Consolidated Variable Interest Entity (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) ft²	Sep. 30, 2020 USD ($) ft²
Consolidated Variable Interest Entity [Abstract]
Rentable square feet of office property under development (in sq feet) | ft²		1,200,000
Assets and liabilities of consolidated variable interest entity [Abstract]
Development in-process	$ 172,706	$ 258,219
Accounts payable, accrued expenses and other liabilities	$ 286,911	299,587
Consolidated Variable Interest Entity [Member]
Consolidated Variable Interest Entity [Abstract]
Rentable square feet of office property under development (in sq feet) | ft²	150,000
Total anticipated development costs	$ 71,300
Contribution of cash to acquire interest in consolidated joint venture	$ 20,000
Interest in consolidated joint venture (in hundredths)	80.00%
Partner's contribution of property to acquire interest in consolidated joint venture	$ 5,000
Advance to consolidated affiliate	$ 46,300
Amount of loan funded to affiliate		13,700
Assets and liabilities of consolidated variable interest entity [Abstract]
Development in-process		42,015
Accounts payable, accrued expenses and other liabilities		$ 2,706
Consolidated Variable Interest Entity [Member] | London Interbank Offered Rate (LIBOR) [Member]
Consolidated Variable Interest Entity [Abstract]
Variable interest rate basis	LIBOR
Interest rate, basis spread (in hundredths)	2.50%
Variable Interest Entity [Member]
Consolidated Variable Interest Entity [Abstract]
Partner's interest in consolidated joint venture (in hundredths)		20.00%
Variable Interest Entity [Member] | Consolidated Variable Interest Entity [Member]
Consolidated Variable Interest Entity [Abstract]
Partner's interest in consolidated joint venture (in hundredths)	20.00%