Total Notional or Contractual Amounts, Fair Value, Collateral and Net Exposure of Derivative Financial Instruments (Parenthetical) (Detail)	3 Months Ended	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2011
Derivative [Line Items]
Credit rating of institutional counterparties	The credit ratings of our institutional counterparties as of September 30, 2011 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, 2011.	The credit ratings of our institutional counterparties as of September 30, 2011 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, 2011.