Total Notional or Contractual Amounts, Fair Value, Collateral and Net Exposure of Derivative Financial Instruments (Parenthetical) (Detail)	3 Months Ended	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2011
Derivative [Line Items]
Credit rating of institutional counterparties	The credit ratings of our institutional counterparties as of June 30, 2011 remain at "A" or higher and there were no material changes in their credit ratings for the three and six months ended June 30, 2011.	The credit ratings of our institutional counterparties as of June 30, 2011 remain at "A" or higher and there were no material changes in their credit ratings for the three and six months ended June 30, 2011.