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Subsequent Events	9 Months Ended
Subsequent Events	Sep. 30, 2019
Subsequent Events [Abstract]
Subsequent Events [Text Block]	13. Subsequent Events On October 1, 2019, a joint venture in which the Company has a 50% interest partially placed in-service Dock 72, a Class A office project with approximately 670,000 net rentable square feet located in Brooklyn, New York. On October 1, 2019, a joint venture in which the Company has a 50% interest partially placed in-service The Hub on Causeway - Residential, an approximately 320,000 square foot project comprised of 440 residential units located in Boston, Massachusetts. On October 24, 2019, the Company completed and placed in-service 145 Broadway, a build-to-suit Class A office project with approximately 485,000 net rentable square feet located in Cambridge, Massachusetts.