Taxable Income (Loss) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Taxable Income
Net income (loss) attributable to common stockholders	$ 2,908		$ 2,011		$ 6,042		$ (3,778)
Reconciling items to taxable income:
Depreciation and amortization	6,557		4,194		13,500		9,519
Gain or loss on disposition of depreciable assets	1,251		78		(4,357)		(2,098)
Straight-line rent	(1,434)		(1,009)		(3,244)		(2,149)
Receivable allowances	109		324		(371)		720
Stock-based compensation	1,443		1,298		2,632		2,684
Other	2,246		5,955		7,505		7,044
Taxable income	13,080	[1]	12,851	[1]	21,707	[1]	11,942	[1]
Dividends paid	$ 23,398		$ 22,325		$ 46,785		$ 42,570

[1] Before REIT dividends paid deduction.