Schedule of Agency Securities Forward Purchase Commitments (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013		Dec. 31, 2012
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Forward purchase agreements - Face	$ 1,056,500		$ 565,000
Forward purchase agreements - Cost	1,090,940		585,100
Forward purchase agreements - Fair Market Value	1,095,098		587,247
Forward purchase agreements - Due To Brokers	1,090,940	[1]	585,100	[1]
Forward purchase agreements - Net Asset	$ 4,158		$ 2,147

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