Dispositions of properties (Details) - USD ($) $ in Thousands						3 Months Ended
	Apr. 05, 2016	Dec. 09, 2015	May. 13, 2015	Mar. 31, 2015	Feb. 23, 2015	Mar. 31, 2015	Mar. 31, 2016
Gain (loss) on sale of property
Gain (loss) on sale of property						$ 10,462
Asset held for sale							$ 15,921
Office Property in Plano, Texas | Disposal group disposed of by sale, not classified as discontinued operations
Gain (loss) on sale of property
Gain (loss) on sale of property					$ 1,500
Office Property in Eden Prairie, Minnesota | Disposal group disposed of by sale, not classified as discontinued operations
Gain (loss) on sale of property
Gain (loss) on sale of property				$ 9,000
Office Property in Charlotte, North Carolina | Disposal group disposed of by sale, not classified as discontinued operations
Gain (loss) on sale of property
Gain (loss) on sale of property			$ 900
Office Property in San Jose, California | Disposal group disposed of by sale, not classified as discontinued operations
Gain (loss) on sale of property
Gain (loss) on sale of property		$ 12,300
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
Asset held for sale							$ 4,100
Office Property In Maryland Heights Missouri | Disposal group disposed of by sale, not classified as discontinued operations
Gain (loss) on sale of property
Gain (loss) on sale of property	$ 4,100