Real Estate Fund Investments - Additional Information (Details) - ft²	Sep. 30, 2017	Jul. 17, 2017	Jul. 16, 2017	Jan. 24, 2017	Dec. 31, 2016
Investment Holdings [Line Items]
Area of office and retail properties	12,500,000
Percentage of ownership interest in new joint venture		36.60%
Property Fund II [Member]
Investment Holdings [Line Items]
Percentage interest in fund	10.00%
Property Fund III [Member]
Investment Holdings [Line Items]
Percentage interest in fund	3.10%
Property Fund VII [Member]
Investment Holdings [Line Items]
Percentage interest in fund	7.50%
Fund VII and Fund VII H [Member]
Investment Holdings [Line Items]
Percentage interest in fund			7.20%
50 Beale Street [Member]
Investment Holdings [Line Items]
Percentage of ownership interest in new joint venture		36.60%
50 Beale Street [Member] | Fund VII and Fund VII H [Member]
Investment Holdings [Line Items]
Area of office and retail properties			660,625
Property ownership, percentage		42.80%	42.80%
Percentage of ownership interest in new joint venture		36.60%
60 Wall Street [Member]
Investment Holdings [Line Items]
Percentage interest in fund				62.30%	62.30%
Percentage of remaining interest sold by other investors				37.70%
Area of office and retail properties				1,600,000	1,600,000
Property ownership, percentage				5.00%