Real Estate Fund Investments - Summary of Ownership Interest in Property Funds and Property Funds Ownership Interest in Underlying Properties (Details)	Dec. 31, 2017	Dec. 31, 2016
0 Bond Street [Member]
Investment Holdings [Line Items]
Property funds, percentage	100.00%	100.00%
60 Wall Street [Member]
Investment Holdings [Line Items]
Property funds, percentage		100.00%
One Market Plaza [Member]
Investment Holdings [Line Items]
Property funds, percentage		100.00%
50 Beale Street [Member]
Investment Holdings [Line Items]
Property funds, percentage		100.00%
Fund II [Member]
Investment Holdings [Line Items]
Property funds, percentage	10.00%
Fund II [Member] | 60 Wall Street [Member]
Investment Holdings [Line Items]
Property funds, percentage		46.30%
Fund III [Member]
Investment Holdings [Line Items]
Property funds, percentage	3.10%
Fund III [Member] | 60 Wall Street [Member]
Investment Holdings [Line Items]
Property funds, percentage		16.00%
Fund III [Member] | One Market Plaza [Member]
Investment Holdings [Line Items]
Property funds, percentage		2.00%
Fund VII and VII-H [Member]
Investment Holdings [Line Items]
Property funds, percentage	7.20%
Fund VII and VII-H [Member] | 0 Bond Street [Member]
Investment Holdings [Line Items]
Property funds, percentage	100.00%	100.00%
Fund VII and VII-H [Member] | 50 Beale Street [Member]
Investment Holdings [Line Items]
Property funds, percentage		42.80%
Property Funds [Member] | 0 Bond Street [Member]
Investment Holdings [Line Items]
Property funds, percentage	100.00%	100.00%
Property Funds [Member] | 60 Wall Street [Member]
Investment Holdings [Line Items]
Property funds, percentage		62.30%
Property Funds [Member] | One Market Plaza [Member]
Investment Holdings [Line Items]
Property funds, percentage		2.00%
Property Funds [Member] | 50 Beale Street [Member]
Investment Holdings [Line Items]
Property funds, percentage		42.80%
Other Investors [Member] | 60 Wall Street [Member]
Investment Holdings [Line Items]
Property funds, percentage		37.70%
Other Investors [Member] | One Market Plaza [Member]
Investment Holdings [Line Items]
Property funds, percentage		98.00%
Other Investors [Member] | 50 Beale Street [Member]
Investment Holdings [Line Items]
Property funds, percentage		57.20%