Summary of Investments in the RIGP (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 2,616		$ 1,369
Level 1 | Mutual Funds | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	880	[1]	609	[1]
Level 1 | Mutual Funds | Money Market Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,736		760
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,468		3,438		3,564
Level 3 | Coal
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 3,468	[2]	$ 3,438	[2]

[1] This mutual fund generally seeks long-term growth of capital and income and invests in a portfolio consisting of 100% in equities.

[2] This value was determined using an expected present value of future cash flows valuation model. This plan asset relates to a 20.8% interest in a coal lease, which derives value from specified minimum royalty payments received from CONSOL Energy Inc. related to coal reserves mined from two Pennsylvania mines owned by the lessor. The coal lease extends through 2023.