Subsequent Events (Notes)	3 Months Ended
Mar. 31, 2018
Subsequent Events [Abstract]
Subsequent Events [Text Block]	12. Subsequent Events On April 19, 2018, a joint venture in which the Company has a 50% interest obtained construction financing with a total commitment of $180.0 million collateralized by its Hub on Causeway - Residential development project. The construction financing bears interest at a variable rate equal to LIBOR plus 2.00% per annum and matures on April 19, 2022, with two, one-year extension options, subject to certain conditions. The joint venture has not yet drawn any funds under the loan. The Hub on Causeway - Residential is an approximately 320,000 square foot project comprised of 440 residential units located in Boston, Massachusetts. On April 23, 2018, the Company entered into an agreement to acquire Santa Monica Business Park in the Ocean Park neighborhood of Santa Monica, California for a net purchase price of approximately $616.0 million. Santa Monica Business Park is a 47-acre office park consisting of 21 buildings totaling approximately 1.2 million net rentable square feet. Approximately 70% of the rentable square footage is subject to a ground lease with 80 years remaining, including renewal periods. The ground lease provides the Company with the right to purchase the land underlying the properties subject to the ground lease in 2028 with subsequent purchase rights every 15 years. The closing is subject to customary closing conditions and termination rights for transactions of this type. There can be no assurance that the acquisition will be completed on the terms currently contemplated, or at all. On April 24, 2018, Boston Properties Limited Partnership exercised its option to draw $500.0 million on its Delayed Draw Facility. The Delayed Draw Facility totaling $500.0 million bears interest at a variable rate equal to LIBOR plus 0.90% per annum based on Boston Properties Limited Partnership's current credit rating and matures on April 24, 2022. On April 27, 2018, a joint venture in which the Company has a 60% interest refinanced the mortgage loan collateralized by its 540 Madison Avenue property located in New York City totaling $120.0 million. The mortgage loan bears interest at a variable rate equal to LIBOR plus 1.10% per annum and matures on June 5, 2023. The previous mortgage loan bore interest at a variable rate equal to LIBOR plus 1.50% per annum and was scheduled to mature on June 5, 2018. 540 Madison Avenue is an approximately 284,000 net rentable square foot Class A office property.

12. Subsequent Events

On April 19, 2018, a joint venture in which the Company has a 50% interest obtained construction financing with a total commitment of $180.0 million collateralized by its Hub on Causeway - Residential development project. The construction financing bears interest at a variable rate equal to LIBOR plus 2.00% per annum and matures on April 19, 2022, with two, one-year extension options, subject to certain conditions. The joint venture has not yet drawn any funds under the loan. The Hub on Causeway - Residential is an approximately 320,000 square foot project comprised of 440 residential units located in Boston, Massachusetts.

On April 23, 2018, the Company entered into an agreement to acquire Santa Monica Business Park in the Ocean Park neighborhood of Santa Monica, California for a net purchase price of approximately $616.0 million. Santa Monica Business Park is a 47-acre office park consisting of 21 buildings totaling approximately 1.2 million net rentable square feet. Approximately 70% of the rentable square footage is subject to a ground lease with 80 years remaining, including renewal periods. The ground lease provides the Company with the right to purchase the land underlying the properties subject to the ground lease in 2028 with subsequent purchase rights every 15 years. The closing is subject to customary closing conditions and termination rights for transactions of this type. There can be no assurance that the acquisition will be completed on the terms currently contemplated, or at all.

On April 24, 2018, Boston Properties Limited Partnership exercised its option to draw $500.0 million on its Delayed Draw Facility. The Delayed Draw Facility totaling $500.0 million bears interest at a variable rate equal to LIBOR plus 0.90% per annum based on Boston Properties Limited Partnership's current credit rating and matures on April 24, 2022.

On April 27, 2018, a joint venture in which the Company has a 60% interest refinanced the mortgage loan collateralized by its 540 Madison Avenue property located in New York City totaling $120.0 million. The mortgage loan bears interest at a variable rate equal to LIBOR plus 1.10% per annum and matures on June 5, 2023. The previous mortgage loan bore interest at a variable rate equal to LIBOR plus 1.50% per annum and was scheduled to mature on June 5, 2018. 540 Madison Avenue is an approximately 284,000 net rentable square foot Class A office property.