Secured and unsecured senior debt Summary of secured and unsecured debt (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2015 USD ($)
Debt Instrument [Line Items]
Fixed Rate/Hedged Variable Rate	$ 3,275,629
Unhedged Variable rate	1,038,603
Total Consolidated	$ 4,314,232
Percentage of Total	100.00%
Weighted-Average Interest Rate at End of Period (as a percent)	2.97%	[1]
Weighted Average Remaining Terms (in years)	5 years 1 month 6 days
Percentage of fixed rate/hedged total debt	76.00%
Percentage of unhedged floating rate total debt	24.00%
Construction Loans [Member]
Debt Instrument [Line Items]
Outstanding Balance	$ 219,603
Secured notes payable
Debt Instrument [Line Items]
Fixed Rate/Hedged Variable Rate	478,016
Unhedged Variable rate	295,603
Total Consolidated	$ 773,619
Percentage of Total	17.90%
Weighted-Average Interest Rate at End of Period (as a percent)	4.23%	[1]
Weighted Average Remaining Terms (in years)	2 years 7 months 6 days
Stated interest rate (as a percent)	4.35%
Senior Notes [Member]
Debt Instrument [Line Items]
Fixed Rate/Hedged Variable Rate	$ 1,747,613
Unhedged Variable rate	0
Total Consolidated	$ 1,747,613
Percentage of Total	40.50%
Weighted-Average Interest Rate at End of Period (as a percent)	3.98%
Weighted Average Remaining Terms (in years)	7 years 7 months 6 days
$1.5 billion unsecured senior line of credit
Debt Instrument [Line Items]
Outstanding Balance	$ 843,000
Fixed Rate/Hedged Variable Rate	100,000
Unhedged Variable rate	743,000
Total Consolidated	$ 843,000
Percentage of Total	19.60%
Weighted-Average Interest Rate at End of Period (as a percent)	1.19%	[1],[2]
Weighted Average Remaining Terms (in years)	3 years 3 months 18 days
2019 Unsecured Senior Bank Term Loan
Debt Instrument [Line Items]
Outstanding Balance	$ 600,000
Fixed Rate/Hedged Variable Rate	600,000
Unhedged Variable rate	0
Total Consolidated	$ 600,000
Percentage of Total	13.90%
Weighted-Average Interest Rate at End of Period (as a percent)	1.72%	[1]
Weighted Average Remaining Terms (in years)	3 years 3 months 18 days
Unsecured Bank Term Loan 2021 [Member]
Debt Instrument [Line Items]
Outstanding Balance	$ 350,000
Fixed Rate/Hedged Variable Rate	350,000
Unhedged Variable rate	0
Total Consolidated	$ 350,000
Percentage of Total	8.10%
Weighted-Average Interest Rate at End of Period (as a percent)	1.52%	[1]
Weighted Average Remaining Terms (in years)	5 years 3 months 18 days
LIBOR | Secured Debt from Bank Maturing on 1 July 2015 [Member]
Debt Instrument [Line Items]
Debt Instrument, Description of Variable Rate Basis	LIBOR
Applicable margin (as a percent)	1.50%
LIBOR | $1.5 billion unsecured senior line of credit
Debt Instrument [Line Items]
Debt Instrument, Description of Variable Rate Basis	LIBOR
Applicable margin (as a percent)	1.10%	[3]
LIBOR | 2019 Unsecured Senior Bank Term Loan
Debt Instrument [Line Items]
Debt Instrument, Description of Variable Rate Basis	LIBOR
Applicable margin (as a percent)	1.20%
LIBOR | Unsecured Bank Term Loan 2021 [Member]
Debt Instrument [Line Items]
Debt Instrument, Description of Variable Rate Basis	LIBOR
Applicable margin (as a percent)	1.10%

[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] Our unsecured senior line of credit contains a feature that allows lenders to competitively bid on the interest rate for borrowings under the facility. This may result in an interest rate that is below the stated rate of LIBOR+1.10%. In addition to the cost of borrowing, the facility is subject to an annual facility fee of 0.20%, based on the aggregate commitments outstanding.

[3] We have two, one-year options to extend the stated maturity date to June 1, 2018, subject to certain conditions.