Reportable Segments (Property Revenue in Excess of Expenses) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Total property revenues in excess of expenses	$ 50,845	$ 71,062
Domestic office investments
Segment Reporting Information [Line Items]
Total property revenues in excess of expenses	20,902	27,934
Domestic other investments
Segment Reporting Information [Line Items]
Total property revenues in excess of expenses	11,439	15,645
International office investments
Segment Reporting Information [Line Items]
Total property revenues in excess of expenses	11,232	17,967
International other investments
Segment Reporting Information [Line Items]
Total property revenues in excess of expenses	$ 7,272	$ 9,516