Real Estate - Real Estate Under Development and Construction in Progress (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) property	Dec. 31, 2017 USD ($) property
Property, Plant and Equipment [Line Items]
Number of properties under development | property	5	5
Real estate under development, beginning balance	$ 173,702
Transfers In	0
Capitalized Costs	8,678
Transfers Out	0
Real estate under development, ending balance	$ 182,380	$ 173,702
Number of unconsolidated project in development | property	1	1
Core Portfolio
Property, Plant and Equipment [Line Items]
Number of properties under development | property	2	2
Real estate under development, beginning balance	$ 21,897
Transfers In	0
Capitalized Costs	2,159
Transfers Out	0
Real estate under development, ending balance	$ 24,056	$ 21,897
Number of projects put into service during period | property		1
Fund Portfolio | Fund II
Property, Plant and Equipment [Line Items]
Number of properties under development | property	0	0
Real estate under development, beginning balance	$ 4,908
Transfers In	0
Capitalized Costs	(175)
Transfers Out	0
Real estate under development, ending balance	$ 4,733	$ 4,908
Fund Portfolio | Fund III
Property, Plant and Equipment [Line Items]
Number of properties under development | property	2	2
Real estate under development, beginning balance	$ 63,939
Transfers In	0
Capitalized Costs	6,431
Transfers Out	0
Real estate under development, ending balance	$ 70,370	$ 63,939
Fund Portfolio | Fund IV
Property, Plant and Equipment [Line Items]
Number of properties under development | property	1	1
Real estate under development, beginning balance	$ 82,958
Transfers In	0
Capitalized Costs	263
Transfers Out	0
Real estate under development, ending balance	$ 83,221	$ 82,958
Number of unconsolidated project in development | property		4
Number of projects put into service during period | property		3