Summary of Quarterly Results (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Summary of Quarterly Results [Abstract]
Revenues	$ 48,491	$ 48,642	$ 46,878	$ 47,301	$ 46,558	$ 46,949	$ 45,377	$ 46,362	$ 191,312	$ 185,246	$ 174,206
Net income attributable to common stockholders	617,157	18,856	18,892	19,482	20,634	20,425	20,157	18,207	674,387	79,423	66,429
Net Income Per Common Share - Basic (in dollars per share)	$ 120.82	$ 3.69	$ 3.70	$ 3.81	$ 4.04	$ 4.00	$ 3.95	$ 3.57	$ 132.04	$ 15.55	$ 13.01
Net Income Per Common Share - Diluted (in dollars per share)	$ 120.82	$ 3.69	$ 3.70	$ 3.81	$ 4.04	$ 4.00	$ 3.95	$ 3.57	$ 132.04	$ 15.55	$ 13.01
Net gain on sale of real estate									$ 599,628