Consolidated Balance Sheets (Parenthetical) (USD $) Share data in Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Mortgage securities and similar investments, pledged under repurchase arrangements	$ 11,930,000,000	$ 8,220,000,000
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	250,000	250,000
Common stock, shares issued	88,287	88,287
Common stock, shares outstanding	70,259	70,259
$1.60 Cumulative Preferred Stock, Series A [Member]
Preferred stock, dividend rate	$ 1.60	$ 1.60
Preferred stock, shares issued	186	186
Preferred stock, shares outstanding	187	187
Preferred stock, aggregate liquidation preference	3,056,000	3,073,000
$1.26 Cumulative Convertible Preferred Stock, Series B [Member]
Preferred stock, dividend rate	$ 1.26	$ 1.26
Preferred stock, shares issued	16,184	16,184
Preferred stock, shares outstanding	15,819	15,819
Preferred stock, aggregate liquidation preference	$ 184,175,000	$ 180,023,000
Preferred Stock [Member]
Preferred stock, par value	$ 0.1	$ 0.1
Preferred stock, shares authorized	100,000	100,000