Real Estate Investments - Schedule of Annualized Rental Income by Major Tenants (Detail) - Worldwide Plaza [Member]	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Major Customer [Line Items]
Percentage of total annualized rental income	50.10%	50.10%	48.90%
Cravath, Swaine & Moore LLP [Member] \| Liquidation Value [Member]
Revenue, Major Customer [Line Items]
Percentage of total annualized rental income	46.00%	17.00%	16.00%
Nomura Holdings America, Inc [Member] \| Liquidation Value [Member]
Revenue, Major Customer [Line Items]
Percentage of total annualized rental income	31.00%	11.00%	11.00%

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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