Other Data (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Income Tax Disclosure [Abstract]
Net income (loss) attributable to common stockholders	$ (6,392)	$ 5,815	$ 2,908	$ 3,134	$ 2,917	$ 647	$ 2,011	$ (5,789)	$ 5,465		$ (214)		$ 8,200
Reconciling items to taxable income:
Depreciation and amortization									28,526		21,479		19,603
Gain or loss on disposition of depreciable assets									922		(85)		6,916
Straight-line rent									(6,075)		(4,142)		(1,520)
Receivable allowances									(74)		(299)		(3,652)
Stock-based compensation									5,400		6,104		1,977
Other									12,724		9,926		4,752
Reconciling items to taxable income, total									41,423		32,983		28,076
Taxable income									46,888	[1]	32,769	[1]	36,276	[1]
Dividends paid									$ 96,356		$ 89,270		$ 75,821

[1] Before REIT dividend paid deduction.