Credit Facility and Commercial Paper Program (Details) £ in Millions	2 Months Ended	9 Months Ended
	Sep. 30, 2020 USD ($) extension	Sep. 30, 2020 USD ($) currency extension	Sep. 30, 2019	Sep. 30, 2020 GBP (£) extension	Dec. 31, 2019 USD ($)	Dec. 31, 2019 GBP (£)
Credit facility
Credit facility origination costs	$ 8,562,000	$ 8,562,000			$ 11,453,000
Maximum aggregate capacity of commercial paper program	1,000,000,000.0	1,000,000,000.0
Commercial paper borrowings outstanding	$ 300,000,000.0	$ 300,000,000.0
Commercial paper
Credit facility
Weighted average borrowing rate during the period (as a percent)	0.30%
Weighted average interest rate at the end of the period (as a percent)	0.20%	0.20%		0.20%
Unsecured debt | Revolving credit facility
Credit facility
Maximum borrowing capacity	$ 3,000,000,000.0	$ 3,000,000,000.0
Number of extensions | extension	2	2		2
Term of extension option		6 months
Number of currencies allowable per facility | currency		14
Credit facility expansion option	$ 1,000,000,000.0	$ 1,000,000,000.0
Current borrowing capacity	2,400,000,000	2,400,000,000
Outstanding balance	$ 556,100,000	$ 556,100,000		£ 430.5	$ 704,300,000	£ 169.2
Weighted average borrowing rate during the period (as a percent)		1.50%	3.20%
Weighted average interest rate at the end of the period (as a percent)	0.80%	0.80%		0.80%	2.20%	2.20%
Unsecured debt | Revolving credit facility | Other assets, net
Credit facility
Credit facility origination costs	$ 8,600,000	$ 8,600,000			$ 11,500,000
Unsecured debt | Revolving credit facility | LIBOR
Credit facility
Basis spread on variable rate (as a percent)		0.775%
Commitment fee (as a percent)		0.125%
All-in drawn variable interest rate (as a percent)		0.90%