Quarterly Financial Statement Information (Unaudited) (Schedule Of Quarterly Financial Information) (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
Quarterly Financial Statement Information [Abstract]
Total revenue	$ 153,267	$ 144,621	$ 140,687	$ 141,398	$ 137,139	$ 128,627	$ 124,946	$ 125,867	$ 579,973	$ 516,579	$ 499,176
Net income attributable to common stockholders	$ 13,929	$ 8,508	$ 8,195	$ 7,224	$ 6,769	$ 5,507	$ 5,168	$ 2,637	$ 37,856	$ 20,081	$ 34,419
Basic earnings per share data: Net income attributable to common stockholders	$ 0.29	$ 0.20	$ 0.21	$ 0.19	$ 0.18	$ 0.15	$ 0.14	$ 0.08	$ 0.90	$ 0.55	$ 1.04
Diluted earnings per share data: Net income attributable to common stockholders	$ 0.29	$ 0.20	$ 0.20	$ 0.19	$ 0.18	$ 0.15	$ 0.14	$ 0.07	$ 0.89	$ 0.54	$ 1.04