Summary of Investments in the RIGP (Parenthetical) (Detail)	12 Months Ended
Dec. 31, 2011
Defined Benefit Plan Disclosure [Line Items]
Mutual fund portfolio in equities securities	100.00%
Coal
Defined Benefit Plan Disclosure [Line Items]
Interest in coal lease	20.80%
Number coal reserve mines in Pennsylvania	2
Coal lease expiration period	2023

Target allocation percentage of investments in equity securities to total plan assets presented on a weighted-average basis as of the measurement date of the latest statement of financial position.

No definition available.

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration