Debt - Narrative (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2020 USD ($) loan	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Mortgages and Line of Credit Facility [Line Items]
Loss on extinguishment of debt		$ 62	$ 5,805	$ 5,153
Mortgages Payable
Mortgages and Line of Credit Facility [Line Items]
Net carrying value of real estate collateralizing the mortgages payable	$ 1,800,000	1,800,000	1,400,000
Aggregate borrowings under mortgages payable			2,200
Repayments of secured debt			709,100
Loss on extinguishment of debt			5,800
Payment for debt extinguishment or debt prepayment cost			2,900
Derivative, loss on derivative			2,900
Mortgages Payable | Interest rate swaps and caps
Mortgages and Line of Credit Facility [Line Items]
Derivative notional amount	1,300,000	1,300,000	867,600
Derivative entered into - aggregate notional value		560,000
Credit Facility
Mortgages and Line of Credit Facility [Line Items]
Credit facility, maximum borrowing capacity	1,400,000	1,400,000
Credit Facility | Revolving Credit Facility
Mortgages and Line of Credit Facility [Line Items]
Credit facility, maximum borrowing capacity	1,000,000	1,000,000
Credit Facility | Tranche A-1 Term Loan
Mortgages and Line of Credit Facility [Line Items]
Credit facility, maximum borrowing capacity	200,000	200,000
Credit Facility | Tranche A-2 Term Loan
Mortgages and Line of Credit Facility [Line Items]
Credit facility, maximum borrowing capacity	200,000	$ 200,000
Minimum | London Interbank Offered Rate (LIBOR) | Credit Facility | Revolving Credit Facility
Mortgages and Line of Credit Facility [Line Items]
Debt Instrument basis spread on variable rate		1.05%
Minimum | London Interbank Offered Rate (LIBOR) | Credit Facility | Tranche A-1 Term Loan
Mortgages and Line of Credit Facility [Line Items]
Debt Instrument basis spread on variable rate		1.20%
Minimum | London Interbank Offered Rate (LIBOR) | Credit Facility | Tranche A-2 Term Loan
Mortgages and Line of Credit Facility [Line Items]
Debt Instrument basis spread on variable rate		1.15%
Maximum | London Interbank Offered Rate (LIBOR) | Credit Facility | Revolving Credit Facility
Mortgages and Line of Credit Facility [Line Items]
Debt Instrument basis spread on variable rate		1.50%
Maximum | London Interbank Offered Rate (LIBOR) | Credit Facility | Tranche A-1 Term Loan
Mortgages and Line of Credit Facility [Line Items]
Debt Instrument basis spread on variable rate		1.70%
Maximum | London Interbank Offered Rate (LIBOR) | Credit Facility | Tranche A-2 Term Loan
Mortgages and Line of Credit Facility [Line Items]
Debt Instrument basis spread on variable rate		1.70%
4747 Bethesda Avenue, The Bartlett, 1221 Van Street and 220 20th Street
Mortgages and Line of Credit Facility [Line Items]
Number of mortgage loans | loan		4
Principal amount	560,000	$ 560,000
RTC West
Mortgages and Line of Credit Facility [Line Items]
Increase in principal balance		$ 20,200
West End25
Mortgages and Line of Credit Facility [Line Items]
Repayments of secured debt	$ 94,700
Central Place Tower | Mortgages Payable | Interest rate swaps and caps
Mortgages and Line of Credit Facility [Line Items]
Derivative terminated - aggregate notional value			$ 220,000