Secured and unsecured senior debt Schedule of secured and unsecured senior debts (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013
Debt Instrument [Line Items]
Fixed Rate/Hedged Variable Rate	$ 2,584,808
Unhedged Variable rate	476,253
Total Consolidated	3,061,061
Percentage of Total	100.00%
Weighted Average Interest Rate at End of Period	3.76%	[1]
Weighted Average Remaining Terms (in years)	5 years 3 months 18 days
Percentage of fixed rate/hedged total debt	84.00%
Percentage of unhedged floating rate total debt	16.00%
Secured notes payable
Debt Instrument [Line Items]
Fixed Rate/Hedged Variable Rate	586,578
Unhedged Variable rate	122,253
Total Consolidated	708,831
Percentage of Total	23.20%
Weighted Average Interest Rate at End of Period	5.45%	[1]
Weighted Average Remaining Terms (in years)	2 years 2 months 12 days
4.60% unsecured senior notes payable
Debt Instrument [Line Items]
Fixed Rate/Hedged Variable Rate	549,603
Unhedged Variable rate	0
Total Consolidated	549,603
Percentage of Total	17.90%
Weighted Average Interest Rate at End of Period	4.61%	[1]
Weighted Average Remaining Terms (in years)	8 years 3 months 18 days
3.90% unsecured senior notes payable
Debt Instrument [Line Items]
Fixed Rate/Hedged Variable Rate	498,627
Unhedged Variable rate	0
Total Consolidated	498,627
Percentage of Total	16.30%
Weighted Average Interest Rate at End of Period	3.94%	[1]
Weighted Average Remaining Terms (in years)	9 years 6 months
$1.5 billion unsecured senior line of credit
Debt Instrument [Line Items]
Fixed Rate/Hedged Variable Rate	0
Unhedged Variable rate	204,000
Total Consolidated	204,000
Percentage of Total	6.70%
Weighted Average Interest Rate at End of Period	1.27%	[1]
Weighted Average Remaining Terms (in years)	5 years
2016 Unsecured Senior Bank Term Loan
Debt Instrument [Line Items]
Fixed Rate/Hedged Variable Rate	350,000
Unhedged Variable rate	150,000
Total Consolidated	500,000
Percentage of Total	16.30%
Weighted Average Interest Rate at End of Period	2.37%	[1]
Weighted Average Remaining Terms (in years)	2 years 7 months 6 days
2019 Unsecured Senior Bank Term Loan
Debt Instrument [Line Items]
Fixed Rate/Hedged Variable Rate	600,000
Unhedged Variable rate	0
Total Consolidated	$ 600,000
Percentage of Total	19.60%
Weighted Average Interest Rate at End of Period	2.85%	[1]
Weighted Average Remaining Terms (in years)	5 years

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