Secured and unsecured senior debt Summary of secured and unsecured debt (Details) (USD $)	6 Months Ended													0 Months Ended
	Jun. 30, 2014		Jun. 30, 2013	Jun. 30, 2014 Secured notes payable		Jun. 30, 2014 Senior Notes [Member]		Jun. 30, 2014 $1.5 billion unsecured senior line of credit		Jun. 30, 2014 2016 Unsecured Senior Bank Term Loan		Jun. 30, 2014 2019 Unsecured Senior Bank Term Loan		Jul. 18, 2014 Unsecured Senior Notes Due in 2020 and 2029 [Member] Subsequent Event [Member] Senior Notes [Member]	Jul. 18, 2014 $1.5 billion unsecured senior line of credit Subsequent Event [Member]	Jul. 18, 2014 2016 Unsecured Senior Bank Term Loan Subsequent Event [Member]
Debt Instrument [Line Items]
Proceeds from Unsecured Notes Payable														$ 694,000,000
Repayments of Unsecured Debt	0		150,000,000												569,000,000	125,000,000
Fixed Rate/Hedged Variable Rate	2,413,965,000			415,655,000		1,048,310,000		0		350,000,000		600,000,000
Unhedged Variable rate	920,896,000			199,896,000		0		571,000,000		150,000,000		0
Total Consolidated	$ 3,334,861,000			$ 615,551,000		$ 1,048,310,000		$ 571,000,000	[1]	$ 500,000,000	[1]	$ 600,000,000
Percentage of Total	100.00%
Total consolidated debt, Effective Interest	3.03%	[2]
Weighted Average Interest Rate at End of Period				4.83%	[2]	4.29%	[2]	1.25%	[2]	1.40%	[2]	2.05%	[2]
Weighted Average Remaining Terms (in years)	5 years 1 month			3 years 2 months		8 years 3 months		4 years 6 months		2 years 1 month		4 years 6 months		9 years 7 months
Percentage of fixed rate/hedged total debt	72.00%
Percentage of unhedged floating rate total debt	28.00%

[1] These amounts do not reflect our unsecured senior notes payable offering completed on July 18, 2014. Net proceeds of $694 million were used to reduce variable-rate debt, including the partial repayment of $125 million of our 2016 Unsecured Senior Bank Term Loan and the reduction of $569 million of borrowings outstanding on our unsecured senior line of credit. See Note 13 – Subsequent Events, to our consolidated financial statements (unaudited) appearing elsewhere in this quarterly report on Form 10-Q for further information.

[2] 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.