Reportable Segments (Narratives) (Details)	3 Months Ended
	Mar. 31, 2018 real_estate_investment segment	Dec. 31, 2015 real_estate_investment
Segment Reporting Information [Line Items]
Number of reportable segments | segment	4
Number of real estate investments	34	45
Domestic office investments
Segment Reporting Information [Line Items]
Number of real estate investments	9
Domestic other investments
Segment Reporting Information [Line Items]
Number of real estate investments	5
International office investments
Segment Reporting Information [Line Items]
Number of real estate investments	9
International other investments
Segment Reporting Information [Line Items]
Number of real estate investments	11