Asset Impairments - Net Book Value and Fair Values of Assets (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Coal and other mineral rights, net book value	$ 1,380,428	$ 1,257,501
Intangible assets, net book value	70,811	75,164
Plant and equipment, net book value	32,401	46,185
West Virginia [Member]
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Coal and other mineral rights, net fair value	4,050	6,618
Intangible assets, net fair value
Plant and equipment, net fair value	1,981	2,600
Total, net fair value	6,031	9,218
Coal and other mineral rights, net book value	6,618	76,003
Intangible assets, net book value		43,855
Plant and equipment, net book value	1,981	7,775
Total net book value	8,599	127,633
Gatling LLC Ohio [Member]
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Coal and other mineral rights, net fair value		20,035
Intangible assets, net fair value
Plant and equipment, net fair value		2,947
Total, net fair value		22,982
Coal and other mineral rights, net book value		56,769
Intangible assets, net book value		33,670
Plant and equipment, net book value		2,947
Total net book value		$ 93,386