Unsecured senior notes payable (Details) - USD ($) $ in Thousands			3 Months Ended		12 Months Ended			24 Months Ended
	Aug. 16, 2019	Jul. 17, 2019	Sep. 30, 2019	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Sep. 12, 2019	Jul. 15, 2019
Debt Instrument [Line Items]
Unsecured senior notes payable					$ 6,100,000
Ownership interest in subsidiary (as a percent)					100.00%
Effective rate (as a percent)					3.91%
Weighted Average Remaining Terms (in years)					10 years 4 months 24 days
Outstanding Balance					$ 6,796,440
Proceeds from issuance of unsecured senior notes payable					2,721,169	$ 899,321	$ 1,023,262
Repayments of Secured Debt					306,199	156,888	396,240
Repayments of Debt								$ 942,000
Loss on early extinguishment of debt					47,570	$ 1,122	$ 3,451
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
Outstanding Balance				$ 350,000	350,000
Proceeds from issuance of unsecured senior notes payable				$ 346,600
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
Outstanding Balance					650,000
Re-opening of 4.00% Unsecured Senior Note Payable
Debt Instrument [Line Items]
Outstanding Balance				$ 200,000
Proceeds from issuance of unsecured senior notes payable				$ 203,000
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
Outstanding Balance				$ 300,000	$ 300,000
Proceeds from issuance of unsecured senior notes payable				$ 296,500
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
Outstanding Balance			$ 750,000		$ 750,000
Proceeds from issuance of unsecured senior notes payable			$ 742,500
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
Outstanding Balance			$ 700,000		$ 700,000				$ 200,000	$ 500,000
Proceeds from issuance of unsecured senior notes payable			$ 711,100
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
Outstanding Balance			$ 400,000		$ 400,000
Proceeds from issuance of unsecured senior notes payable			$ 395,800
2019 Unsecured Senior Notes Issuances
Debt Instrument [Line Items]
Weighted average stated interest rate					3.71%
Weighted Average Effective Interest Rate					3.77%
Weighted Average Remaining Terms (in years)					16 years 10 months 24 days
Outstanding Balance					$ 2,700,000
Proceeds from issuance of unsecured senior notes payable					2,695,500
Secured notes payable maturing on 4/1/20
Debt Instrument [Line Items]
Stated interest rate (as a percent)				7.75%
Effective rate (as a percent)				8.15%
Maturity date				Apr. 01, 2020
Repayments of Secured Debt				$ 106,700
Loss on early extinguishment of debt				$ (7,100)
Secured notes payable maturing on 1/28/20
Debt Instrument [Line Items]
Stated interest rate (as a percent)				3.29%
Effective rate (as a percent)				3.29%
Maturity date				Jan. 28, 2020
Repayments of Secured Debt				$ 193,100
Loss on early extinguishment of debt				$ (269)
2.75% Unsecured Senior Notes Due in 2020
Debt Instrument [Line Items]
Stated interest rate (as a percent)			2.75%
Effective rate (as a percent)			2.96%
Maturity date			Jan. 15, 2020
Repayments of Debt			$ 400,000
4.60% Unsecured Senior Notes Due in 2022
Debt Instrument [Line Items]
Stated interest rate (as a percent)			4.60%
Effective rate (as a percent)			4.75%
Maturity date			Apr. 01, 2022
Repayments of Debt			$ 550,000
2.75% and 4.60% Unsecured Senior Notes Payable
Debt Instrument [Line Items]
Cash tender offer settlement date		Jul. 17, 2019
Unsecured senior notes payable redemption date	Aug. 16, 2019
Loss on early extinguishment of debt					$ (40,200)
Unsecured Senior Bank Term Loan
Debt Instrument [Line Items]
Stated interest rate (as a percent)			3.62%
Effective rate (as a percent)			3.62%
Maturity date			Jan. 02, 2025
Repayments of Debt			$ 350,000
2019 Secured and Unsecured Debt Repayments
Debt Instrument [Line Items]
Weighted average stated interest rate					3.97%
Weighted Average Effective Interest Rate					4.11%
Weighted Average Remaining Terms (in years)					2 years 4 months 24 days
Repayments of Debt					$ 1,600,000