Schedule of Agency Securities Forward Purchase Commitments (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2013		Dec. 31, 2012
Net Investment Income [Line Items]
Forward purchase agreements - Face	$ 350,000		$ 565,000
Forward purchase agreements - Cost	361,070		585,100
Forward purchase agreements - Fair Market Value	362,951		587,247
Forward purchase agreements - Due To Brokers	361,070	[1]	585,100	[1]
Forward purchase agreements - Net Asset	$ 1,881		$ 2,147

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