Reportable Segments (Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Revenues	$ 84,499	$ 106,366
Domestic office investments
Segment Reporting Information [Line Items]
Revenues	32,662	44,095
Domestic other investments
Segment Reporting Information [Line Items]
Revenues	20,104	24,796
International office investments
Segment Reporting Information [Line Items]
Revenues	21,509	24,343
International other investments
Segment Reporting Information [Line Items]
Revenues	$ 10,224	$ 13,132

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Name:	us-gaap_Revenues
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration