Real Estate - Real Estate Under Development and Construction in Progress (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) property	Dec. 31, 2016 property
Property, Plant and Equipment [Line Items]
Depreciation and amortization expense related to building under development demolished, Write-offs	$ 2,000
Number of real estate properties | property	5	10
Real estate under development, beginning balance	$ 543,486
Transfers In	102,930
Capitalized Costs	32,400
Transfers Out	505,114
Real estate under development, ending balance	$ 173,702
Core Portfolio
Property, Plant and Equipment [Line Items]
Number of real estate properties | property	2	1
Real estate under development, beginning balance	$ 3,499
Transfers In	22,422
Capitalized Costs	819
Transfers Out	4,843
Real estate under development, ending balance	$ 21,897
Fund Portfolio | Fund II
Property, Plant and Equipment [Line Items]
Number of real estate properties | property	0	2
Real estate under development, beginning balance	$ 443,012
Transfers In	0
Capitalized Costs	6,851
Transfers Out	444,955
Real estate under development, ending balance	$ 4,908
Fund Portfolio | Fund III
Property, Plant and Equipment [Line Items]
Number of real estate properties | property	2	3
Real estate under development, beginning balance	$ 50,452
Transfers In	0
Capitalized Costs	22,572
Transfers Out	9,085
Real estate under development, ending balance	$ 63,939
Fund Portfolio | Fund IV
Property, Plant and Equipment [Line Items]
Number of real estate properties | property	1	4
Real estate under development, beginning balance	$ 46,523
Transfers In	80,508
Capitalized Costs	2,158
Transfers Out	46,231
Real estate under development, ending balance	$ 82,958