Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Balance Sheets [Abstract]
Mortgage-backed securities, pledged assets	$ 17,377,453	$ 17,012,472
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	97,779,404	76,823,220
Common stock, shares outstanding	97,779,404	76,823,220