Secured and unsecured senior debt Schedule of secured and unsecured senior debts (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013
Debt Instrument [Line Items]
Fixed Rate/Hedged Variable Rate	$ 2,587,316
Unhedged Variable rate	283,527
Principal amount	2,870,843
Percentage of Total	100.00%
Effective rate (as a percent)	3.91%	[1]
Weighted Average Remaining Terms (in years)	5 years 6 months 12 days
Percentage of fixed rate/hedged total debt	90.00%
Percentage of unhedged floating rate total debt	10.00%
Secured Debt
Debt Instrument [Line Items]
Fixed Rate/Hedged Variable Rate	589,126
Unhedged Variable rate	119,527
Principal amount	708,653
Percentage of Total	24.70%
Effective rate (as a percent)	5.47%	[1]
Weighted Average Remaining Terms (in years)	2 years 6 months
Senior Notes
Debt Instrument [Line Items]
Fixed Rate/Hedged Variable Rate	1,048,190
Unhedged Variable rate	0
Principal amount	1,048,190
Percentage of Total	36.50%
Effective rate (as a percent)	4.29%	[1]
Weighted Average Remaining Terms (in years)	9 years 1 month 6 days
Line of Credit
Debt Instrument [Line Items]
Fixed Rate/Hedged Variable Rate	0
Unhedged Variable rate	14,000
Principal amount	14,000
Percentage of Total	0.50%
Effective rate (as a percent)	1.28%	[1]
Weighted Average Remaining Terms (in years)	5 years 3 months 12 days
Unsecured Bank Term Loan 2016
Debt Instrument [Line Items]
Fixed Rate/Hedged Variable Rate	350,000
Unhedged Variable rate	150,000
Principal amount	500,000
Percentage of Total	17.40%
Effective rate (as a percent)	1.70%	[1]
Weighted Average Remaining Terms (in years)	2 years 9 months 12 days
Unsecured Bank Term Loan 2019
Debt Instrument [Line Items]
Fixed Rate/Hedged Variable Rate	600,000
Unhedged Variable rate	0
Principal amount	$ 600,000
Percentage of Total	20.90%
Effective rate (as a percent)	3.30%	[1]
Weighted Average Remaining Terms (in years)	5 years 3 months 12 days

[1] 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.