Equity Investments (Details3) (USD $) In Thousands, except Per Share data	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Income Statements:
Rental income	$ 222,181	$ 195,817	$ 436,697	$ 390,896
Operating expenses	(90,161)	(81,352)	(180,568)	(162,822)
Depreciation and amortization	(51,973)	(45,233)	(105,814)	(90,673)
Acquisition related costs	(2,415)	(1,103)	(5,060)	(1,413)
General and administrative	(11,618)	(9,554)	(22,569)	(18,804)
Operating income	66,014	37,084	122,686	95,693
Interest and other income	392	447	1,062	1,565
Interest expense	(48,200)	(44,659)	(95,614)	(89,524)
Equity in earnings (losses) of an investee	2,910	2,305	5,622	4,644
Income tax expense	(90)	(181)	(436)	(363)
Net income	19,964	9,998	66,576	47,295
Weighted average common shares outstanding (in shares)	72,144	64,595	72,142	60,685
GOV
Income Statements:
Rental income	41,923	25,940	80,999	49,295
Operating expenses	(15,253)	(8,460)	(29,986)	(16,262)
Depreciation and amortization	(9,097)	(5,401)	(17,483)	(10,281)
Acquisition related costs	(1,009)	(1,011)	(1,838)	(1,855)
General and administrative	(2,566)	(1,623)	(4,909)	(3,082)
Operating income	13,998	9,445	26,783	17,815
Interest and other income	20	16	35	68
Interest expense	(3,076)	(1,678)	(5,613)	(3,209)
Equity in earnings (losses) of an investee	46	(23)	83	(52)
Income before income tax expense	10,988	7,760	21,288	14,622
Income tax expense	(56)	(25)	(102)	(36)
Net income	$ 10,932	$ 7,735	$ 21,186	$ 14,586
Weighted average common shares outstanding (in shares)	40,506	31,261	40,503	30,178
Net income per common share (in dollars per share)	$ 0.27	$ 0.25	$ 0.52	$ 0.48