Other Data (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Taxable income
Net income (loss) attributable to common stockholders	$ 2,917	$ 647	$ 2,011	$ (5,789)	$ 380	$ (3,248)	$ 6,474	$ 4,594	$ (214)	$ 8,200	$ 51,091
Reconciling items to taxable income:
Depreciation and amortization									21,479	19,603	17,196
Gain or loss on disposition of depreciable assets									(85)	6,916	8,549
Straight-line rent									(4,142)	(1,520)	(1,623)
Receivable allowances									(299)	(3,652)	523
Stock-based compensation									6,104	1,977	9,323
Other									9,926	4,752	(4,104)
Reconciling items to taxable income, total									32,983	28,076	29,864
Taxable income									32,769	36,276	80,955
Dividends paid									$ 89,270	$ 75,821	$ 91,385