Subsequent Events (Details) $ in Thousands	Aug. 02, 2021 USD ($) ft² a Vehicles	Jul. 20, 2021 USD ($) ft²	Jul. 13, 2021 USD ($) ft²	Jun. 30, 2021 USD ($)	Jun. 29, 2021 USD ($) ft²	Jun. 07, 2021 USD ($) a ft²	Dec. 31, 2020 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Subsequent Event [Line Items]
Cash and Cash Equivalents, at Carrying Value				$ 557,307			$ 1,668,742	$ 1,691,047	$ 644,950
Safeco Plaza
Subsequent Event [Line Items]
Net Rentable Area (in sf) | ft²					800,000
Cash and Cash Equivalents, at Carrying Value					$ 46,500
Shady Grove Bio+Tech Campus
Subsequent Event [Line Items]
Net Rentable Area (in sf) | ft²						435,000
Cash and Cash Equivalents, at Carrying Value						$ 10,000
Area of Land | a						31
Subsequent Event [Member] | 181, 191 and 201 Spring Street
Subsequent Event [Line Items]
Contractual Sales Price			$ 191,500
Net Rentable Area (in sf) | ft²			333,000
Leased percentage			100.00%
Subsequent Event [Member] | Safeco Plaza
Subsequent Event [Line Items]
Net Rentable Area (in sf) | ft²		800,000
Payments to Acquire Real Estate		$ 465,000
Cash and Cash Equivalents, at Carrying Value		51,500
Subsequent Event [Member] | Safeco Plaza | Company's Share [Member]
Subsequent Event [Line Items]
Cash and Cash Equivalents, at Carrying Value		$ 26,300
Subsequent Event [Member] | Shady Grove Bio+Tech Campus
Subsequent Event [Line Items]
Net Rentable Area (in sf) | ft²	435,000
Payments to Acquire Real Estate	$ 116,500
Number of real estate properties | Vehicles	7
Area of Land | a	31