Fair Value Measurements (Summary of Derivative Instruments) (Details) - Interest rate swaps - Designated as Hedging Instrument - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Other Assets
Derivative [Line Items]
Derivative Asset, Fair Value		$ 4,327,000
Derivative Asset, Notional Amount		1,250,000,000
Other liabilities
Derivative [Line Items]
Derivative Liability, Fair Value	$ 73,536,000	40,354,000
Derivative Liability, Notional Amount	1,225,000,000	850,000,000
Other | Other Assets
Derivative [Line Items]
Derivative Asset, Fair Value	17,000	64,000
Derivative Asset, Notional Amount	175,000,000	175,000,000
Unsecured term loan | Other liabilities
Derivative [Line Items]
Derivative Liability, Fair Value	63,437,000	36,809,000
Derivative Liability, Notional Amount	$ 750,000,000	$ 750,000,000
Derivative, Interest Rate	1.15%	2.80%
Derivative, Swap Rate	3.87%	3.87%
Derivative expiration date	2023-10	2023-10
33-00 Northern Boulevard mortgage loan | Other liabilities
Derivative [Line Items]
Derivative Liability, Fair Value	$ 9,065,000	$ 3,545,000
Derivative Liability, Notional Amount	$ 100,000,000	$ 100,000,000
Derivative, Interest Rate	1.96%	3.52%
Derivative, Swap Rate	4.14%	4.14%
Derivative expiration date	2025-01	2025-01
888 Seventh Avenue mortgage loan | Other Assets
Derivative [Line Items]
Derivative Asset, Fair Value		$ 218,000
Derivative Asset, Notional Amount		$ 375,000,000
Derivative, Interest Rate		3.44%
Derivative, Swap Rate		3.25%
888 Seventh Avenue mortgage loan | Other liabilities
Derivative [Line Items]
Derivative Liability, Fair Value	$ 1,034,000
Derivative Liability, Notional Amount	$ 375,000,000
Derivative, Interest Rate	1.84%
Derivative, Swap Rate	3.25%
Derivative expiration date	2020-12	2020-12
770 Broadway mortgage loan | Other Assets
Derivative [Line Items]
Derivative Asset, Fair Value		$ 4,045,000
Derivative Asset, Notional Amount		$ 700,000,000
Derivative, Interest Rate		3.46%
Derivative, Swap Rate		2.56%
770 Broadway mortgage loan | Other liabilities
Derivative [Line Items]
Derivative expiration date		2020-09
LIBOR | Unsecured term loan | Other liabilities
Derivative [Line Items]
Derivative, Basis Spread on Variable Rate	1.00%	1.00%
LIBOR | 33-00 Northern Boulevard mortgage loan | Other liabilities
Derivative [Line Items]
Derivative, Basis Spread on Variable Rate	1.80%	1.80%
LIBOR | 888 Seventh Avenue mortgage loan | Other Assets
Derivative [Line Items]
Derivative, Basis Spread on Variable Rate		1.70%
LIBOR | 888 Seventh Avenue mortgage loan | Other liabilities
Derivative [Line Items]
Derivative, Basis Spread on Variable Rate	1.70%
LIBOR | 770 Broadway mortgage loan | Other Assets
Derivative [Line Items]
Derivative, Basis Spread on Variable Rate		1.75%
Floating | Unsecured term loan | Other liabilities
Derivative [Line Items]
Derivative Liability, Notional Amount	$ 50,000,000
Floating | LIBOR | Unsecured term loan | Other liabilities
Derivative [Line Items]
Derivative, Basis Spread on Variable Rate	1.00%