Quarterly Results of Operations (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Results of Operations (Unaudited) [Abstract]
Total property revenues	$ 141,628	$ 135,070	$ 129,764	$ 125,474	$ 122,373	$ 117,226	$ 114,906	$ 111,208	$ 531,936	$ 465,713	$ 405,728
Income before discontinued operations	49,640	20,221	42,490	17,202	14,493	11,767	10,502	12,106	129,553	48,868	49,162
Net income	49,640	20,221	42,490	27,239	17,868	11,085	16,052	12,511	139,590	57,516	50,782
Net income available to common stockholders	$ 43,793	$ 16,219	$ 37,078	$ 22,722	$ 13,937	$ 7,687	$ 10,326	$ 8,418	$ 119,812	$ 40,368	$ 33,764
Net income:
Basic (in dollars per share)	$ 1.22	$ 0.46	$ 1.07	$ 0.67	$ 0.42	$ 0.23	$ 0.32	$ 0.27	$ 3.42	$ 1.24	$ 1.14
Diluted (in dollars per share)	$ 1.22	$ 0.45	$ 1.07	$ 0.67	$ 0.42	$ 0.23	$ 0.32	$ 0.27	$ 3.41	$ 1.24	$ 1.14
Market price:
High (in dollars per share)	$ 150.71	$ 160.64	$ 161.53	$ 151.54	$ 148.44	$ 145.40	$ 138.31	$ 124.41
Low (in dollars per share)	$ 136.38	$ 147.38	$ 146.05	$ 136.43	$ 111.25	$ 119.15	$ 122.67	$ 109.98
Close (in dollars per share)	$ 146.65	$ 148.24	$ 153.92	$ 151.51	$ 140.51	$ 120.04	$ 135.29	$ 124.00
Dividends declared (in dollars per share)	$ 1.10	$ 1.10	$ 1.10	$ 1.10	$ 1.04	$ 1.04	$ 1.04	$ 1.04