CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME
Rental income	$ 211,076	$ 178,950	$ 117,219
Expenses:
Real estate taxes	23,413	19,345	12,177
Utility expenses	16,810	15,316	9,064
Other operating expenses	38,558	31,784	19,937
Depreciation and amortization	51,166	40,089	24,239
Loss on asset impairment	494
Acquisition related costs	1,614	3,504	5,750
General and administrative	12,355	10,898	7,061
Total expenses	144,410	120,936	78,228
Operating income	66,666	58,014	38,991
Interest and other income	29	104	103
Interest expense (including net amortization of debt premiums and deferred financing fees of $1,332, $1,045 and $2,283, respectively)	(16,892)	(12,057)	(7,351)
Loss on extinguishment of debt			(3,786)
Equity in earnings (losses) of an investee	316	139	(1)
Income before income tax expense	50,119	46,200	27,956
Income tax expense	(159)	(203)	(161)
Net income	49,960	45,997	27,795
Other Comprehensive income:
Equity in unrealized gain of an investee	22	75	2
Other comprehensive income	22	75	2
Comprehensive income	$ 49,982	$ 46,072	$ 27,797
Weighted average common shares outstanding (in shares)	48,617	43,368	34,341
Net income per common share (in dollars per share)	$ 1.03	$ 1.06	$ 0.81