Real Estate Properties - Schedule of Unconsolidated Joint Venture: Mortgage Debt (Details) - Mortgages - Unconsolidated Joint Ventures - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Real Estate [Line Items]
Interest rate (as a percent)	3.93%
Principal balance	$ 82,000	$ 82,000
Prosperity Metro Plaza
Real Estate [Line Items]
Interest rate (as a percent)	4.09%
Principal balance	$ 50,000	50,000
1750 H Street, NW
Real Estate [Line Items]
Interest rate (as a percent)	3.69%
Principal balance	$ 32,000	$ 32,000