Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Current assets:
Cash and cash equivalents	$ 138,149	$ 93,593
Restricted cash	10,322
Trade accounts receivable	380,595	208,060
Other receivables	88,584	44,260
Inventories	377,490	235,616
Prepaid royalties	21,944	33,932
Deferred income taxes	42,051
Coal derivative assets	13,335	15,191
Other	110,304	104,262
Total current assets	1,182,774	734,914
Property, plant and equipment:
Coal lands and mineral rights	6,578,430	2,523,172
Plant and equipment	3,225,985	2,397,444
Deferred mine development	1,064,279	872,329
Property, plant and equipment	10,868,694	5,792,945
Less accumulated depreciation, depletion and amortization	(2,919,544)	(2,484,053)
Property, plant and equipment, net	7,949,150	3,308,892
Other assets:
Prepaid royalties	86,626	66,525
Goodwill	596,103	114,963
Deferred income taxes	(976,753)	361,556
Equity investments	225,605	177,451
Other	173,701	116,468
Total other assets	1,082,035	836,963
Total assets	10,213,959	4,880,769
Current liabilities:
Accounts payable	383,782	198,216
Coal derivative liabilities	7,828	4,947
Deferred income taxes	(42,051)	7,775
Accrued expenses and other current liabilities	348,207	245,411
Current maturities of debt and short-term borrowings	280,851	70,997
Total current liabilities	1,020,668	527,346
Long-term debt	3,762,297	1,538,744
Asset retirement obligations	446,784	334,257
Accrued pension benefits	48,244	49,154
Accrued postretirement benefits other than pension	42,309	37,793
Accrued workers' compensation	71,948	35,290
Deferred income taxes	976,753
Other noncurrent liabilities	255,382	110,234
Total liabilities	6,624,385	2,632,818
Redeemable noncontrolling interest	11,534	10,444
Stockholders' equity:
Common stock, $0.01 par value, authorized 260,000 shares, issued 213,183 and 164,117 shares at December 31, 2011 and 2010, respectively	2,136	1,645
Paid-in capital	3,015,349	1,734,709
Treasury stock, 1,512 shares at December 31, 2011 and 2010, at cost	(53,848)	(53,848)
Retained earnings	622,353	561,418
Accumulated other comprehensive loss	(7,950)	(6,417)
Total stockholders' equity	3,578,040	2,237,507
Total liabilities and stockholders' equity	$ 10,213,959	$ 4,880,769