Acquisitions (Details) $ in Thousands	3 Months Ended			12 Months Ended
	Sep. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) ft²	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Number of Real Estate Properties Acquired				47
Purchase Price | $				$ 2,259,778	$ 1,037,180	$ 675,584
10260 Campus Point Drive and 4161 Campus Point Court
Business Acquisition [Line Items]
Purchase Price | $			$ 80,000
10260 Campus Point Drive and 4161 Campus Point Court | First Installment Payment
Business Acquisition [Line Items]
Purchase Price | $			$ 15,000
10260 Campus Point Drive and 4161 Campus Point Court | Second Installment Payment
Business Acquisition [Line Items]
Purchase Price | $		$ 65,000
4224/4242 Campus Point Court and 10210 Campus Point Drive
Business Acquisition [Line Items]
Payment to acquire real estate, noncontrolling interest share | $	$ 114,800
Future development
Business Acquisition [Line Items]
Area of Real Estate Property				2,350,338
Active redevelopment
Business Acquisition [Line Items]
Area of Real Estate Property				501,069
Operating with future development and redevelopment
Business Acquisition [Line Items]
Area of Real Estate Property				879,433
Operating property
Business Acquisition [Line Items]
Area of Real Estate Property				1,949,100
Acquired in-place leases | $				$ 197,800
Below-market leases | $				$ 90,200
Weighted average remaining amortization period, acquired in-place and below-market leases				9 years 10 months 24 days
Operating property | Acquired-in-Place Leases
Business Acquisition [Line Items]
Weighted average remaining amortization period, acquired in-place and below-market leases				9 years
Operating property | Acquired Below-Market Leases
Business Acquisition [Line Items]
Weighted average remaining amortization period, acquired in-place and below-market leases				11 years 10 months 24 days
Greater Boston
Business Acquisition [Line Items]
Number of Real Estate Properties Acquired				12
Purchase Price | $				$ 858,600
Greater Boston | Future development
Business Acquisition [Line Items]
Area of Real Estate Property				668,000
Greater Boston | Active redevelopment
Business Acquisition [Line Items]
Area of Real Estate Property				153,157
Greater Boston | Operating with future development and redevelopment
Business Acquisition [Line Items]
Area of Real Estate Property				154,855
Greater Boston | Operating property
Business Acquisition [Line Items]
Area of Real Estate Property				615,439
San Francisco
Business Acquisition [Line Items]
Number of Real Estate Properties Acquired				8
Purchase Price | $				$ 735,450
San Francisco | Future development
Business Acquisition [Line Items]
Area of Real Estate Property				0
San Francisco | Active redevelopment
Business Acquisition [Line Items]
Area of Real Estate Property				347,912
San Francisco | Operating with future development and redevelopment
Business Acquisition [Line Items]
Area of Real Estate Property				478,000
San Francisco | Operating property
Business Acquisition [Line Items]
Area of Real Estate Property				247,770
New York City
Business Acquisition [Line Items]
Number of Real Estate Properties Acquired				0
Purchase Price | $				$ 25,000
New York City | Future development
Business Acquisition [Line Items]
Area of Real Estate Property				135,938
New York City | Active redevelopment
Business Acquisition [Line Items]
Area of Real Estate Property				0
New York City | Operating with future development and redevelopment
Business Acquisition [Line Items]
Area of Real Estate Property				0
New York City | Operating property
Business Acquisition [Line Items]
Area of Real Estate Property				0
San Diego
Business Acquisition [Line Items]
Number of Real Estate Properties Acquired				16
Purchase Price | $				$ 474,214
San Diego | Future development
Business Acquisition [Line Items]
Area of Real Estate Property				869,000
San Diego | Active redevelopment
Business Acquisition [Line Items]
Area of Real Estate Property				0
San Diego | Operating with future development and redevelopment
Business Acquisition [Line Items]
Area of Real Estate Property				93,220
San Diego | Operating property
Business Acquisition [Line Items]
Area of Real Estate Property				912,419
Seattle
Business Acquisition [Line Items]
Number of Real Estate Properties Acquired				1
Purchase Price | $				$ 28,500
Seattle | Future development
Business Acquisition [Line Items]
Area of Real Estate Property				188,400
Seattle | Active redevelopment
Business Acquisition [Line Items]
Area of Real Estate Property				0
Seattle | Operating with future development and redevelopment
Business Acquisition [Line Items]
Area of Real Estate Property				18,680
Seattle | Operating property
Business Acquisition [Line Items]
Area of Real Estate Property				0
Maryland
Business Acquisition [Line Items]
Number of Real Estate Properties Acquired				3
Purchase Price | $				$ 76,130
Maryland | Future development
Business Acquisition [Line Items]
Area of Real Estate Property				435,000
Maryland | Active redevelopment
Business Acquisition [Line Items]
Area of Real Estate Property				0
Maryland | Operating with future development and redevelopment
Business Acquisition [Line Items]
Area of Real Estate Property				0
Maryland | Operating property
Business Acquisition [Line Items]
Area of Real Estate Property				138,938
Other markets
Business Acquisition [Line Items]
Number of Real Estate Properties Acquired				7
Purchase Price | $				$ 77,000
Other markets | Future development
Business Acquisition [Line Items]
Area of Real Estate Property				54,000
Other markets | Active redevelopment
Business Acquisition [Line Items]
Area of Real Estate Property				0
Other markets | Operating with future development and redevelopment
Business Acquisition [Line Items]
Area of Real Estate Property				134,678
Other markets | Operating property
Business Acquisition [Line Items]
Area of Real Estate Property				34,534
North America
Business Acquisition [Line Items]
Area of Real Estate Property				27,000,000.0
North America | Operating and Development/Redevelopment Properties
Business Acquisition [Line Items]
Purchase Price | $				$ 2,274,894