Secured and unsecured senior debt (Details) (USD $)																9 Months Ended		9 Months Ended							1 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012 2012	Sep. 30, 2012 2013	Sep. 30, 2012 2014	Sep. 30, 2012 2015	Sep. 30, 2012 2016	Sep. 30, 2012 Thereafter	Sep. 30, 2012 Secured notes payable	Sep. 30, 2012 Secured notes payable from insurance co. maturing on 3/1/13	Sep. 30, 2012 CMBS maturing on 9/1/13	Sep. 30, 2012 Other secured notes payable maturing on 11/16/13	Sep. 30, 2012 Secured notes payable from insurance co. maturing on 4/1/14	Sep. 30, 2012 Secured notes payable from bank maturing on 4/20/14	Sep. 30, 2012 Secured notes payable from bank maturing on 7/1/14	Sep. 30, 2012 Secured notes payable from bank maturing on 11/1/14	Sep. 30, 2012 Other secured notes payable maturing on 11/18/14	Sep. 30, 2012 Secured notes payable from bank maturing on 6/1/15	Sep. 30, 2012 Secured notes payable from bank maturing on 7/1/15	Sep. 30, 2012 CMBS maturing on 1/1/16	Sep. 30, 2012 CMBS maturing on 4/1/16	Sep. 30, 2012 CMBS maturing on 8/1/16	Sep. 30, 2012 Other secured notes payable maturing on 4/1/20	Sep. 30, 2012 Secured notes payable from bank maturing on 6/1/37	Sep. 30, 2012 Unsecured Senior Line of Credit and Bank Term Loans	Apr. 30, 2012 $1.5 billion unsecured senior line of credit item	Sep. 30, 2012 $1.5 billion unsecured senior line of credit	Sep. 30, 2012 2016 Unsecured Senior Bank Term Loan	Sep. 30, 2012 2017 Unsecured Senior Bank Term Loan	Sep. 30, 2012 Unsecured Senior Notes Payable	Sep. 30, 2012 8.00% Unsecured Senior Convertible Notes	Dec. 31, 2011 8.00% Unsecured Senior Convertible Notes	Sep. 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	$ 2,590,609,000	$ 2,569,000	$ 51,565,000	$ 229,547,000	$ 12,819,000	$ 983,276,000	$ 1,310,833,000	$ 640,815,000																		$ 50,000,000	$ 750,000,000	$ 600,000,000	$ 549,794,000
Unhedged Variable rate	441,535,000	60,000	240,000	76,240,000	1,995,000		363,000,000	78,535,000																		363,000,000
Total consolidated debt	3,032,144,000	2,629,000	51,805,000	305,787,000	14,814,000	983,276,000	1,673,833,000	719,350,000	8,012,000	26,228,000	7,527,000	213,451,000	76,000,000	6,622,000	7,701,000	540,000	6,077,000	1,995,000	81,039,000	32,394,000	134,728,000	116,685,000	874,000			413,000,000	750,000,000	600,000,000	549,794,000
Percentage of Total	100.00%							23.70%																		13.60%	24.80%	19.80%	18.10%
Weighted average interest rate (as a percent)	3.93%							5.76%																		1.46%	3.12%	3.84%	4.61%
Weighted Average Remaining Term	4 years 10 months 24 days							3 years 2 months 12 days																		4 years 7 months 6 days	3 years 9 months 18 days	4 years 3 months 18 days	9 years 6 months
Percentage of fixed rate/hedged total debt	85.00%
Percentage of unhedged floating rate total debt	15.00%
Unsecured credit facility maximum permitted borrowings																										1,100,000,000
Amount available for borrowing																											1,500,000,000
Stated interest rate (as a percent)								5.70%	6.21%	6.36%	6.14%	5.26%	2.33%	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																U.S. treasury bills Rate		LIBOR								LIBOR	LIBOR	LIBOR
Applicable margin (as a percent)																3.00%		1.50%								1.20%	1.75%	1.50%
Effective rate (as a percent)	4.01%							5.76%	6.21%	6.36%	6.14%	5.59%	2.33%	4.88%	4.00%	6.00%	4.50%	1.74%	5.73%	5.82%	6.35%	7.75%	6.50%	3.93%		1.46%	3.12%	3.84%				4.61%
Total consolidated debt, Effective Interest Rate (as a percent)	3.93%
Future principal payments due on secured and unsecured debt
2012								2,741,000	78,000	135,000		929,000		22,000	29,000	60,000	21,000		393,000	208,000	542,000	320,000	4,000
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								14,826,000									5,788,000	1,995,000	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								111,102,000														110,301,000	801,000			413,000,000		600,000,000	550,000,000
Average/Subtotal	3,033,123,000	2,741,000	52,269,000	305,865,000	14,826,000	983,320,000	1,674,102,000																							250,000	250,000
Unamortized discount	(979,000)	(112,000)	(464,000)	(78,000)	(12,000)	(44,000)	(269,000)																							(10,000)	(15,000)
Average/Total	3,032,144,000	2,629,000	51,805,000	305,787,000	14,814,000	983,276,000	1,673,833,000	719,350,000	8,012,000	26,228,000	7,527,000	213,451,000	76,000,000	6,622,000	7,701,000	540,000	6,077,000	1,995,000	81,039,000	32,394,000	134,728,000	116,685,000	874,000			413,000,000	750,000,000	600,000,000	549,794,000
Balloon payments	2,993,038,000		41,165,000	297,330,000	7,723,000	980,029,000	1,666,791,000
Principal amortization	$ 39,106,000	$ 2,629,000	$ 10,640,000	$ 8,457,000	$ 7,091,000	$ 3,247,000	$ 7,042,000
Term of U.S. treasury bills																10 years
Interest rate, variable interest rate floor (as a percent)																6.00%
Interest rate, variable interest rate ceiling (as a percent)																8.50%
Annual facility fee (as a percent)																										0.25%
Number of extensions to maturity date																									2
Length of maturity date extension period																									6 months