CONDENSED CONSOLIDATED BALANCE SHEETS (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Current assets
Cash and cash equivalents	$ 186,232	$ 168,745
Accounts receivable	70,192	86,838
Due from related parties	1,520	227
Inventories, net	77,975	79,802
Deferred income taxes	21,670	21,670
Derivative financial instruments	14,875	17,111
Other assets	24,178	9,840
Total current assets	396,642	384,233
Noncurrent assets
Property, plant and equipment, net	1,557,331	1,589,138
Port access contract rights, net	52,852	53,780
Goodwill	35,634	35,634
Deferred income taxes	53,725	56,468
Other assets	47,457	40,665
Total assets	2,143,641	2,159,918
Current liabilities
Accounts payable	47,341	52,035
Royalties and production taxes	132,252	126,212
Accrued expenses	47,888	52,213
Current portion of federal coal lease obligations	63,970	63,970
Other liabilities	1,632	1,632
Total current liabilities	293,083	296,062
Noncurrent liabilities
Senior notes	498,541	498,480
Asset retirement obligations, net of current portion	205,898	216,241
Accumulated postretirement benefit obligation, net of tax	51,992	50,276
Other liabilities	10,699	11,025
Total liabilities	1,060,213	1,072,084
Commitments and Contingencies (Note 12)
Equity
Common stock ($0.01 par value; 200,000 shares authorized; 61,509 and 61,434 shares issued and 61,034 and 61,022 outstanding at March 31, 2015 and December 31, 2014, respectively)	610	610
Treasury stock, at cost (475 shares and 432 shares at March 31, 2015 and December 31, 2014, respectively)	(6,490)	(6,243)
Additional paid-in capital	568,346	568,022
Retained earnings	532,064	536,744
Accumulated other comprehensive income (loss)	(11,102)	(11,299)
Total equity	1,083,428	1,087,834
Total liabilities and equity	$ 2,143,641	$ 2,159,918