Interest Rate Swap Terms and Positions (Parenthetical) (Detail)	9 Months Ended
Sep. 30, 2012
Interest rate swaps
Derivative [Line Items]
LIBOR rate used	References to LIBOR represent the one-month rate.
Commodity Contract [Member]
Derivative [Line Items]
Number Of Derivative Counterparty	6
Credit Concentration Risk [Member] \| Commodity Contract [Member]
Derivative [Line Items]
Number Of Derivative Counterparty	3

The reference rate for the variable rate of the interest rate derivative, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

No definition available.

Name:	us-gaap_DerivativeDescriptionOfVariableRateBasis
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration