Real Estate Assets (Details) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2021 USD ($) ft² numberOfOfficeBuildings	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($) ft² numberOfBuildings	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) ft² numberOfOfficeBuildings	Sep. 30, 2020 USD ($)
Acquisitions [Abstract]
Rentable square feet of commercial real estate properties (in sq feet) | ft²	29,000,000				29,000,000
Dispositions [Abstract]
Gains on disposition of property	$ 38,572			$ 10,012	$ 80,371	$ 163,397
Nashville TN Land Acquisitions [Member]
Acquisitions [Abstract]
Acquisition purchase price	$ 22,900	$ 16,100
Expected term of purchase price payment (in years)		2 years
Forum Buildings Acquisition [Member]
Acquisitions [Abstract]
Acquisition purchase price			$ 131,300
Percentage of partner's interest in asset acquired (in hundredths)			75.00%
Number of buildings acquired | numberOfBuildings			5
Rentable square feet of commercial real estate properties (in sq feet) | ft²			636,000
Dispositions [Abstract]
Percentage of equity interest in joint venture (in hundredths)			25.00%
2021 Dispositions
Dispositions [Abstract]
Number of buildings sold | numberOfOfficeBuildings	2				2
Sale price of real estate	$ 119,700	$ 43,000	$ 30,700
Closing credits excluded for unfunded building and/or tenant improvements	4,400	900
Gains on disposition of property	$ 37,300	$ 22,900	$ 18,900
Markel Land Disposition [Member]
Dispositions [Abstract]
Percentage of equity interest in joint venture (in hundredths)	50.00%				50.00%
Joint venture sale price of real estate	$ 3,000
Joint venture gain on disposition of property	$ 1,300