Summary of Significant Accounting Policies (Details) (USD $)	1 Months Ended				3 Months Ended		6 Months Ended
	Apr. 30, 2012	May 31, 2011	Mar. 31, 2011	Feb. 28, 2011	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 entity segment M	Jun. 30, 2011	Dec. 31, 2011
Summary of Significant Accounting Policies
Number of reportable segments							1
Original maturities of short-term investments, maximum (in months)							3
Maximum ownership percentage to classify investments as cost method investments							5.00%
U.S. Treasury Securities Sold Short
Short sale of U.S. Treasury securities				$ 112,700,000
Realized gain from settlement of short position			122,000
Favorable movement in prices of U.S. Treasury securities (from short position)			194,000
Interest accrued and transaction costs on securities			72,000
Capitalized financing costs, net of amortization					6,000,000		6,000,000		5,000,000
Amortization of financing costs included in interest expense					1,300,000	1,000,000	2,500,000	1,400,000
Income Taxes
Penalty payable on prohibited transactions (as a percent)							100.00%
Number of taxable REIT subsidiaries							3
Provision for income taxes					140,000	823,000	539,000	1,204,000
Federal income tax rate (as a percent)							34.00%	34.00%
State income tax rate (as a percent)							7.50%	7.50%
Underwriting Commissions and Offering Costs
Underwriting and offering costs	$ 642,000	$ 1,100,000					$ 642,000	$ 880,000