Summary of Significant Accounting Policies (Details) (USD $)	1 Months Ended				4 Months Ended	12 Months Ended
	May 31, 2011	Mar. 31, 2011	Feb. 28, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 entity segment M	Dec. 31, 2010	Dec. 31, 2009
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%
Capitalized financing costs, net of amortization				$ 4,100,000		$ 5,000,000	$ 4,100,000
Amortization of financing costs included in interest expense						3,800,000	600,000
Income Taxes
Penalty payable on prohibited transactions (as a percent)						100.00%
Number of taxable REIT subsidiaries						2
Provision for income taxes						790,000	426,000
Federal income tax rate (as a percent)						34.00%	33.00%
State income tax rate (as a percent)						7.50%	7.50%
Underwriting Commissions and Offering Costs
Underwriting and offering costs	1,100,000			18,900,000	57,588,000	1,091,000	18,986,000	57,600,000
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