Total Notional or Contractual Amounts, Fair Value, Collateral and Net Exposure of Derivative Financial Instruments (Parenthetical) (Detail)	9 Months Ended
Sep. 30, 2012
Disclosure Total Notional Or Contractual Amounts Fair Value Collateral And Net Exposure Of Derivative Financial Instruments [Abstract]
Credit rating of institutional counterparties	The credit ratings of our institutional counterparties as of September 30, 2012 remain at investment grade or higher and there were no material changes in their credit ratings for the three and nine months ended September 30, 2012

Identification of the name or type of counterparty to the derivative instrument.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 12

-Section 13

-Sentence Column A

-Footnote 1

Name:	invest_DerivativeCounterparty
Namespace Prefix:	invest_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration