Summary of Quarterly Results (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Summary of Quarterly Results [Abstract]
Revenues	$ 64,607	$ 64,737	$ 62,036	$ 62,872	$ 62,250	$ 61,390	$ 59,166	$ 58,544	$ 254,252	$ 241,350	$ 223,529
Net income attributable to common stockholders	$ 20,634	$ 20,425	$ 20,157	$ 18,207	$ 17,891	$ 17,875	$ 15,549	$ 15,114	$ 79,423	$ 66,429	$ 132,190
Income per common share - basic	$ 4.04	$ 4.00	$ 3.95	$ 3.57	$ 3.50	$ 3.50	$ 3.05	$ 2.96	$ 15.55	$ 13.01	$ 25.90
Income per common share - diluted	$ 4.04	$ 4.00	$ 3.95	$ 3.57	$ 3.50	$ 3.50	$ 3.05	$ 2.96	$ 15.55	$ 13.01	$ 25.89