Fair Value Measurements (Details) (USD $)					12 Months Ended		1 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2011 Bank Borrowings	Jan. 31, 2012 Bank Borrowings Repayment of debt	Jan. 31, 2012 Bank Borrowings Issuance of debt	Dec. 31, 2011 Senior unsecured notes	Dec. 31, 2010 Senior unsecured notes	Dec. 31, 2011 Carrying Value	Dec. 31, 2010 Carrying Value		Dec. 31, 2011 Carrying Value Bonds payable	Dec. 31, 2010 Carrying Value Bonds payable	Dec. 31, 2011 Carrying Value Bank Borrowings	Dec. 31, 2010 Carrying Value Bank Borrowings	Dec. 31, 2011 Carrying Value Senior unsecured notes	Dec. 31, 2010 Carrying Value Senior unsecured notes	Dec. 31, 2011 Fair Value Bonds payable	Dec. 31, 2010 Fair Value Bonds payable	Dec. 31, 2011 Fair Value Bank Borrowings	Dec. 31, 2010 Fair Value Bank Borrowings	Dec. 31, 2011 Fair Value Level 2	Dec. 31, 2010 Fair Value Level 2	Dec. 31, 2011 Fair Value Level 3	Dec. 31, 2010 Fair Value Level 3	Dec. 31, 2011 Fair Value Level 3 Senior unsecured notes	Dec. 31, 2010 Fair Value Level 3 Senior unsecured notes
Fair Value Measurements
Mortgage loans receivable	$ 53,081,000		$ 59,026,000							$ 53,081,000	$ 59,026,000														$ 61,844,000	$ 67,697,000
Marketable securities	6,485,000	[1]	6,478,000	[1]						6,485,000	6,478,000	[1]											6,500,000	6,695,000
Debt obligations													3,200,000	3,730,000	56,000,000	37,700,000	100,000,000	50,000,000	3,200,000	3,730,000	56,000,000	37,700,000					101,223,000	49,943,000
Discount rate used to value future cash inflows of the mortgage loans receivable (as a percent)	6.00%		7.50%
Fair value of marketable debt securities as a percentage of quoted market price (as a percent)	100.00%		103.00%
Discount rate used to value future cash outflow (as a percent)								4.80%	5.50%
Amount borrowed under Unsecured Credit Agreement					167,600,000		4,000,000
Amount outstanding under Unsecured Credit Agreement	56,000,000		37,700,000		56,000,000	60,000,000
Amount available for borrowing under Unsecured Credit Agreement					$ 154,000,000	$ 150,000,000

[1] At December 31, 2011 and 2010, we had a $6,500 face value investment in marketable securities issued by an entity that qualifies as a related party because the entity's Chief Executive Officer is on our Board of Directors. See Note 12. Transactions with Related Party for further discussion.