Credit Facility - Narrative (Details) £ in Millions		3 Months Ended
	Apr. 09, 2020 USD ($)	Mar. 31, 2020 USD ($) extension	Mar. 31, 2019 USD ($) currency	May 01, 2020 GBP (£)	May 01, 2020 USD ($)	Dec. 31, 2019 GBP (£)	Dec. 31, 2019 USD ($)
Credit facility
Credit facility origination costs		$ 10,274,000					$ 11,453,000
Outstanding balance		615,176,000					704,335,000
Borrowings on line of credit		846,532,000	$ 732,000,000
Cash and cash equivalents		41,804,000	$ 2,997,000				54,011,000
Unsecured debt | Revolving Credit Facility
Credit facility
Maximum borrowing capacity		$ 3,000,000,000.0
Number of extensions | extension		2
Term of extension option		6 months
Number of currencies allowable per facility | currency			14
Credit facility expansion option		$ 1,000,000,000.0
Current borrowing capacity		2,400,000,000
Outstanding balance		$ 615,200,000				£ 169.2	$ 704,300,000
Weighted average borrowing rate during the period (as a percent)		2.10%	3.30%
Weighted average interest rate at the end of the period (as a percent)		1.30%				2.20%	2.20%
Unsecured debt | Revolving Credit Facility | Other assets, net
Credit facility
Credit facility origination costs		$ 10,300,000					$ 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%
Subsequent event
Credit facility
Cash and cash equivalents	$ 1,250,000,000
Subsequent event | Unsecured debt | Revolving Credit Facility
Credit facility
Credit facility expansion option					$ 1,000,000,000.0
Current borrowing capacity					1,100,000,000
Outstanding balance				£ 325.5	$ 1,900,000,000
Borrowings on line of credit	$ 1,200,000,000