Fair Value Measurements (Summary of Derivative Instruments) (Details) - Interest rate swaps - Designated as Hedging Instrument	3 Months Ended
	Mar. 31, 2020 USD ($)
Other liabilities
Derivative [Line Items]
Derivative Liability, Fair Value	$ 81,601,000
Derivative Liability, Notional Amount	1,925,000,000
Other | Other Assets
Derivative [Line Items]
Derivative Asset, Fair Value	94,000
Derivative Asset, Notional Amount	175,000,000
Unsecured term loan | Other liabilities
Derivative [Line Items]
Derivative Liability, Fair Value	68,439,000
Derivative Liability, Notional Amount	$ 750,000,000
Derivative, Interest Rate	1.94%
Derivative, Swap Rate	3.87%
Derivative expiration date	2023-10
33-00 Northern Boulevard mortgage loan | Other liabilities
Derivative [Line Items]
Derivative Liability, Fair Value	$ 9,141,000
Derivative Liability, Notional Amount	$ 100,000,000
Derivative, Interest Rate	2.81%
Derivative, Swap Rate	4.14%
Derivative expiration date	2025-01
888 Seventh Avenue mortgage loan | Other liabilities
Derivative [Line Items]
Derivative Liability, Fair Value	$ 3,077,000
Derivative Liability, Notional Amount	$ 375,000,000
Derivative, Interest Rate	2.62%
Derivative, Swap Rate	3.25%
Derivative expiration date	2020-12
770 Broadway mortgage loan | Other liabilities
Derivative [Line Items]
Derivative Liability, Fair Value	$ 944,000
Derivative Liability, Notional Amount	$ 700,000,000
Derivative, Interest Rate	2.76%
Derivative, Swap Rate	2.56%
Derivative expiration date	2020-09
LIBOR | Unsecured term loan | Other liabilities
Derivative [Line Items]
Derivative, Basis Spread on Variable Rate	1.00%
LIBOR | 33-00 Northern Boulevard mortgage loan | Other liabilities
Derivative [Line Items]
Derivative, Basis Spread on Variable Rate	1.80%
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 liabilities
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
Floating | LIBOR | Unsecured term loan | Other liabilities
Derivative [Line Items]
Derivative, Basis Spread on Variable Rate	1.00%