STOCKHOLDERS' EQUITY (Details) (USD $) In Thousands, except Share data, unless otherwise specified	0 Months Ended	2 Months Ended	1 Months Ended			3 Months Ended							5 Months Ended	8 Months Ended	16 Months Ended
	Dec. 03, 2009	Jun. 30, 2012	May 11, 2012	May 11, 2010	Dec. 31, 2009	Dec. 31, 2012	Sep. 30, 2012	Mar. 31, 2012	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Mar. 31, 2010	May 11, 2010	Dec. 31, 2012	May 11, 2012	May 14, 2012	Sep. 17, 2010
STOCKHOLDERS' EQUITY
Common stock and UPREIT units authorized for repurchase						2,291,160								2,291,160
Common stock, shares authorized	3,700,000															4,400,000	3,600,000
Maximum authorized gross proceeds from common stock issued under public equity offering	$ 150,000
Number of Shares Sold		698,599	207,500	1,021,400	871,600	10,100	1,262,125	188,393	877,400	1,485,707	841,000	1,285,700	3,178,700	1,970,824	3,600,000
Gross Proceeds		42,528	13,224	50,078	39,830	628	80,303	11,156	56,542	90,102	47,524	60,092	150,000	123,459	218,548
Net Proceeds		$ 41,617	$ 12,957	$ 49,273	$ 38,916	$ 609	$ 78,682	$ 10,897	$ 55,273	$ 88,299	$ 46,572	$ 58,856	$ 147,045	$ 120,908	$ 213,998
Average Sales Price (in dollars per share)		$ 60.88	$ 63.73	$ 49.03	$ 45.70	$ 62.18	$ 63.63	$ 59.22	$ 64.44	$ 60.65	$ 56.51	$ 46.74	$ 47.19	$ 62.64	$ 60.71