Unsecured senior line of credit and commercial paper program (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)
Debt Instrument [Line Items]
Effective rate (as a percent)	3.91%
Outstanding Balance	$ 6,796,440
$2.2 billion unsecured senior line of credit
Debt Instrument [Line Items]
Annual facility fee (as a percent)	0.15%
Effective rate (as a percent)	2.89%
Outstanding Balance	$ 384,000
Commercial paper program
Debt Instrument [Line Items]
Commercial paper program issuance balance	750,000
Outstanding Balance	$ 0
Maximum | Commercial paper program
Debt Instrument [Line Items]
Number of maturity days from date of issuance	397
Effective rate (as a percent)	2.29%
Minimum | Commercial paper program
Debt Instrument [Line Items]
Effective rate (as a percent)	1.83%