Secured and unsecured senior debt (Details) (USD $)																								1 Months Ended	6 Months Ended
	Jun. 30, 2012 Y	Jun. 30, 2012 2012	Jun. 30, 2012 2013	Jun. 30, 2012 2014	Jun. 30, 2012 2015	Jun. 30, 2012 2016	Jun. 30, 2012 Thereafter	Jun. 30, 2012 Secured notes payable Y	Jun. 30, 2012 Secured notes payable from insurance co. maturing on 3/1/13	Jun. 30, 2012 CMBS maturing on 9/1/13	Jun. 30, 2012 Other secured notes payable maturing on 11/16/13	Jun. 30, 2012 Secured notes payable from insurance co. maturing on 4/1/14	Jun. 30, 2012 Secured notes payable from bank maturing on 4/20/14	Jun. 30, 2012 Secured notes payable from bank maturing on 7/1/14	Jun. 30, 2012 Secured notes payable from bank maturing on 11/1/14	Jun. 30, 2012 Other secured notes payable maturing on 11/18/14	Jun. 30, 2012 Secured notes payable from bank maturing on 6/1/15	Jun. 30, 2012 CMBS maturing on 1/1/16	Jun. 30, 2012 CMBS maturing on 4/1/16	Jun. 30, 2012 CMBS maturing on 8/1/16	Jun. 30, 2012 Other secured notes payable maturing on 4/1/20	Jun. 30, 2012 Secured notes payable from bank maturing on 6/1/37	Jun. 30, 2012 Unsecured Senior Line of Credit and Bank Term Loans	Apr. 30, 2012 $1.5 Billion unsecured senior line of credit extension	Jun. 30, 2012 $1.5 Billion unsecured senior line of credit Y	Jun. 30, 2012 2016 Unsecured Senior Bank Term Loan Y	Jun. 30, 2012 2017 Unsecured Senior Bank Term Loan Y	Jun. 30, 2012 Unsecured Senior Notes Payable Y	Jun. 30, 2012 8.00% Unsecured Senior Convertible Notes	Dec. 31, 2011 8.00% Unsecured Senior Convertible Notes	Jun. 30, 2012 Unsecured senior notes payable	Feb. 29, 2012 Unsecured senior notes payable
Summary of fixed rate/hedged and floating rate debt
Fixed Rate/Hedged Variable Rate	$ 2,643,160,000	$ 5,120,000	$ 51,565,000	$ 229,547,000	$ 12,819,000	$ 983,276,000	$ 1,360,833,000	$ 643,377,000																	$ 100,000,000	$ 750,000,000	$ 600,000,000	$ 549,783,000
Unhedged Variable rate	355,600,000	120,000	240,000	76,240,000			279,000,000	76,600,000																	279,000,000
Total consolidated debt	2,998,760,000	5,240,000	51,805,000	305,787,000	12,819,000	983,276,000	1,639,833,000	719,977,000	8,090,000	26,361,000	7,527,000	214,368,000	76,000,000	6,655,000	7,742,000	600,000	6,107,000	81,413,000	32,598,000	135,261,000	117,000,000	878,000			379,000,000	750,000,000	600,000,000	550,250,000
Percentage of Total	100.00%							24.00%																	12.70%	25.00%	20.00%	18.30%
Weighted average interest rate (as a percent)	4.01%							5.76%																	1.51%	3.29%	3.84%	4.61%
Weighted Average Remaining Term (in years)	5.1							3.5																	4.8	4.0	4.6	9.7
Percentage of fixed rate/hedged total debt	88.00%
Percentage of unhedged floating rate total debt	12.00%
Unsecured credit facility maximum permitted borrowings																									1,500,000,000
Amount available for borrowing																									1,100,000,000
Stated interest rate (as a percent)								5.70%	6.21%	6.36%	6.14%	5.26%	2.24%	6.05%	5.39%	6.00%	5.64%	5.73%	5.82%	6.35%	7.75%	6.50%							8.00%	8.00%	4.60%	4.60%
Base rate																									LIBOR	LIBOR	LIBOR
Applicable margin (as a percent)																									1.20%	1.65%	1.50%
Effective rate (as a percent)	4.01%							5.77%	6.21%	6.36%	6.14%	5.59%	2.24%	4.88%	4.00%	6.00%	4.50%	5.73%	5.82%	6.35%	7.75%	6.50%	4.01%		1.51%	3.29%	3.84%				4.61%
Total consolidated debt, Effective Interest Rate (as a percent)	4.01%
Future principal payments due on secured and unsecured debt
2012								5,463,000	156,000	268,000		1,846,000		55,000	70,000	120,000	51,000	767,000	412,000	1,075,000	635,000	8,000
2013								52,269,000	7,934,000	26,093,000	7,527,000	3,839,000		142,000	177,000	240,000	130,000	1,616,000	878,000	2,332,000	1,345,000	16,000
2014								305,615,000				208,683,000	76,000,000	6,458,000	7,495,000	240,000	138,000	1,713,000	931,000	2,487,000	1,453,000	17,000						250,000
2015								12,831,000									5,788,000	1,816,000	988,000	2,652,000	1,570,000	17,000
2016								233,320,000										75,501,000	29,389,000	126,715,000	1,696,000	19,000				750,000,000
Thereafter								111,182,000													110,301,000	801,000			379,000,000		600,000,000	550,000,000
Average/Subtotal	2,999,850,000	5,463,000	52,269,000	305,865,000	12,831,000	983,320,000	1,640,102,000																						250,000	250,000
Unamortized discount	(1,090,000)	(223,000)	(464,000)	(78,000)	(12,000)	(44,000)	(269,000)																						(12,000)	(15,000)
Average/Total	2,998,760,000	5,240,000	51,805,000	305,787,000	12,819,000	983,276,000	1,639,833,000	719,977,000	8,090,000	26,361,000	7,527,000	214,368,000	76,000,000	6,655,000	7,742,000	600,000	6,107,000	81,413,000	32,598,000	135,261,000	117,000,000	878,000			379,000,000	750,000,000	600,000,000	550,250,000
Balloon payments	2,956,793,000		41,165,000	297,080,000	5,728,000	980,029,000	1,632,791,000
Principal amortization	$ 41,967,000	$ 5,240,000	$ 10,640,000	$ 8,707,000	$ 7,091,000	$ 3,247,000	$ 7,042,000
Number of extensions to maturity date																								2
Length of maturity date extension period																								6 months