Earnings from Discontinued Operations (Summary of Discontinued Operations) (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 Property	Dec. 31, 2011 Property	Dec. 31, 2010
Revenues
Rental revenues									$ 456,202	$ 393,142	$ 348,522
Other revenues									40,064	36,647	30,936
Total operating revenues	131,394	127,452	122,033	116,286	113,040	110,696	105,398	101,672	497,165	430,806	380,138
Expenses
Property operating expenses									329,462	293,447	261,972
Interest expense	15,773	14,698	14,170	14,110	14,734	15,183	13,845	13,653	58,751	57,415	54,632	[1]
Income from discontinued operations before gain on sale	77	(451)	467	484	847	806	984	988	577	3,625	2,051
Net gain (loss) on insurance and other settlement proceeds on discontinued operations									48	(12)	0
Gain (loss) on sale of discontinued operations	3,161	16,092	12,953	9,429	7,872	4,927	0	0	41,635	12,799	(2)
Income from discontinued operations									42,260	16,412	2,049	[1]
Number of properties sold (properties)									9	2
Discontinued Operation
Revenues
Rental revenues									8,054	20,578	21,025
Other revenues									974	2,507	2,423
Total operating revenues									9,028	23,085	23,448
Expenses
Property operating expenses									5,299	12,662	14,353
Depreciation and amortization									2,507	5,557	5,680
Interest expense									645	1,241	1,364
Total expense									$ 8,451	$ 19,460	$ 21,397

[1] The 2010 column shows the segment break down based on the 2011 same store portfolios. A comparison using the 2012 same store portfolio would not be comparative due to the nature of the classifications.