Secured and unsecured senior debt Detail of secured and unsecured debt (Details) (USD $)	9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013
Summary of fixed rate/hedged and floating rate debt
Fixed Rate/Hedged Variable Rate	$ 3,103,415,000
Unhedged Variable rate	397,700,000
Principal amount	3,501,115,000
Total consolidated debt, Effective Interest	3.39%	[1]
Debt Instrument, Periodic Payment Terms, Balloon Payment to be Paid	3,470,517,000
Debt Instrument, Annual Principal Payment	30,598,000
Future principal payments due on secured and unsecured debt
Repayments of Unsecured Debt	125,000,000		250,000,000
2014
Summary of fixed rate/hedged and floating rate debt
Fixed Rate/Hedged Variable Rate	2,055,000
Unhedged Variable rate	60,000
Principal amount	2,115,000
Debt Instrument, Periodic Payment Terms, Balloon Payment to be Paid	0
Debt Instrument, Annual Principal Payment	2,115,000
2015
Summary of fixed rate/hedged and floating rate debt
Fixed Rate/Hedged Variable Rate	14,290,000
Unhedged Variable rate	46,596,000
Principal amount	60,886,000
Debt Instrument, Periodic Payment Terms, Balloon Payment to be Paid	52,336,000
Debt Instrument, Annual Principal Payment	8,550,000
2016
Summary of fixed rate/hedged and floating rate debt
Fixed Rate/Hedged Variable Rate	591,423,000
Unhedged Variable rate	42,952,000
Principal amount	634,375,000
Debt Instrument, Periodic Payment Terms, Balloon Payment to be Paid	629,851,000
Debt Instrument, Annual Principal Payment	4,524,000
2017
Summary of fixed rate/hedged and floating rate debt
Fixed Rate/Hedged Variable Rate	3,042,000
Unhedged Variable rate	166,092,000
Principal amount	169,134,000
Debt Instrument, Periodic Payment Terms, Balloon Payment to be Paid	166,092,000
Debt Instrument, Annual Principal Payment	3,042,000
2018
Summary of fixed rate/hedged and floating rate debt
Fixed Rate/Hedged Variable Rate	3,253,000
Unhedged Variable rate	0
Principal amount	3,253,000
Debt Instrument, Periodic Payment Terms, Balloon Payment to be Paid	0
Debt Instrument, Annual Principal Payment	3,253,000
Thereafter
Summary of fixed rate/hedged and floating rate debt
Fixed Rate/Hedged Variable Rate	2,489,352,000
Unhedged Variable rate	142,000,000
Principal amount	2,631,352,000
Debt Instrument, Periodic Payment Terms, Balloon Payment to be Paid	2,622,238,000
Debt Instrument, Annual Principal Payment	9,114,000
Secured notes payable
Summary of fixed rate/hedged and floating rate debt
Fixed Rate/Hedged Variable Rate	406,125,000
Unhedged Variable rate	230,700,000
Principal amount	636,825,000
Stated interest rate (as a percent)	4.71%
Effective rate (as a percent)	4.67%	[1]
Future principal payments due on secured and unsecured debt
2014	2,194,000
2015	61,212,000
2016	259,712,000
2017	169,484,000
2018	3,615,000
Thereafter	140,608,000
Other secured notes payable maturing on 11/18/14
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)	6.00%
Effective rate (as a percent)	6.00%	[1]
Maturity Date	Nov. 18, 2014	[2]
Future principal payments due on secured and unsecured debt
2014	60,000
2015	0
2016	0
2017	0
2018	0
Thereafter	0
Outstanding Balance	60,000
Secured notes payable from bank maturing on 6/1/15
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)	5.64%
Effective rate (as a percent)	4.50%	[1]
Maturity Date	Jun. 01, 2015	[2]
Future principal payments due on secured and unsecured debt
2014	34,000
2015	5,777,000
2016	0
2017	0
2018	0
Thereafter	0
Outstanding Balance	5,811,000
Secured notes payable maturing on 7/1/15
Summary of fixed rate/hedged and floating rate debt
Effective rate (as a percent)	1.66%	[1]
Maturity Date	Jul. 01, 2015	[2]
Number of extensions available under line of credit	2
Future principal payments due on secured and unsecured debt
2014	0
2015	46,596,000
2016	0
2017	0
2018	0
Thereafter	0
Outstanding Balance	46,596,000
CMBS maturing on 1/1/16
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)	5.73%
Effective rate (as a percent)	5.73%	[1]
Maturity Date	Jan. 01, 2016	[2]
Future principal payments due on secured and unsecured debt
2014	440,000
2015	1,816,000
2016	75,501,000
2017	0
2018	0
Thereafter	0
Outstanding Balance	77,757,000
CMBS maturing on 4/1/16
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)	5.82%
Effective rate (as a percent)	5.82%	[1]
Maturity Date	Apr. 01, 2016	[2]
Future principal payments due on secured and unsecured debt
2014	234,000
2015	988,000
2016	29,389,000
2017	0
2018	0
Thereafter	0
Outstanding Balance	30,611,000
Secured Debt Maturing On 15 April 2016
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)	5.74%
Effective rate (as a percent)	3.00%	[1]
Maturity Date	Apr. 15, 2016	[2]
Future principal payments due on secured and unsecured debt
2014	42,000
2015	175,000
2016	6,916,000
2017	0
2018	0
Thereafter	0
Outstanding Balance	7,133,000
Secured notes payable maturing on 06/1/16
Summary of fixed rate/hedged and floating rate debt
Effective rate (as a percent)	1.56%	[1]
Maturity Date	Jun. 01, 2016	[2]
Number of extensions available under line of credit	2
Future principal payments due on secured and unsecured debt
2014	0
2015	0
2016	17,952,000
2017	0
2018	0
Thereafter	0
Outstanding Balance	17,952,000
CMBS maturing on 8/1/16
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)	6.35%
Effective rate (as a percent)	6.35%	[1]
Maturity Date	Aug. 01, 2016	[2]
Future principal payments due on secured and unsecured debt
2014	619,000
2015	2,652,000
2016	126,715,000
2017	0
2018	0
Thereafter	0
Outstanding Balance	129,986,000
Secured Debt from Bank Maturing on 20 January 2017
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)	2.14%
Effective rate (as a percent)	2.14%	[1]
Maturity Date	Jan. 20, 2017	[2]
Future principal payments due on secured and unsecured debt
2014	0
2015	0
2016	0
2017	76,000,000
2018	0
Thereafter	0
Outstanding Balance	76,000,000
Secured notes payable maturing on 8/23/17
Summary of fixed rate/hedged and floating rate debt
Effective rate (as a percent)	1.50%	[1]
Maturity Date	Aug. 23, 2017	[2]
Future principal payments due on secured and unsecured debt
2014	0
2015	0
2016	0
2017	90,092,000
2018	0
Thereafter	0
Outstanding Balance	90,092,000
Secured Debt Other Maturing 1 April 2020
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)	7.75%
Effective rate (as a percent)	7.75%	[1]
Maturity Date	Apr. 01, 2020	[2]
Future principal payments due on secured and unsecured debt
2014	374,000
2015	1,570,000
2016	1,696,000
2017	1,832,000
2018	1,979,000
Thereafter	106,490,000
Outstanding Balance	113,941,000
Secured Debt Maturing on January 2023
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)	4.66%
Effective rate (as a percent)	4.66%	[1]
Maturity Date	Jan. 01, 2023	[2]
Future principal payments due on secured and unsecured debt
2014	337,000
2015	1,402,000
2016	1,464,000
2017	1,540,000
2018	1,614,000
Thereafter	33,367,000
Outstanding Balance	39,724,000
Secured notes payable from bank maturing on 6/1/37
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)	6.50%
Effective rate (as a percent)	6.50%	[1]
Maturity Date	Jun. 01, 2037	[2]
Future principal payments due on secured and unsecured debt
2014	0
2015	18,000
2016	19,000
2017	20,000
2018	22,000
Thereafter	751,000
Outstanding Balance	830,000
Unamortized Premiums
Future principal payments due on secured and unsecured debt
Unamortized premium	332,000
Unamortized Premiums | 2014
Future principal payments due on secured and unsecured debt
Unamortized premium	54,000
Unamortized Premiums | 2015
Future principal payments due on secured and unsecured debt
Unamortized premium	218,000
Unamortized Premiums | 2016
Future principal payments due on secured and unsecured debt
Unamortized premium	60,000
Unamortized Premiums | 2017
Future principal payments due on secured and unsecured debt
Unamortized premium	0
Unamortized Premiums | 2018
Future principal payments due on secured and unsecured debt
Unamortized premium	0
Unamortized Premiums | Thereafter
Future principal payments due on secured and unsecured debt
Unamortized premium	0
Unsecured Debt
Summary of fixed rate/hedged and floating rate debt
Principal amount	2,864,290,000
Effective rate (as a percent)	3.11%	[1]
Unsecured Debt | 2014
Future principal payments due on secured and unsecured debt
2014	(79,000)
Unsecured Debt | 2015
Future principal payments due on secured and unsecured debt
2015	(326,000)
Unsecured Debt | 2016
Future principal payments due on secured and unsecured debt
2016	374,663,000
Unsecured Debt | 2017
Future principal payments due on secured and unsecured debt
2017	(350,000)
Unsecured Debt | 2018
Future principal payments due on secured and unsecured debt
2018	(362,000)
Unsecured Debt | Thereafter
Future principal payments due on secured and unsecured debt
Thereafter	2,490,744,000
2016 Unsecured Senior Bank Term Loan
Summary of fixed rate/hedged and floating rate debt
Fixed Rate/Hedged Variable Rate	350,000,000
Unhedged Variable rate	25,000,000
Principal amount	375,000,000
Effective rate (as a percent)	1.42%	[1]
Maturity Date	Jul. 31, 2016	[2]
Future principal payments due on secured and unsecured debt
2014	0
2015	0
2016	375,000,000
2017	0
2018	0
Thereafter	0
Outstanding Balance	375,000,000
2019 Unsecured Senior Bank Term Loan
Summary of fixed rate/hedged and floating rate debt
Fixed Rate/Hedged Variable Rate	600,000,000
Unhedged Variable rate	0
Principal amount	600,000,000
Effective rate (as a percent)	2.05%	[1]
Maturity Date	Jan. 03, 2019	[2]
Future principal payments due on secured and unsecured debt
2014	0
2015	0
2016	0
2017	0
2018	0
Thereafter	600,000,000
Outstanding Balance	600,000,000
$1.5 billion unsecured senior line of credit
Summary of fixed rate/hedged and floating rate debt
Fixed Rate/Hedged Variable Rate	0
Unhedged Variable rate	142,000,000
Principal amount	142,000,000
Effective rate (as a percent)	1.25%	[1]
Maturity Date	Jan. 03, 2019	[2]
Annual facility fee (as a percent)	0.20%
Future principal payments due on secured and unsecured debt
2014	0
2015	0
2016	0
2017	0
2018	0
Thereafter	142,000,000
Outstanding Balance	142,000,000
Unsecured Senior Notes Due in January 2020 [Member]
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)	2.75%
Effective rate (as a percent)	2.79%	[1]
Maturity Date	Jan. 15, 2020	[2]
Future principal payments due on secured and unsecured debt
2014	0
2015	0
2016	0
2017	0
2018	0
Thereafter	400,000,000
Outstanding Balance	400,000,000
4.60% unsecured senior notes payable
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)	4.60%
Effective rate (as a percent)	4.61%	[1]
Maturity Date	Apr. 01, 2022	[2]
Future principal payments due on secured and unsecured debt
2014	0
2015	0
2016	0
2017	0
2018	0
Thereafter	550,000,000
Outstanding Balance	550,000,000
3.90% unsecured senior notes payable
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)	3.90%
Effective rate (as a percent)	3.94%	[1]
Maturity Date	Jun. 15, 2023	[2]
Future principal payments due on secured and unsecured debt
2014	0
2015	0
2016	0
2017	0
2018	0
Thereafter	500,000,000
Outstanding Balance	500,000,000
Unsecured Senior Notes Due in July 2029 [Member]
Summary of fixed rate/hedged and floating rate debt
Stated interest rate (as a percent)	4.50%
Effective rate (as a percent)	4.51%	[1]
Maturity Date	Jul. 30, 2029	[2]
Future principal payments due on secured and unsecured debt
2014	0
2015	0
2016	0
2017	0
2018	0
Thereafter	300,000,000
Outstanding Balance	300,000,000
Unamortized Discounts
Future principal payments due on secured and unsecured debt
Unamortized discount	(2,710,000)
Unamortized Discounts | 2014
Future principal payments due on secured and unsecured debt
Unamortized discount	(79,000)
Unamortized Discounts | 2015
Future principal payments due on secured and unsecured debt
Unamortized discount	(326,000)
Unamortized Discounts | 2016
Future principal payments due on secured and unsecured debt
Unamortized discount	(337,000)
Unamortized Discounts | 2017
Future principal payments due on secured and unsecured debt
Unamortized discount	(350,000)
Unamortized Discounts | 2018
Future principal payments due on secured and unsecured debt
Unamortized discount	(362,000)
Unamortized Discounts | Thereafter
Future principal payments due on secured and unsecured debt
Unamortized discount	(1,256,000)
Senior Notes [Member]
Summary of fixed rate/hedged and floating rate debt
Fixed Rate/Hedged Variable Rate	1,747,290,000
Unhedged Variable rate	0
Principal amount	1,747,290,000
Stated interest rate (as a percent)	3.50%
Effective rate (as a percent)	3.98%
Future principal payments due on secured and unsecured debt
Debt Instrument, Face Amount	$ 700,000,000
LIBOR | Secured notes payable maturing on 7/1/15
Summary of fixed rate/hedged and floating rate debt
Base rate	LIBOR
Basis spread on LIBOR (as a percent)	1.50%
LIBOR | Secured notes payable maturing on 06/1/16
Summary of fixed rate/hedged and floating rate debt
Base rate	LIBOR
Basis spread on LIBOR (as a percent)	1.40%
LIBOR | Secured notes payable maturing on 8/23/17
Summary of fixed rate/hedged and floating rate debt
Base rate	LIBOR
Basis spread on LIBOR (as a percent)	1.35%
LIBOR | 2016 Unsecured Senior Bank Term Loan
Summary of fixed rate/hedged and floating rate debt
Base rate	LIBOR
Basis spread on LIBOR (as a percent)	1.20%
LIBOR | 2019 Unsecured Senior Bank Term Loan
Summary of fixed rate/hedged and floating rate debt
Base rate	LIBOR
Basis spread on LIBOR (as a percent)	1.20%
LIBOR | $1.5 billion unsecured senior line of credit
Summary of fixed rate/hedged and floating rate debt
Base rate	LIBOR
Basis spread on LIBOR (as a percent)	1.10%	[3]

[1] Represents the weighted average 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.

[2] Includes any extension options that we control.

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