurrent liabilities	205,103	221,030
Total liabilities	7,019,815	7,152,210
Stockholders’ equity
Common stock, $0.01 par value, authorized 260,000 shares, issued 213,753 and 213,759 shares at June 30, 2013 and December 31, 2012	2,141	2,141
Paid-in capital	3,032,626	3,026,823
Treasury stock, at cost	(53,848)	(53,848)
Accumulated deficit	(259,032)	(104,042)
Accumulated other comprehensive loss	(10,973)	(16,507)
Total stockholders’ equity	2,710,914	2,854,567
Total liabilities and stockholders’ equity	$ 9,730,729	$ 10,006,777