Schedule of Agency Securities Forward Purchase Commitments (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2012		Dec. 31, 2011
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Forward purchase agreements - Face	$ 1,358,000		$ 340,000
Forward purchase agreements - Cost	1,409,047		348,937
Forward purchase agreements - Fair Market Value	1,423,058		351,249
Forward purchase agreements - Due To Brokers	1,409,047	[1]	348,937	[1]
Forward purchase agreements - Net Asset	$ 14,011		$ 2,312

[1] Amounts due to brokers are usually settled within 30-90 days after period end.