Transactions with Affiliates - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended		9 Months Ended		21 Months Ended
	Jul. 02, 2012 sqft	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013
Related Party Transaction [Line Items]
Description of management fee						The Manager is supervised and directed by our board of directors and is responsible for (i) the selection, purchase and sale of our investment portfolio; (ii) our financing and hedging activities; and (iii) providing us with management services. The Manager will also perform such other services and activities relating to our assets and operations as may be appropriate. In exchange for these services, the Manager receives a management fee paid monthly in arrears in an amount equal to one-twelfth of 1.20% of our Equity (as defined in the Management Agreement).
Monthly management fee payment in arrears, one-twelfth of percentage of equity						1.20%
Maximum percentage of investment in U.S. agency residential MBS of other advising REITs under Management Agreement				75.00%
Number of days prior notice of non-renewal of the Management Agreement				180 days
Termination fees description				We are required to provide 180-days prior notice of non-renewal of the Management Agreement and must pay a termination fee on the last day of the initial term or any automatic renewal term, equal to three times the average annual management fee earned by the Manager during the prior 24-month period immediately preceding the most recently completed month prior to the effective date of termination.
Change in Control and Arbitration Agreements, description				Under the amended agreements, in the event that a change in control occurs, each of these officers will receive a lump sum payment equal to (i) 12 months annual base salary in effect on December 31, 2011, plus (ii) the average annual incentive compensation received for the two complete fiscal years prior December 31, 2011, plus (iii) the average annual bonus received for the two complete fiscal years prior to December 31, 2011, as well as other benefits.
Sublease agreement, sq. ft. leased	7,300
Rent paid for leased office space per square foot	59.87
Sublease with PIA, expiration date	Jun. 30, 2022
Rent		$ 123,000	$ 106,000	$ 366,000	$ 275,000
New sublease agreement, base monthly rent	36,426.47
Sublease agreement, base monthly rent percentage increase starting July 1, 2013	3.00%
Fees paid for administrative services				On July 25, 2008, we entered into an administrative services agreement with PIA, which was amended and restated on August 20, 2010. Under this agreement, PIA provides administrative services and equipment to us including human resources, operational support and information technology, and we pay an annual fee of 5 basis points on the first $225 million of stockholders’ equity and 1.25 basis points thereafter (paid quarterly in arrears) for those services. The administrative services agreement had an initial term of one year and renews for successive one-year terms thereafter unless either party gives notice of termination no less than 30 days before the expiration of the then-current annual term.
Basis points on equity for the annual fee				5 basis points on the first $225 million of stockholders’ equity and 1.25 basis points thereafter (paid quarterly in arrears) for those services
Prior written notice to terminate administrative agreement		30 days		30 days		30 days
Administrative service fees		53,000	52,000	159,000	154,000
Stockholders equity amount used to calculate administrative service fees		225,000,000		225,000,000		225,000,000
First $225 million of Stockholders' Equity
Related Party Transaction [Line Items]
Basis points on equity for the annual fee				0.05%
Above $225 million of Stockholders' Equity
Related Party Transaction [Line Items]
Basis points on equity for the annual fee				0.0125%
Mr. Lloyd McAdams
Related Party Transaction [Line Items]
Outstanding membership interests				50.00%
Mr. Joseph E. McAdams
Related Party Transaction [Line Items]
Outstanding membership interests				45.00%
Mr. Thad M. Brown
Related Party Transaction [Line Items]
Outstanding membership interests				5.00%
Sublease
Related Party Transaction [Line Items]
Rent		$ 123,000	$ 106,000	$ 366,000	$ 275,000