Schedule of Agency Securities Forward Purchase Commitments (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Forward purchase agreements - Face	$ 565,000		$ 340,000
Forward purchase agreements - Cost	585,100		348,937
Forward purchase agreements - Fair Market Value	587,247		351,249
Forward purchase agreements - Due To Brokers	585,100	[1]	348,937	[1]
Forward purchase agreements - Net Asset	$ 2,147		$ 2,312

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