Reportable Segments (Assets) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total Assets	$ 3,457,535	$ 3,726,197
Domestic office investments
Segment Reporting Information [Line Items]
Total Assets	1,141,163	1,146,312
Domestic other investments
Segment Reporting Information [Line Items]
Total Assets	778,785	794,558
International office investments
Segment Reporting Information [Line Items]
Total Assets	1,063,283	1,053,971
International other investments
Segment Reporting Information [Line Items]
Total Assets	437,901	429,827
Corporate Segment
Segment Reporting Information [Line Items]
Total Assets	$ 36,403	$ 301,529