Summary of Restructuring Related Obligations (Detail) (USD $)	3 Months Ended	9 Months Ended
Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Restructuring Cost and Reserve [Line Items]
Restructuring Reserve, Current	$ 3,200,000		$ 3,200,000
FirmTransportationLossUpperRange	17,300,000
Balance at beginning of period	251,000	0	576,000	64,000
Employee, office and other costs accrued	1,431,000	1,553,000	1,283,000	1,623,000
Firm transportation charge	17,332,000	0	17,332,000	0
Cash payments, net	(1,359,000)	(286,000)	(1,536,000)	(420,000)
Balance at end of period	17,655,000	1,267,000	17,655,000	1,267,000
Restructuring Reserve, Noncurrent	$ 14,443,000		$ 14,443,000		$ 0
Appalachian Organization Restructuring [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Number of Positions Eliminated	30

Carrying amount as of the balance sheet date of known and estimated obligations associated with exit from or disposal of business activities or restructurings pursuant to a duly authorized plan, which are expected to be paid in the next twelve months or in the normal operating cycle if longer. Costs of such activities include those for one-time termination benefits, termination of an operating lease or other contract, consolidating or closing facilities, relocating employees, and costs associated with an ongoing benefit arrangement, but excludes costs associated with the retirement of a long-lived asset.

+ References+ Details

Carrying amount as of the balance sheet date of known and estimated costs associated with exit from or disposal of business activities or restructurings pursuant to a duly authorized plan, which are expected to be paid after one year or beyond the next operating cycle, if longer. Costs of such activities include those for one-time termination benefits, termination of an operating lease or other contract, consolidating or closing facilities, and relocating employees, and costs associated with an ongoing benefit arrangement, but excludes costs associated with the retirement of a long-lived asset.

+ References+ Details

- Definition

Amount of cash paid in the period to fully or partially settle a specified, previously accrued type of restructuring cost.

+ References+ Details