Subsequent Events	6 Months Ended
Jun. 30, 2018
Subsequent Events [Abstract]
Subsequent Events [Text Block]	12. Subsequent Events On July 13, 2018, the Company entered into a joint venture with a third party to acquire a development site at 3 Hudson Boulevard that, upon the future acquisition of additional available development rights, can accommodate a Class A office tower with up to 2.0 million net rentable square feet located on the entire square block between 11th Avenue and Hudson Boulevard Park from West 34th Street to West 35th Street in New York City. The Company owns a 25% interest in and will be the managing member of the joint venture. The acquisition includes improvements consisting of excavation work and foundation elements that are currently being constructed on the site. The Company contributed cash totaling approximately $45.6 million at closing and will contribute in the future approximately $62.2 million for its initial capital contribution, a portion of which will fund the remaining costs to complete the foundation elements to grade for the future office building. In addition, the Company has provided $80.0 million of mortgage financing to the joint venture which bears interest at a variable rate equal to LIBOR plus 3.50% per annum and matures on July 13, 2023, with extension options, subject to certain conditions. On July 19, 2018, the Company completed the acquisition of Santa Monica Business Park in the Ocean Park neighborhood of Santa Monica, California for a purchase price of approximately $627.5 million, including $11.5 million of seller funded leasing costs after the effective date of the purchase and sale agreement. 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 in 2028 with subsequent purchase rights every 15 years. The acquisition was completed in a joint venture with Canada Pension Plan Investment Board, which invested approximately $147.4 million for a 45% ownership interest in the joint venture. The Company will provide customary operating, property management and leasing services to, and invested approximately $180.1 million in the joint venture. The acquisition was completed with $300.0 million of financing. The mortgage financing bears interest at a variable rate equal to LIBOR plus 1.28% per annum and matures on July 19, 2025. At closing, the borrower under the loan, which is a subsidiary of the joint venture, entered into interest rate swap contracts with notional amounts aggregating $300.0 million through April 1, 2025, resulting in a fixed rate of approximately 4.063% per annum through the expiration of the interest rate swap contracts. On July 27, 2018, the Company entered into a joint venture with its partner at The Hub on Causeway mixed-use development in Boston, Massachusetts to acquire the air rights for the development of an approximately 627,000 net rentable square foot Class A office tower at the site to be known as 100 Causeway Street. The joint venture entered into a lease agreement with an affiliate of Verizon Communications, Inc. under which Verizon will lease approximately 70% of the office tower for a term of 20 years. With the execution of the lease, the joint venture commenced development of the project. The Company will serve as co-development manager for the project. The joint venture partner contributed an air rights parcel and improvements, with a fair value of approximately $41.3 million, for its initial 50% interest in the joint venture. The Company contributed improvements totaling approximately $3.9 million and will contribute cash totaling approximately $37.4 million for its initial 50% interest. On August 7, 2018, the Company entered into an agreement with a third party to (1) agree to share certain pre-development costs during the pre-lease period and (2) to agree to form a joint venture to thereafter own and develop a leasehold interest in 343 Madison Avenue located in New York City. The Company will serve as development manager of the project and will own a 55% interest in the joint venture. In 2016, the Company was selected by the Metropolitan Transportation Authority ("MTA") as the developer of the project and will enter into a pre-lease agreement and a 99-year ground lease with the MTA for the site. The site will support a Class A office tower with approximately 900,000 net rentable square feet. There can be no assurances that the transaction will be completed on the terms currently contemplated, or at all.

12. Subsequent Events

On July 13, 2018, the Company entered into a joint venture with a third party to acquire a development site at 3 Hudson Boulevard that, upon the future acquisition of additional available development rights, can accommodate a Class A office tower with up to 2.0 million net rentable square feet located on the entire square block between 11th Avenue and Hudson Boulevard Park from West 34th Street to West 35th Street in New York City. The Company owns a 25% interest in and will be the managing member of the joint venture. The acquisition includes improvements consisting of excavation work and foundation elements that are currently being constructed on the site. The Company contributed cash totaling approximately $45.6 million at closing and will contribute in the future approximately $62.2 million for its initial capital contribution, a portion of which will fund the remaining costs to complete the foundation elements to grade for the future office building. In addition, the Company has provided $80.0 million of mortgage financing to the joint venture which bears interest at a variable rate equal to LIBOR plus 3.50% per annum and matures on July 13, 2023, with extension options, subject to certain conditions.

On July 19, 2018, the Company completed the acquisition of Santa Monica Business Park in the Ocean Park neighborhood of Santa Monica, California for a purchase price of approximately $627.5 million, including $11.5 million of seller funded leasing costs after the effective date of the purchase and sale agreement. 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 in 2028 with subsequent purchase rights every 15 years. The acquisition was completed in a joint venture with Canada Pension Plan Investment Board, which invested approximately $147.4 million for a 45% ownership interest in the joint venture. The Company will provide customary operating, property management and leasing services to, and invested approximately $180.1 million in the joint venture. The acquisition was completed with $300.0 million of financing. The mortgage financing bears interest at a variable rate equal to LIBOR plus 1.28% per annum and matures on July 19, 2025. At closing, the borrower under the loan, which is a subsidiary of the joint venture, entered into interest rate swap contracts with notional amounts aggregating $300.0 million through April 1, 2025, resulting in a fixed rate of approximately 4.063% per annum through the expiration of the interest rate swap contracts.

On July 27, 2018, the Company entered into a joint venture with its partner at The Hub on Causeway mixed-use development in Boston, Massachusetts to acquire the air rights for the development of an approximately 627,000 net rentable square foot Class A office tower at the site to be known as 100 Causeway Street. The joint venture entered into a lease agreement with an affiliate of Verizon Communications, Inc. under which Verizon will lease approximately 70% of the office tower for a term of 20 years. With the execution of the lease, the joint venture commenced development of the project. The Company will serve as co-development manager for the project. The joint venture partner contributed an air rights parcel and improvements, with a fair value of approximately $41.3 million, for its initial 50% interest in the joint venture. The Company contributed improvements totaling approximately $3.9 million and will contribute cash totaling approximately $37.4 million for its initial 50% interest.

On August 7, 2018, the Company entered into an agreement with a third party to (1) agree to share certain pre-development costs during the pre-lease period and (2) to agree to form a joint venture to thereafter own and develop a leasehold interest in 343 Madison Avenue located in New York City. The Company will serve as development manager of the project and will own a 55% interest in the joint venture. In 2016, the Company was selected by the Metropolitan Transportation Authority ("MTA") as the developer of the project and will enter into a pre-lease agreement and a 99-year ground lease with the MTA for the site. The site will support a Class A office tower with approximately 900,000 net rentable square feet. There can be no assurances that the transaction will be completed on the terms currently contemplated, or at all.