Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Balance Sheets
Residential mortgage-backed securities, at fair value pledged as collateral (in dollars)	$ 4,146,646	$ 5,043,824
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	500,000,000	500,000,000
Common stock, shares issued	24,304,503	24,143,944
Common stock, shares outstanding	24,304,503	24,143,944
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized	100,000,000	100,000,000
Preferred stock, shares outstanding	0	0