Detail of secured and unsecured senior debt (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Sep. 12, 2019 USD ($)	Jul. 15, 2019 USD ($)
Debt Instrument [Line Items]
Effective rate (as a percent)			3.91%
Future principal payments due on secured and unsecured debt
2020			$ 6,418
2021			6,901
2022			7,227
2023			604,548
2024			1,217,559
Thereafter			4,953,787
Outstanding Balance			6,796,440
Unamortized Discount (Premium) and Debt Issuance Costs, Net			(18,961)
Total Consolidated			$ 6,777,479
Secured notes payable
Debt Instrument [Line Items]
Stated interest rate (as a percent)			4.55%
Effective rate (as a percent)			3.57%
Future principal payments due on secured and unsecured debt
2020			$ 6,418
2021			6,901
2022			7,227
2023			104,548
2024			183,559
Thereafter			28,787
Outstanding Balance			337,440
Unamortized Discount (Premium) and Debt Issuance Costs, Net			11,912
Total Consolidated			$ 349,352
Secured notes payable maturing on 1/1/23
Debt Instrument [Line Items]
Stated interest rate (as a percent)			4.66%
Effective rate (as a percent)			4.90%
Maturity date			Jan. 01, 2023
Future principal payments due on secured and unsecured debt
2020			$ 1,621
2021			1,852
2022			1,942
2023			26,259
2024			0
Thereafter			0
Outstanding Balance			31,674
Unamortized Discount (Premium) and Debt Issuance Costs, Net			(198)
Total Consolidated			$ 31,476
Secured notes payable maturing on 3/10/23
Debt Instrument [Line Items]
Stated interest rate (as a percent)			3.93%
Effective rate (as a percent)			3.19%
Maturity date			Mar. 10, 2023
Future principal payments due on secured and unsecured debt
2020			$ 1,565
2021			1,629
2022			1,693
2023			74,517
2024			0
Thereafter			0
Outstanding Balance			79,404
Unamortized Discount (Premium) and Debt Issuance Costs, Net			1,771
Total Consolidated			$ 81,175
Secured notes payable maturing on 2/6/24
Debt Instrument [Line Items]
Stated interest rate (as a percent)			4.82%
Effective rate (as a percent)			3.40%
Maturity date			Feb. 06, 2024
Future principal payments due on secured and unsecured debt
2020			$ 3,207
2021			3,394
2022			3,564
2023			3,742
2024			183,527
Thereafter			0
Outstanding Balance			197,434
Unamortized Discount (Premium) and Debt Issuance Costs, Net			10,978
Total Consolidated			$ 208,412
Secured Notes Payable Maturing on 7/1/26
Debt Instrument [Line Items]
Stated interest rate (as a percent)			4.14%
Effective rate (as a percent)			4.42%
Maturity date			Jul. 01, 2026
Future principal payments due on secured and unsecured debt
2020			$ 0
2021			0
2022			0
2023			0
2024			0
Thereafter			28,200
Outstanding Balance			28,200
Unamortized Discount (Premium) and Debt Issuance Costs, Net			(639)
Total Consolidated			$ 27,561
Secured notes payable maturing on 7/1/36
Debt Instrument [Line Items]
Stated interest rate (as a percent)			6.50%
Effective rate (as a percent)			6.50%
Maturity date			Jul. 01, 2036
Future principal payments due on secured and unsecured debt
2020			$ 25
2021			26
2022			28
2023			30
2024			32
Thereafter			587
Outstanding Balance			728
Unamortized Discount (Premium) and Debt Issuance Costs, Net			0
Total Consolidated			$ 728
Unsecured Debt
Debt Instrument [Line Items]
Effective rate (as a percent)			3.93%
Future principal payments due on secured and unsecured debt
2020			$ 0
2021			0
2022			0
2023			500,000
2024			1,034,000
Thereafter			4,925,000
Outstanding Balance			6,459,000
Unamortized Discount (Premium) and Debt Issuance Costs, Net			(30,873)
Total Consolidated			$ 6,428,127
$2.2 billion unsecured senior line of credit
Debt Instrument [Line Items]
Effective rate (as a percent)			2.89%
Maturity date			Jan. 28, 2024
Future principal payments due on secured and unsecured debt
2020			$ 0
2021			0
2022			0
2023			0
2024			384,000
Thereafter			0
Outstanding Balance			384,000
Unamortized Discount (Premium) and Debt Issuance Costs, Net			0
Total Consolidated			$ 384,000
$2.2 billion unsecured senior line of credit | LIBOR
Debt Instrument [Line Items]
Applicable margin (as a percent)			0.825%
3.90% unsecured senior notes payable
Debt Instrument [Line Items]
Stated interest rate (as a percent)			3.90%
Effective rate (as a percent)			4.04%
Maturity date			Jun. 15, 2023
Future principal payments due on secured and unsecured debt
2020			$ 0
2021			0
2022			0
2023			500,000
2024			0
Thereafter			0
Outstanding Balance			500,000
Unamortized Discount (Premium) and Debt Issuance Costs, Net			(2,065)
Total Consolidated			497,935
4.00% unsecured senior note payable
Debt Instrument [Line Items]
Stated interest rate (as a percent)		4.00%
Effective rate (as a percent)		4.03%
Maturity date		Jan. 15, 2024
Future principal payments due on secured and unsecured debt
2020			0
2021			0
2022			0
2023			0
2024			650,000
Thereafter			0
Outstanding Balance			650,000
Unamortized Discount (Premium) and Debt Issuance Costs, Net			(548)
Total Consolidated			$ 649,452
3.45% unsecured senior notes payable
Debt Instrument [Line Items]
Stated interest rate (as a percent)			3.45%
Effective rate (as a percent)			3.62%
Maturity date			Apr. 30, 2025
Future principal payments due on secured and unsecured debt
2020			$ 0
2021			0
2022			0
2023			0
2024			0
Thereafter			600,000
Outstanding Balance			600,000
Unamortized Discount (Premium) and Debt Issuance Costs, Net			(4,667)
Total Consolidated			$ 595,333
4.30% unsecured senior notes payable
Debt Instrument [Line Items]
Stated interest rate (as a percent)			4.30%
Effective rate (as a percent)			4.50%
Maturity date			Jan. 15, 2026
Future principal payments due on secured and unsecured debt
2020			$ 0
2021			0
2022			0
2023			0
2024			0
Thereafter			300,000
Outstanding Balance			300,000
Unamortized Discount (Premium) and Debt Issuance Costs, Net			(2,942)
Total Consolidated			297,058
3.80% Unsecured Senior Notes Payable
Debt Instrument [Line Items]
Stated interest rate (as a percent)		3.80%
Effective rate (as a percent)		3.96%
Maturity date		Apr. 15, 2026
Future principal payments due on secured and unsecured debt
2020			0
2021			0
2022			0
2023			0
2024			0
Thereafter			350,000
Outstanding Balance		$ 350,000	350,000
Unamortized Discount (Premium) and Debt Issuance Costs, Net			(3,081)
Total Consolidated			$ 346,919
3.95% unsecured senior notes payable due in 2027
Debt Instrument [Line Items]
Stated interest rate (as a percent)			3.95%
Effective rate (as a percent)			4.13%
Maturity date			Jan. 15, 2027
Future principal payments due on secured and unsecured debt
2020			$ 0
2021			0
2022			0
2023			0
2024			0
Thereafter			350,000
Outstanding Balance			350,000
Unamortized Discount (Premium) and Debt Issuance Costs, Net			(3,552)
Total Consolidated			$ 346,448
3.95% unsecured senior notes payable due in 2028
Debt Instrument [Line Items]
Stated interest rate (as a percent)			3.95%
Effective rate (as a percent)			4.07%
Maturity date			Jan. 15, 2028
Future principal payments due on secured and unsecured debt
2020			$ 0
2021			0
2022			0
2023			0
2024			0
Thereafter			425,000
Outstanding Balance			425,000
Unamortized Discount (Premium) and Debt Issuance Costs, Net			(3,403)
Total Consolidated			$ 421,597
4.50% unsecured senior notes payable
Debt Instrument [Line Items]
Stated interest rate (as a percent)			4.50%
Effective rate (as a percent)			4.60%
Maturity date			Jul. 30, 2029
Future principal payments due on secured and unsecured debt
2020			$ 0
2021			0
2022			0
2023			0
2024			0
Thereafter			300,000
Outstanding Balance			300,000
Unamortized Discount (Premium) and Debt Issuance Costs, Net			(2,126)
Total Consolidated			$ 297,874
2.75% Unsecured Senior Notes Payable Due 2029
Debt Instrument [Line Items]
Stated interest rate (as a percent)	2.75%
Effective rate (as a percent)	2.87%
Maturity date	Dec. 15, 2029		Dec. 15, 2029
Future principal payments due on secured and unsecured debt
2020			$ 0
2021			0
2022			0
2023			0
2024			0
Thereafter			400,000
Outstanding Balance	$ 400,000		400,000
Unamortized Discount (Premium) and Debt Issuance Costs, Net			(4,089)
Total Consolidated			$ 395,911
4.70% unsecured senior note payable
Debt Instrument [Line Items]
Stated interest rate (as a percent)			4.70%
Effective rate (as a percent)			4.81%
Maturity date			Jul. 01, 2030
Future principal payments due on secured and unsecured debt
2020			$ 0
2021			0
2022			0
2023			0
2024			0
Thereafter			450,000
Outstanding Balance			450,000
Unamortized Discount (Premium) and Debt Issuance Costs, Net			(3,903)
Total Consolidated			$ 446,097
3.375% Unsecured Senior Notes Payable
Debt Instrument [Line Items]
Stated interest rate (as a percent)	3.375%
Effective rate (as a percent)	3.48%
Maturity date	Aug. 15, 2031		Aug. 15, 2031
Future principal payments due on secured and unsecured debt
2020			$ 0
2021			0
2022			0
2023			0
2024			0
Thereafter			750,000
Outstanding Balance	$ 750,000		750,000
Unamortized Discount (Premium) and Debt Issuance Costs, Net			(7,527)
Total Consolidated			742,473
4.85% Unsecured Senior Note Payable
Debt Instrument [Line Items]
Stated interest rate (as a percent)		4.85%
Effective rate (as a percent)		4.93%
Maturity date		Apr. 15, 2049
Future principal payments due on secured and unsecured debt
2020			0
2021			0
2022			0
2023			0
2024			0
Thereafter			300,000
Outstanding Balance		$ 300,000	300,000
Unamortized Discount (Premium) and Debt Issuance Costs, Net			(3,446)
Total Consolidated			$ 296,554
4.00% Unsecured Senior Notes Payables Due 2050
Debt Instrument [Line Items]
Stated interest rate (as a percent)	4.00%
Effective rate (as a percent)	3.91%
Maturity date	Feb. 01, 2050		Feb. 01, 2050
Future principal payments due on secured and unsecured debt
2020			$ 0
2021			0
2022			0
2023			0
2024			0
Thereafter			700,000
Outstanding Balance	$ 700,000		700,000	$ 200,000	$ 500,000
Unamortized Discount (Premium) and Debt Issuance Costs, Net			10,476
Total Consolidated			710,476
Commercial paper program
Future principal payments due on secured and unsecured debt
2020			0
2021			0
2022			0
2023			0
2024			0
Thereafter			0
Outstanding Balance			0
Unamortized Discount (Premium) and Debt Issuance Costs, Net			0
Total Consolidated			0
Commercial paper program issuance balance			$ 750,000
Maximum | Commercial paper program
Debt Instrument [Line Items]
Effective rate (as a percent)			2.29%
Future principal payments due on secured and unsecured debt
Number of maturity days from date of issuance			397
Minimum | Commercial paper program
Debt Instrument [Line Items]
Effective rate (as a percent)			1.83%