Consolidated Balance Sheets (Parenthetical) (USD $) In Millions, except Per Share data, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Agency securities, pledged assets	$ 72,598	$ 50,725
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	10	10
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	300	300
Common stock, shares issued	300	224
Common stock, shares outstanding	300	224