Summary of Significant Accounting Policies (Investment Impairment) (Details) $ in Thousands, € in Millions				3 Months Ended					12 Months Ended
	Feb. 22, 2016 USD ($)	Feb. 01, 2015 USD ($)	Feb. 01, 2015 EUR (€)	Sep. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Summary of significant accounting policies
Impairment charge									$ 9,247	$ 11,540	$ 9,371
Sales price									2,271	0	0
Net proceeds | €			€ 0.2
Land
Summary of significant accounting policies
Impairment charge									7,000
Northpoint Central
Summary of significant accounting policies
Impairment charge				$ 2,200			$ 2,700		2,200	2,700
Unconsolidated Investment in Central Europe
Summary of significant accounting policies
Impairment charge				$ 7,000					7,000
Sales price		$ 900	€ 0.7
Northborough Tower
Summary of significant accounting policies
Impairment charge							2,100			2,100	$ 9,371
Chase- The Private Residences
Summary of significant accounting policies
Impairment charge						$ 100		$ 600		700
Chase - The Private Residences
Summary of significant accounting policies
Sales price	$ 2,500				$ 2,500
Net proceeds	$ 2,200				$ 2,200
Frisco Square | Land
Summary of significant accounting policies
Impairment charge							$ 6,800			6,800
Fair Value, Measurements, Nonrecurring
Summary of significant accounting policies
Impairment charge									$ 9,200	$ 12,300