Schedule Of Agency MBS Forward Purchase Commitments (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended		12 Months Ended
	Jun. 30, 2012 Day		Dec. 31, 2011
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Forward Purchase agreements - Face	$ 1,289,000		$ 340,000
Forward Purchase agreements - Cost	1,332,916		348,937
Forward Purchase agreements - Fair Market Value	1,336,938		351,249
Forward Purchase agreements - Due To Brokers	1,332,916	[1]	348,937	[1]
Forward Purchase agreements - Net Asset	$ 4,022		$ 2,312
Days to coupon reset or contractual payment, minimum range	30
Days to coupon reset or contractual payment, maximum range	90

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