Total Notional or Contractual Amounts, Fair Value, Collateral and Net Exposure of Derivative Financial Instruments (Parenthetical) (Detail)	12 Months Ended
Dec. 31, 2011
Derivative [Line Items]
Credit rating of institutional counterparties	The credit ratings of our institutional counterparties as of December 31, 2011 remain at investment grade or higher and there were no material changes in their credit ratings for the year ended December 31, 2011.

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

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