Schedule III - Real Estate and Accumulated Depreciation Schedule III - Real Estate and Accumulated Depreciation (Changes in Total Real Estate Assets and Accumulated Depreciation) (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule III - Real Estate and Accumulated Depreciation [Abstract]
Balance, beginning of period	$ 5,819,540,000	$ 5,647,677,000	$ 5,461,626,000
Acquisition of operating properties and joint ventures	797,477,000	0	238,885,000
Development	232,296,000	180,028,000	21,798,000
Improvements	60,426,000	61,037,000	44,405,000
Cost of real estate sold contributed to joint venture	0	(12,578,000)	0
Cost of real estate sold – other	(176,872,000)	(32,673,000)	(119,037,000)
Classification to held for sale	(58,953,000)	(23,951,000)	0
Balance, end of period	6,673,914,000	5,819,540,000	5,647,677,000
Balance, Beginning of period	1,432,799,000	1,292,924,000	1,149,056,000
Depreciation	185,546,000	171,009,000	166,867,000
Dispositions	(72,465,000)	(18,877,000)	(22,999,000)
Transfers to held for sale	(26,984,000)	(12,257,000)	0
Balance, End of period	1,518,896,000	1,432,799,000	1,292,924,000
Real Estate, Federal Income Tax Basis	$ 5,900,000,000