Secured and unsecured senior debt Outstanding consolidated indebtedness (Details) (USD $)											9 Months Ended																												1 Months Ended		3 Months Ended		7 Months Ended	9 Months Ended		0 Months Ended	1 Months Ended	3 Months Ended		9 Months Ended		1 Months Ended	3 Months Ended		9 Months Ended			9 Months Ended
	Sep. 30, 2013		Sep. 30, 2013 2013	Sep. 30, 2013 2014	Sep. 30, 2013 2015	Sep. 30, 2013 2016	Sep. 30, 2013 2017	Sep. 30, 2013 Thereafter	Sep. 30, 2013 Secured notes payable		Sep. 30, 2013 Secured notes payable from insurance co. maturing on 4/1/14		Sep. 30, 2013 Secured notes payable from bank maturing on 4/20/14		Sep. 30, 2013 Secured notes payable from bank maturing on 7/1/14		Sep. 30, 2013 Secured notes payable from bank maturing on 11/1/14		Sep. 30, 2013 Other secured notes payable maturing on 11/18/14		Sep. 30, 2013 Secured notes payable from bank maturing on 6/1/15		Sep. 30, 2013 CMBS maturing on 1/1/16		Sep. 30, 2013 CMBS maturing on 4/1/16		Sep. 30, 2013 CMBS maturing on 8/1/16		Sep. 30, 2013 Secured notes payable maturing on 7/1/15 Option Extension_Option		Sep. 30, 2013 Secured notes payable maturing on 06/1/16 Option Extension_Option		Sep. 30, 2013 Secured notes payable maturing on 8/23/17 Extension_Option		Sep. 30, 2013 Other secured notes payable maturing on 4/1/20		Sep. 30, 2013 Secured notes payable from bank maturing on 6/1/37		Aug. 30, 2013 $1.5 billion unsecured senior line of credit	Apr. 30, 2012 $1.5 billion unsecured senior line of credit	Sep. 30, 2013 $1.5 billion unsecured senior line of credit		Jul. 31, 2013 $1.5 billion unsecured senior line of credit	Sep. 30, 2013 $1.5 billion unsecured senior line of credit		Jul. 26, 2013 2016 Unsecured Senior Bank Term Loan	Aug. 30, 2013 2016 Unsecured Senior Bank Term Loan	Sep. 30, 2013 2016 Unsecured Senior Bank Term Loan		Sep. 30, 2013 2016 Unsecured Senior Bank Term Loan		Aug. 30, 2013 Unsecured Bank Term Loan 2019	Sep. 30, 2013 Unsecured Bank Term Loan 2019		Sep. 30, 2013 Unsecured Bank Term Loan 2019		Jul. 31, 2013 Unsecured Bank Term Loan 2019	Sep. 30, 2013 Unsecured Senior Notes Due in April 2022		Sep. 30, 2013 Unsecured Senior Notes 2023
Summary of fixed rate/hedged and floating rate debt
Fixed Rate/Hedged Variable Rate	$ 2,587,316,000		$ 2,471,000	$ 229,176,000	$ 12,693,000	$ 583,143,000	$ 1,668,000	$ 1,758,165,000	$ 589,126,000																																$ 0			$ 0				$ 350,000,000		$ 350,000,000			$ 600,000,000		$ 600,000,000
Unhedged Variable rate	283,527,000		60,000	76,240,000	43,227,000	150,000,000	0	14,000,000	119,527,000																																14,000,000			14,000,000				150,000,000		150,000,000			0		0
Principal amount	2,870,843,000		2,531,000	305,416,000	55,920,000	733,143,000	1,668,000	1,772,165,000	708,653,000																																14,000,000			14,000,000				500,000,000		500,000,000			600,000,000		600,000,000
Stated interest rate (as a percent)									5.41%		5.26%		2.17%		6.05%		5.39%		6.00%		5.64%		5.73%		5.82%		6.35%								7.75%		6.50%																					4.60%		3.90%
Base rate																													LIBOR		LIBOR		LIBOR						LIBOR				LIBOR	LIBOR	[1]	LIBOR	LIBOR			LIBOR		LIBOR			LIBOR
Basis spread on LIBOR (as a percent)																													1.50%		1.40%		1.35%								1.10%	[1]	1.20%	1.10%	[1]	1.75%		1.20%		1.20%			1.20%		1.20%		1.50%
Effective rate (as a percent)	3.91%	[2]							5.47%	[2]	5.59%	[2]	2.17%	[2]	4.88%	[2]	4.00%	[2]	6.00%	[2]	4.50%	[2]	5.73%	[2]	5.82%	[2]	6.35%	[2]	1.69%	[2]	1.59%	[2]	1.54%	[2]	7.75%	[2]	6.50%	[2]			1.28%	[2]		1.28%	[2]			1.70%	[2]	1.70%	[2]		3.30%	[2]	3.30%	[2]		4.61%		3.94%
Maturity date											Apr. 01, 2014	[3]	Apr. 20, 2014	[3],[4]	Jul. 01, 2014	[3]	Nov. 01, 2014	[3]	Nov. 18, 2014	[3]	Jun. 01, 2015	[3]	Jan. 01, 2016	[3]	Apr. 01, 2016	[3]	Aug. 01, 2016	[3]	Jul. 01, 2015	[3],[5]	Jun. 01, 2016	[3],[6]	Aug. 23, 2017	[3],[7]	Apr. 01, 2020	[3]	Jun. 01, 2037	[3]			Jan. 03, 2019	[3]						Jul. 31, 2016	[3]				Jan. 03, 2019	[3]				Apr. 01, 2022	[3]	Jun. 15, 2023	[3]
Aggregate commitments of construction loan																													55,000,000		33,000,000		245,400,000
Annual facility fee (as a percent)																																							0.20%	0.20%	0.20%		0.25%
Number of extensions on line of credit																													2		2		1
Future principal payments due on secured and unsecured debt
2013									2,677,000		979,000		0		24,000		30,000		60,000		22,000		416,000		221,000		580,000		0		0		0		345,000		0				0			0				0		0			0		0			0		0
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		0		0		0		1,453,000		17,000				0			0				0		0			0		0			0		0
2015									56,059,000		0		0		0		0		0		5,788,000		1,816,000		988,000		2,652,000		43,227,000		0		0		1,570,000		18,000				0			0				0		0			0		0			0		0
2016									233,320,000		0		0		0		0		0		0		75,501,000		29,389,000		126,715,000		0		0		0		1,696,000		19,000				0			0				500,000,000		500,000,000			0		0			0		0
2017									1,852,000		0		0		0		0		0		0		0		0		0		0		0		0		1,832,000		20,000				0			0				0		0			0		0			0		0
Thereafter									109,242,000		0		0		0		0		0		0		0		0		0		0		0		0		108,469,000		773,000				14,000,000			14,000,000				0		0			600,000,000		600,000,000			550,000,000		500,000,000
Unamortized discount	(1,922,000)		(146,000)	(199,000)	(139,000)	(177,000)	(184,000)	(1,077,000)
Principal amount	2,872,765,000		2,677,000	305,615,000	56,059,000	733,320,000	1,852,000	1,773,242,000	708,765,000		209,662,000		76,000,000		6,482,000		7,525,000		300,000		5,948,000		79,446,000		31,529,000		132,434,000		43,227,000		0		0		115,365,000		847,000				14,000,000			14,000,000				500,000,000		500,000,000			600,000,000		600,000,000			550,000,000		500,000,000
Total consolidated debt, Effective Interest	3.91%
Balloon payments	2,844,416,000		0	297,080,000	48,955,000	730,029,000	0	1,768,352,000
Principal amortization	$ 26,427,000		$ 2,531,000	$ 8,336,000	$ 6,965,000	$ 3,114,000	$ 1,668,000	$ 3,813,000

[1] In addition to the stated rate, the line of credit is subject to an annual facility fee of 0.20%.

[2] Represents the weighted average contractual interest rate as of the end of the period plus the impact of debt premiums/discounts and our interest rate swap agreements. The weighted average interest rate excludes bank fees and amortization of loan fees.

[3] Includes any extension options that we control.

[4] We are having discussions with the lender on an extension of the maturity date.

[5] Secured construction loan with aggregate commitments of $55.0 million. We have two, one-year options to extend the stated maturity date to July 1, 2017, subject to certain conditions.

[6] Secured construction loan with aggregate commitments of $33.0 million. We have two, one-year options to extend the stated maturity date to June 1, 2018, subject to certain conditions. As of September 30, 2013, we had not drawn on the loan.

[7] Secured construction loan with aggregate commitments of $245.4 million. We have a one-year option to extend the stated maturity date to August 23, 2018, subject to certain conditions. As of September 30, 2013, we had not drawn on the loan.