Related Parties - Additional Information (Details) - USD ($)		3 Months Ended		9 Months Ended		12 Months Ended
	Mar. 27, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Related Party Transaction [Line Items]
Fee and other income		$ 11,355,000	$ 10,138,000	$ 27,045,000	$ 26,485,000
Accounts and other receivables, net		15,289,000		15,289,000		$ 19,231,000
Property management, asset management, leasing and other related services fee income		9,153,000	7,159,000	21,692,000	17,371,000
Transaction related costs		81,000	786,000	542,000	1,704,000
Accounts payable and accrued expenses		$ 109,228,000		109,228,000		117,356,000
Due from Affiliates					181,000,000
Repayment of note receivables				$ 36,918,000	181,000,000
Fund X [Member]
Related Party Transaction [Line Items]
Due from Affiliates						36,918,000
Repayment of note receivables	$ 36,918,000
Repayment of accrued interest	$ 349,000
Note receivables basis spread on variable rate		2.20%		2.20%
712 Fifth Avenue [Member]
Related Party Transaction [Line Items]
Equity method paramount ownership percentage		50.00%		50.00%
CNBB-RDF Holdings Otto Family [Member] | Management Agreements [Member]
Related Party Transaction [Line Items]
Fee and other income		$ 260,000	212,000	$ 967,000	632,000
Accounts and other receivables, net		0		0		0
Unconsolidated Joint Ventures and Real Estate Funds [Member] | Management Agreements [Member]
Related Party Transaction [Line Items]
Accounts and other receivables, net		5,060,000		5,060,000		2,734,000
Property management, asset management, leasing and other related services fee income		$ 6,841,000	4,880,000	$ 17,550,000	12,938,000
Hamburg Trust Consulting GMBH ("HTC") [Member]
Related Party Transaction [Line Items]
Mark-up cost percentage		10.00%		10.00%
Transaction related costs		$ 124,000	72,000	$ 389,000	758,000
Accounts payable and accrued expenses		$ 124,000		$ 124,000		$ 38,000
Hamburg Trust Consulting GMBH ("HTC") [Member] | Chairman, Chief Executive Officer and President [Member]
Related Party Transaction [Line Items]
Percentage of ownership		100.00%		100.00%
Mannheim Trust [Member] | Board of Director [Member] | 712 Fifth Avenue [Member]
Related Party Transaction [Line Items]
Equity method paramount ownership percentage		50.00%		50.00%
Lease rental income		$ 91,000	90,000	$ 272,000	271,000
Kramer Design Services [Member]
Related Party Transaction [Line Items]
Accounts payable and accrued expenses		19,000		19,000
Development expenses		$ 47,000		$ 187,000
Kramer Design Services [Member] | 712 Fifth Avenue [Member]
Related Party Transaction [Line Items]
Equity method paramount ownership percentage		50.00%		50.00%
Expense recognized			$ 183,000	$ 29,000	$ 286,000