Investments in Unconsolidated Real Estate Funds - Additional Information (Details)		1 Months Ended		12 Months Ended
	Jan. 25, 2019 USD ($)	Dec. 31, 2018 USD ($) ft²	Apr. 30, 2016 USD ($)	Dec. 31, 2018 USD ($) ft²	Dec. 31, 2017 USD ($)
Investment Holdings [Line Items]
Area of office and retail properties | ft²		11,900,000		11,900,000
Alternative Investment Fund, Interest Rate		1.30%		1.30%
Proceeds from sale of real estate				$ 349,013,000	$ 540,333,000
Capital commitments			$ 775,200,000
Capital commitments invested		$ 614,450,000		614,450,000
Additional amount reserved for funding future draws		$ 74,391,000		$ 74,391,000
Investment maturity date range start				2019-10
Investment maturity date range end				2027-12
Investment period end date				2019-04
Investment Extended Period End Date				2020-04
Minimum [Member]
Investment Holdings [Line Items]
Stated interest rates on invested capital commitments				5.50%
Maximum [Member]
Investment Holdings [Line Items]
Stated interest rates on invested capital commitments				9.61%
Fund VII and VII-H [Member]
Investment Holdings [Line Items]
Alternative Investment Fund, Interest Rate		7.20%		7.20%
Fund VII and VII-H [Member] | Office Building [Member]
Investment Holdings [Line Items]
Area of office and retail properties | ft²		64,532		64,532
Fund VII and VII-H [Member] | Zero Bond Street [Member]
Investment Holdings [Line Items]
Property funds, percentage		100.00%		100.00%
Fund VII and VII-H [Member] | Zero Bond Street [Member] | Subsequent Event [Member]
Investment Holdings [Line Items]
Proceeds from sale of real estate	$ 130,500,000
Fund X [Member]
Investment Holdings [Line Items]
Capital commitments		$ 167,000,000
Capital commitments invested		0		$ 0
Ownership value of contributed capital in capital commitments		$ 10,000,000