Significant Accounting Policies - Estimated Useful Lives of Depreciable Property (Detail)	12 Months Ended
	Dec. 31, 2012
Minimum [Member] | Building and improvements [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciable property estimated useful lives	5
Minimum [Member] | Office equipment and furniture [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciable property estimated useful lives	3
Minimum [Member] | Vehicles, mining and other equipment [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciable property estimated useful lives	2
Minimum [Member] | Coal processing facilities [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciable property estimated useful lives	7
Minimum [Member] | Computer software [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciable property estimated useful lives	2
Maximum [Member] | Building and improvements [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciable property estimated useful lives	40
Maximum [Member] | Office equipment and furniture [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciable property estimated useful lives	30
Maximum [Member] | Vehicles, mining and other equipment [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciable property estimated useful lives	15
Maximum [Member] | Coal processing facilities [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciable property estimated useful lives	20
Maximum [Member] | Computer software [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciable property estimated useful lives	5