Significant Accounting Policies - Estimated Useful Lives of Depreciable Property (Detail)	12 Months Ended
	Dec. 31, 2013
Minimum [Member] | Building and Improvements [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciable property estimated useful lives	5 years
Minimum [Member] | Office Equipment and Furniture [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciable property estimated useful lives	3 years
Minimum [Member] | Vehicles, Mining and Other Equipment [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciable property estimated useful lives	2 years
Minimum [Member] | Coal Processing Facilities [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciable property estimated useful lives	7 years
Minimum [Member] | Computer Software [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciable property estimated useful lives	2 years
Maximum [Member] | Building and Improvements [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciable property estimated useful lives	40 years
Maximum [Member] | Office Equipment and Furniture [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciable property estimated useful lives	30 years
Maximum [Member] | Vehicles, Mining and Other Equipment [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciable property estimated useful lives	15 years
Maximum [Member] | Coal Processing Facilities [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciable property estimated useful lives	20 years
Maximum [Member] | Computer Software [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciable property estimated useful lives	15 years