SCHEDULE III REAL ESTATE AND ACCUMULATED DEPRECIATION (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Real Estate Properties
Balance at the beginning of the period	$ 1,354,668	$ 977,493	$ 576,757
Additions	192,778	378,489	400,736
Loss on asset impairment	(626)
Disposals	(13,352)	(1,314)
Balance at the end of the period	1,533,468	1,354,668	977,493
Accumulated Depreciation
Balance at the beginning of the period	156,618	131,046	113,027
Additions	32,348	26,886	18,019
Loss on asset impairment	(132)
Disposals	(13,352)	(1,314)
Balance at the end of the period	$ 175,482	$ 156,618	$ 131,046