Other Data (Details Textual) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Other Data (Textual) [Abstract]
Condition to qualify as a REIT as defined under the Internal Revenue Code	Company must meet a number of organizational and operational requirements, including a requirement that it currently distribute at least 90% of its taxable income to its stockholders.
Taxable income to be distributed to stockholders	90.00%
Estimated aggregate total cost of total assets for federal income tax purposes	$ 2,800,000,000	$ 2,700,000,000	$ 2,500,000,000
Number of preferred shares outstanding
Dividends distributed to preferred shares
Reversal of income tax expense		200,000
Income tax expense	$ 100,000