Summary of secured and unsecured senior debts (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)
Debt Instrument [Line Items]
Fixed rate debt	$ 6,393,479
Variable-rate debt	384,000
Total Consolidated	$ 6,777,479
Percentage of Total	100.00%
Weighted Average Interest Rate at End of Period	3.91%
Weighted Average Remaining Terms (in years)	10 years 4 months 24 days
Percentage of fixed rate to total debt	94.00%
Percentage of variable-rate to total debt	6.00%
Secured notes payable
Debt Instrument [Line Items]
Fixed rate debt	$ 349,352
Variable-rate debt	0
Total Consolidated	$ 349,352
Percentage of Total	5.20%
Weighted Average Interest Rate at End of Period	3.57%
Weighted Average Remaining Terms (in years)	4 years
Unsecured senior notes
Debt Instrument [Line Items]
Fixed rate debt	$ 6,044,127
Variable-rate debt	0
Total Consolidated	$ 6,044,127
Percentage of Total	89.10%
Weighted Average Interest Rate at End of Period	3.99%
Weighted Average Remaining Terms (in years)	11 years 2 months 12 days
$2.2 billion unsecured senior line of credit
Debt Instrument [Line Items]
Fixed rate debt	$ 0
Variable-rate debt	384,000
Total Consolidated	$ 384,000
Percentage of Total	5.70%
Weighted Average Interest Rate at End of Period	2.89%
Weighted Average Remaining Terms (in years)	4 years 1 month 6 days
Commercial paper program
Debt Instrument [Line Items]
Fixed rate debt	$ 0
Variable-rate debt	0
Total Consolidated	$ 0
Percentage of Total	0.00%