Summary of Quarterly Results (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Summary of Quarterly Results [Abstract]
Revenues	$ 51,286	$ 50,077	$ 49,983	$ 49,468	$ 50,496	$ 49,886	$ 47,302	$ 48,775	$ 200,814	$ 196,459	$ 191,312
Net Income Attributable to Common Stockholders	$ 18,161	$ 17,692	$ 16,828	$ 15,244	$ 15,790	$ 13,824	$ 13,139	$ 14,162	$ 67,925	$ 56,915	$ 674,387
Net Income Per Common Share - Basic (in dollars per share)	$ 3.55	$ 3.46	$ 3.29	$ 2.98	$ 3.09	$ 2.71	$ 2.57	$ 2.77
Net Income Per Common Share - Diluted (in dollars per share)	$ 3.55	$ 3.46	$ 3.29	$ 2.98	$ 3.09	$ 2.71	$ 2.57	$ 2.77