Real Estate - Under Development and Construction in Progress (Details) $ in Thousands	9 Months Ended
	Sep. 30, 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	13	14
Real estate under development, beginning balance	$ 543,486
Transfers In	86,882
Capitalized Costs	39,250
Transfers Out	432,184
Real estate under development, ending balance	$ 237,434
Core Portfolio
Property, Plant and Equipment [Line Items]
Number of real estate properties | property	2	1
Real estate under development, beginning balance	$ 2,530
Transfers In	7,258
Capitalized Costs	3,852
Transfers Out	5,441
Real estate under development, ending balance	$ 8,199
Fund II | Fund Portfolio
Property, Plant and Equipment [Line Items]
Number of real estate properties | property	1	2
Real estate under development, beginning balance	$ 443,012
Transfers In	0
Capitalized Costs	7,677
Transfers Out	414,000
Real estate under development, ending balance	$ 36,689
Fund III | Fund Portfolio
Property, Plant and Equipment [Line Items]
Number of real estate properties | property	2	3
Real estate under development, beginning balance	$ 51,421
Transfers In	0
Capitalized Costs	13,838
Transfers Out	8,146
Real estate under development, ending balance	$ 57,113
Fund IV | Fund Portfolio
Property, Plant and Equipment [Line Items]
Number of real estate properties | property	8	8
Real estate under development, beginning balance	$ 46,523
Transfers In	79,624
Capitalized Costs	13,883
Transfers Out	4,597
Real estate under development, ending balance	$ 135,433