Leases - Operating Leases - Additional Information (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) shopping_center	Jun. 30, 2019 USD ($)	Sep. 30, 2019 USD ($) item shopping_center	Sep. 30, 2018 USD ($)
Operating Leased Assets [Line Items]
Variable lease revenues	$ 14,700		$ 41,200
Cumulative effect adjustment to retained earnings required upon adoption of the new standard	0		0
Operating lease, right-of-use asset	55,717		$ 55,717
Number of operating lease | item			9
Number of finance lease | item			4
Finance lease, right-of-use asset	93,796		$ 93,796
Finance lease, liability	88,137		88,137
Operating lease liabilities	$ 57,093		57,093
Acquisition of real estate			$ 256,647	$ 104,902
Number of shopping centers with land leases | shopping_center	6		6
Ground lease expense				1,200
Rent expense capitalized				500
Rent expense				700
Capital lease term			49 years
1238 Wisconsin Avenue | Core Portfolio
Operating Leased Assets [Line Items]
Finance lease, right-of-use asset		$ 11,200
Finance lease, liability		10,700
110 University Place
Operating Leased Assets [Line Items]
Operating lease, right-of-use asset		45,300
Operating lease liabilities		45,300
Acquisition of real estate		$ 10,500
Capital Lease Liability
Operating Leased Assets [Line Items]
Finance lease liabilities	$ 71,100		$ 71,100
Finance lease, liability	71,100		71,100
Repayments capital lease obligations				$ 1,900
Ground, Office and Equipment Leases
Operating Leased Assets [Line Items]
Operating lease, right-of-use asset	82,600		82,600
Operating lease liabilities	76,600		76,600
Capital Leases Asset
Operating Leased Assets [Line Items]
Finance lease, right-of-use asset	77,000		77,000
Accounting Standards Update 2016-02
Operating Leased Assets [Line Items]
Operating lease, right-of-use asset	11,900		11,900
Operating lease liabilities	12,800		12,800
Finance lease, right-of-use asset	5,700		5,700
Finance lease, liability	$ 5,700		$ 5,700
Minimum
Operating Leased Assets [Line Items]
Operating lease term	1 month		1 month
Period of lease term			25 years
Maximum
Operating Leased Assets [Line Items]
Operating lease term	60 years		60 years
Period of lease term			71 years