Secured and unsecured senior debt Summary of secured and unsecured debt (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($)
Debt Instrument
Long-term Debt	$ 4,251,357
Long-term Debt, Percentage Bearing Variable Interest, Amount, Net	565,683
Total Consolidated	$ 4,817,040
Percentage of Total	100.00%
Weighted-Average Interest Rate at End of Period (as a percent)	3.76%
Weighted Average Remaining Terms (in years)	5 years 3 months 18 days
Percentage of fixed rate/hedged total debt	88.00%
Percentage of unhedged floating rate total debt	12.00%
Secured notes payable
Debt Instrument
Long-term Debt	$ 902,207
Long-term Debt, Percentage Bearing Variable Interest, Amount, Net	251,683
Total Consolidated	$ 1,153,890
Percentage of Total	24.00%
Weighted-Average Interest Rate at End of Period (as a percent)	3.80%
Weighted Average Remaining Terms (in years)	2 years 9 months 18 days
Unsecured senior notes payable
Debt Instrument
Long-term Debt	$ 2,801,290
Long-term Debt, Percentage Bearing Variable Interest, Amount, Net	0
Total Consolidated	$ 2,801,290
Percentage of Total	58.20%
Weighted-Average Interest Rate at End of Period (as a percent)	4.16%
Weighted Average Remaining Terms (in years)	7 years
$1.65 billion unsecured senior line of credit
Debt Instrument
Long-term Debt	$ 0
Long-term Debt, Percentage Bearing Variable Interest, Amount, Net	314,000
Total Consolidated	$ 314,000
Percentage of Total	6.50%
Weighted-Average Interest Rate at End of Period (as a percent)	2.00%
Weighted Average Remaining Terms (in years)	4 years 1 month 6 days
2019 Unsecured Senior Bank Term Loan
Debt Instrument
Long-term Debt	$ 199,543
Long-term Debt, Percentage Bearing Variable Interest, Amount, Net	0
Total Consolidated	$ 199,543
Percentage of Total	4.10%
Weighted-Average Interest Rate at End of Period (as a percent)	2.84%
Weighted Average Remaining Terms (in years)	1 year 3 months 18 days
2021 Unsecured Senior Bank Term Loan
Debt Instrument
Long-term Debt	$ 348,317
Long-term Debt, Percentage Bearing Variable Interest, Amount, Net	0
Total Consolidated	$ 348,317
Percentage of Total	7.20%
Weighted-Average Interest Rate at End of Period (as a percent)	2.56%
Weighted Average Remaining Terms (in years)	3 years 3 months 18 days