Quarterly Results of Operations (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
Quarterly Results of Operations (Unaudited) [Abstract]
Total property revenues	$ 123,151	$ 118,002	$ 115,657	$ 111,968	$ 107,465	$ 103,225	$ 98,959	$ 99,084	$ 468,778	$ 408,734	$ 404,556
Income before discontinued operations	14,713	11,992	10,720	12,357	8,494	10,260	13,643	17,680	49,782	50,077	44,282
Net income	17,867	11,085	16,053	12,511	8,628	10,426	13,869	17,859	57,516	50,782	53,739
Net income available to common stockholders	$ 13,937	$ 7,688	$ 10,325	$ 8,418	$ 4,778	$ 6,377	$ 9,482	$ 13,127	$ 40,368	$ 33,764	$ 82,200
Net income [Abstract]
Basic (in dollars per share)	$ 0.42	$ 0.23	$ 0.32	$ 0.27	$ 0.16	$ 0.21	$ 0.32	$ 0.45	$ 1.24	$ 1.14	$ 3.01
Diluted (in dollars per share)	$ 0.42	$ 0.23	$ 0.32	$ 0.27	$ 0.16	$ 0.21	$ 0.32	$ 0.45	$ 1.24	$ 1.14	$ 2.91
Market price [Abstract]
High (in dollars per share)	$ 148.44	$ 145.40	$ 138.31	$ 124.41	$ 117.12	$ 115.08	$ 113.03	$ 93.98
Low (in dollars per share)	$ 111.25	$ 119.15	$ 122.67	$ 109.98	$ 105.60	$ 92.62	$ 89.23	$ 76.35
Close (in dollars per share)	$ 140.51	$ 120.04	$ 135.29	$ 124.00	$ 114.22	$ 109.44	$ 97.54	$ 89.95	$ 140.51	$ 114.22
Dividends declared (in dollars per share)	$ 1.04	$ 1.04	$ 1.04	$ 1.04	$ 1.03	$ 1.03	$ 1.03	$ 1.03