Consolidated Balance Sheets (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Parent Company [Member]
Real estate investments at cost:
Land	$ 1,168,160	$ 1,093,700
Buildings and improvements	2,408,966	2,284,522
Properties in development	499,584	610,932
Gross real estate investments at cost	4,076,710	3,989,154
Less: accumulated depreciation	755,378	700,878
Net real estate investments at cost	3,321,332	3,288,276
Investments in real estate partnerships	425,559	428,592
Net real estate investments	3,746,891	3,716,868
Cash and cash equivalents	18,408	22,460
Accounts receivable, net of allowance for doubtful accounts of $4,827 and $4,819 at June 30, 2011 and December 31, 2010, respectively	27,301	36,600
Straight-line rent receivable, net of allowance of $1,598 and $1,396 at June 30, 2011 and December 31, 2010, respectively	47,768	45,241
Notes receivable	35,931	35,931
Deferred costs, less accumulated amortization of $68,172 and $69,158 at June 30, 2011 and December 31, 2010, respectively	62,912	63,165
Acquired lease intangible assets, less accumulated amortization of $13,707 and $13,996 at June 30, 2011 and December 31, 2010, respectively	18,320	18,219
Trading securities held in trust, at fair value	23,285	20,891
Other assets	39,474	35,164
Total assets	4,020,290	3,994,539
Liabilities:
Notes payable	1,940,145	2,084,469
Unsecured line of credit	30,000	10,000
Accounts payable and other liabilities	121,617	138,196
Acquired lease intangible liabilities, less accumulated accretion of $4,434 and $11,010 at June 30, 2011 and December 31, 2010, respectively	8,682	6,682
Tenants' security and escrow deposits	11,152	10,790
Total liabilities	2,111,596	2,250,137
Commitments and contingencies
Stockholders’ equity/Partners' capital:
Preferred stock, $.01 par value per share, 30,000,000 shares authorized; 11,000,000 Series 3-5 shares issued and outstanding at June 30, 2011 and December 31, 2010 with liquidation preferences of $25 per share	275,000	275,000
Common stock $.01 par value per share, 150,000,000 shares authorized; 89,905,318 and 81,886,872 shares issued at June 30, 2011 and December 31, 2010, respectively	899	819
Treasury stock at cost, 330,620 and 347,482 shares held at June 30, 2011 and December 31, 2010, respectively	(14,985)	(16,175)
Additional paid in capital	2,259,745	2,039,612
Accumulated other comprehensive loss	(76,162)	(80,885)
Distributions in excess of net income	(596,898)	(533,194)
Total stockholders' equity	1,847,599	1,685,177
Noncontrolling interests:
Series D preferred units, aggregate redemption value of $50,000 at June 30, 2011 and December 31, 2010	49,158	49,158
Exchangeable operating partnership units, aggregate redemption value of $7,790 and $7,483 at June 30, 2011 and December 31, 2010, respectively	(862)	(762)
Limited partners' interests in consolidated partnerships	12,799	10,829
Total noncontrolling interests	61,095	59,225
Total equity/capital	1,908,694	1,744,402
Total liabilities and equity/capital	4,020,290	3,994,539
Partnership Interest [Member]
Real estate investments at cost:
Land	1,168,160	1,093,700
Buildings and improvements	2,408,966	2,284,522
Properties in development	499,584	610,932
Gross real estate investments at cost	4,076,710	3,989,154
Less: accumulated depreciation	755,378	700,878
Net real estate investments at cost	3,321,332	3,288,276
Investments in real estate partnerships	425,559	428,592
Net real estate investments	3,746,891	3,716,868
Cash and cash equivalents	18,408	22,460
Accounts receivable, net of allowance for doubtful accounts of $4,827 and $4,819 at June 30, 2011 and December 31, 2010, respectively	27,301	36,600
Straight-line rent receivable, net of allowance of $1,598 and $1,396 at June 30, 2011 and December 31, 2010, respectively	47,768	45,241
Notes receivable	35,931	35,931
Deferred costs, less accumulated amortization of $68,172 and $69,158 at June 30, 2011 and December 31, 2010, respectively	62,912	63,165
Acquired lease intangible assets, less accumulated amortization of $13,707 and $13,996 at June 30, 2011 and December 31, 2010, respectively	18,320	18,219
Trading securities held in trust, at fair value	23,285	20,891
Other assets	39,474	35,164
Total assets	4,020,290	3,994,539
Liabilities:
Notes payable	1,940,145	2,084,469
Unsecured line of credit	30,000	10,000
Accounts payable and other liabilities	121,617	138,196
Acquired lease intangible liabilities, less accumulated accretion of $4,434 and $11,010 at June 30, 2011 and December 31, 2010, respectively	8,682	6,682
Tenants' security and escrow deposits	11,152	10,790
Total liabilities	2,111,596	2,250,137
Commitments and contingencies
Stockholders’ equity/Partners' capital:
Preferred stock, $.01 par value per share, 30,000,000 shares authorized; 11,000,000 Series 3-5 shares issued and outstanding at June 30, 2011 and December 31, 2010 with liquidation preferences of $25 per share	49,158	49,158
Preferred units of general partner, $.01 par value per unit, 11,000,000 units issued and outstanding at June 30, 2011 and December 31, 2010, liquidation preference of $25 per unit	275,000	275,000
General partner; 89,905,318 and 81,886,872 units outstanding at June 30, 2011 and December 31, 2010, respectively	1,648,761	1,491,062
Limited partners; 177,164 units outstanding at June 30, 2011 and December 31, 2010	(862)	(762)
Accumulated other comprehensive loss	(76,162)	(80,885)
Total partners' capital	1,895,895	1,733,573
Noncontrolling interests:
Limited partners' interests in consolidated partnerships	12,799	10,829
Total noncontrolling interests	12,799	10,829
Total equity/capital	1,908,694	1,744,402
Total liabilities and equity/capital	$ 4,020,290	$ 3,994,539