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Employee Benefit Plans | 12 Months Ended
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Dec. 31, 2017
Employee Benefit Plans [Abstract] |
Compensation and Employee Benefit Plans [Text Block] | 16. Employee Benefit Plans Effective January 1, 1985, the predecessor of the Company adopted a 401(k) Savings Plan (the “Plan”) for its employees. Under the Plan, as amended, employees, as defined, are eligible to participate in the Plan after they have completed three months of service. Upon formation, the Company adopted the Plan and the terms of the Plan. Under the Plan, as amended, the Company’s matching contribution equals 200% of the first 3% of participant’s eligible earnings contributed (utilizing earnings that are not in excess of an amount established by the IRS ($270,000, $265,000 and $265,000 in 2017, 2016 and 2015, respectively), indexed for inflation) with no vesting requirement. The Company’s aggregate matching contribution for the years ended December 31, 2017, 2016 and 2015 was $4.1 million, $4.0 million and $3.7 million, respectively. The Plan also provides for supplemental retirement contributions to certain employees who had at least ten years of service on January 1, 2001, and who were 40 years of age or older as of January 1, 2001. The maximum supplemental retirement contribution will not exceed the annual limit on contributions established by the IRS. The Company will record an annual supplemental retirement credit for the benefit of each participant. The Company’s supplemental retirement contribution and credit for the years ended December 31, 2017, 2016 and 2015 was $18,000, $21,000 and $42,000, respectively. The Company also maintains a deferred compensation plan that is designed to allow officers of Boston Properties, Inc. to defer a portion of their current income on a pre-tax basis and receive a tax-deferred return on these deferrals. The Company’s obligation under the plan is that of an unsecured promise to pay the deferred compensation to the plan participants in the future. At December 31, 2017 and 2016, the Company had maintained approximately $29.2 million and $23.8 million, respectively, in a separate account, which is not restricted as to its use. The Company’s liability under the plan is equal to the total amount of compensation deferred by the plan participants and earnings on the deferred compensation pursuant to investments elected by the plan participants. The Company’s liability as of December 31, 2017 and 2016 was $29.2 million and $23.8 million, respectively, which are included in the accompanying Consolidated Balance Sheets.

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- Definition Employee Benefit Plans [Abstract] + References No definition available. + Details
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- Definition The entire disclosure for an entity's employee compensation and benefit plans, including, but not limited to, postemployment and postretirement benefit plans, defined benefit pension plans, defined contribution plans, non-qualified and supplemental benefit plans, deferred compensation, share-based compensation, life insurance, severance, health care, unemployment and other benefit plans. + References No definition available. + Details
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