Notes Payable - Additional Information (Detail) (USD $) In Millions, unless otherwise specified			12 Months Ended
	Dec. 31, 2011 CreditFacility	Dec. 31, 2010 CreditFacility	Dec. 31, 2011 Minimum	Dec. 31, 2011 Maximum	Dec. 31, 2011 Line of Credit Expires August 2012	Dec. 31, 2011 Line of Credit Expires July 2012
Debt Disclosure [Line Items]
Line of credit expired amount					$ 75	$ 150
Number of lines of credit	2	2
Aggregate borrowing capacity	225	225
Note payable	104	49				104
Expiration date of Line of credit					August 2012	July 2012
Compensating balance	$ 0	$ 0
Line of credit, interest rate			0.58%	2.58%