Real Estate - Schedule of Development in Process Activities (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($) property	Dec. 31, 2019 USD ($) property	Dec. 31, 2018 USD ($) property
Property, Plant and Equipment [Line Items]
Number of properties under development | property	3	3	3
Real estate under development, beginning balance	$ 268,298	$ 253,402	$ 120,297
Transfers In	33,057	117,344
Capitalized Costs	10,876	25,580
Transfers Out	$ 29,037	$ 9,819
Core Portfolio
Property, Plant and Equipment [Line Items]
Number of properties under development | property	0	0	1
Real estate under development, beginning balance	$ 63,946	$ 60,863	$ 7,759
Transfers In	0	57,342
Capitalized Costs	3,083	5,581
Transfers Out	$ 0	$ 9,819
Fund Portfolio | Fund II
Property, Plant and Equipment [Line Items]
Number of properties under development | property	0	0	0
Real estate under development, beginning balance	$ 39,921	$ 10,703	$ 7,462
Transfers In	33,057	0
Capitalized Costs	2,631	3,241
Transfers Out	$ 6,470	$ 0
Fund Portfolio | Fund III
Property, Plant and Equipment [Line Items]
Number of properties under development | property	1	1	1
Real estate under development, beginning balance	$ 23,266	$ 36,240	$ 21,242
Transfers In	0	12,313
Capitalized Costs	1,071	2,685
Transfers Out	$ 14,045	$ 0
Fund Portfolio | Fund IV
Property, Plant and Equipment [Line Items]
Number of properties under development | property	2	2	1
Real estate under development, beginning balance	$ 141,165	$ 145,596	$ 83,834
Transfers In	0	47,689
Capitalized Costs	4,091	14,073
Transfers Out	$ 8,522	$ 0