Dispositions of Properties (Details) - USD ($) $ in Thousands					3 Months Ended		9 Months Ended
	Oct. 20, 2017	Jan. 06, 2017	Dec. 16, 2016	Apr. 05, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Gain (loss) on sale of property
Gain (loss) on sale of property					$ (257)	$ (523)	$ (18,460)	$ (1,166)
Office Property in Baltimore Maryland
Gain (loss) on sale of property
Provision for loss on property held					$ 300	$ 500	$ 20,700	$ 5,300
Office Property in Baltimore Maryland | Subsequent Events.
Gain (loss) on sale of property
Net Proceeds	$ 31,600
Office Property In Milpitas, California | Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations
Gain (loss) on sale of property
Gain (loss) on sale of property		$ 2,300
Office Property In Maryland Heights Missouri | Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations
Gain (loss) on sale of property
Gain (loss) on sale of property				$ 4,200
Office Property In Federal Way Washington | Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations
Gain (loss) on sale of property
Gain (loss) on sale of property			$ (7,100)
Net Proceeds			$ 7,300