Summary of Quarterly Results (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Summary of Quarterly Results [Abstract]
Revenues	$ 50,496	$ 49,886	$ 47,302	$ 48,775	$ 48,491	$ 48,642	$ 46,878	$ 47,301	$ 196,459	$ 191,312	$ 185,246
Net Income Attributable to Common Stockholders	15,790	13,824	13,139	14,162	617,157	18,856	18,892	19,482	56,915	674,387	79,423
Net Income Per Common Share - Basic (in dollars per share)	$ 3.09	$ 2.71	$ 2.57	$ 2.77	$ 120.82	$ 3.69	$ 3.70	$ 3.81
Net Income Per Common Share - Diluted (in dollars per share)	$ 3.09	$ 2.71	$ 2.57	$ 2.77	$ 120.82	$ 3.69	$ 3.70	$ 3.81
Net gain on sale of real estate					$ 599,628					$ 599,628