Stock Option and Incentive Plan	3 Months Ended
Mar. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock Option and Incentive Plan	10. Stock Option and Incentive Plan On February 6, 2018, Boston Properties, Inc.’s Compensation Committee approved the 2018 MYLTIP awards under Boston Properties, Inc.’s 2012 Plan to certain officers and employees of Boston Properties, Inc. The 2018 MYLTIP awards utilize Boston Properties, Inc.’s TSR over a three-year measurement period, on an annualized, compounded basis, as the performance metric. Earned awards will be based on Boston Properties, Inc.’s TSR relative to (i) the Cohen & Steers Realty Majors Portfolio Index (50% weight) and (ii) the Nareit Office Index adjusted to include Vornado Realty Trust (50% weight). Earned awards will range from zero to a maximum of approximately $32.3 million depending on Boston Properties, Inc.’s TSR relative to the two indices, with a target of approximately $16.2 million and linear interpolation between zero and maximum. Earned awards measured on the basis of relative TSR performance are subject to an absolute TSR component in the form of relatively simple modifiers that (A) reduce the level of earned awards in the event Boston Properties, Inc.’s annualized TSR is less than 0% and (B) cause some awards to be earned in the event Boston Properties, Inc.’s annualized TSR is more than 12% even though on a relative basis alone Boston Properties, Inc.’s TSR would not result in any earned awards. Earned awards (if any) will vest 50% on February 5, 2021 and 50% on February 5, 2022, based on continued employment. Vesting will be accelerated in the event of a change in control, termination of employment by Boston Properties, Inc. without cause, or termination of employment by the award recipient for good reason, death, disability or retirement. If there is a change of control prior to February 5, 2021, earned awards will be calculated based on TSR performance up to the date of the change of control. The 2018 MYLTIP awards are in the form of LTIP Units issued on the grant date which (i) are subject to forfeiture to the extent awards are not earned and (ii) prior to the performance measurement date are only entitled to one-tenth (10%) of the regular quarterly distributions payable on common partnership units. Under ASC 718, the 2018 MYLTIP awards have an aggregate value of approximately $13.3 million, which amount will generally be amortized into earnings over the four-year plan period under the graded vesting method. On February 4, 2018, the measurement period for the Company’s 2015 MYLTIP awards ended and, based on Boston Properties, Inc.’s relative TSR performance, the final awards were determined to be 22.0% of target or an aggregate of approximately $3.6 million (after giving effect to voluntary employee separations). As a result, an aggregate of 337,847 2015 MYLTIP Units that had been previously granted were automatically forfeited. During the three months ended March 31, 2018, Boston Properties, Inc. issued 18,226 shares of restricted common stock and Boston Properties Limited Partnership issued 195,546 LTIP Units and 342,659 2018 MYLTIP Units to employees under the 2012 Plan. Employees paid $0.01 per share of restricted common stock and $0.25 per LTIP Unit and 2018 MYLTIP Unit. When issued, LTIP Units are not economically equivalent in value to a share of Common Stock, but over time can increase in value to one-for-one parity with Common Stock if there is sufficient appreciation in the value of the Company’s assets. The aggregate value of the LTIP Units is included in noncontrolling interests in the Consolidated Balance Sheets. Grants of restricted stock and LTIP Units to employees vest in four equal annual installments. Restricted stock is measured at fair value on the date of grant based on the number of shares granted and the closing price of Boston Properties, Inc.’s Common Stock on the date of grant as quoted on the New York Stock Exchange. Such value is recognized as an expense ratably over the corresponding employee service period. The shares of restricted stock granted during the three months ended March 31, 2018 were valued at approximately $2.2 million ($118.98 per share weighted-average). The LTIP Units granted were valued at approximately $21.5 million (approximately $109.88 per unit weighted-average fair value) using a Monte Carlo simulation method model. The per unit fair values of the LTIP Units granted were estimated on the dates of grant and for a substantial majority of such units were valued using the following assumptions: an expected life of 5.7 years, a risk-free interest rate of 2.63% and an expected price volatility of 27.0%. Because the 2012 OPP Units, 2013 MYLTIP Units, 2014 MYLTIP Units, 2015 MYLTIP Units, 2016 MYLTIP Units, 2017 MYLTIP Units and 2018 MYLTIP Units are subject to both a service condition and a market condition, the Company recognizes the related compensation expense under the graded vesting attribution method. Under the graded vesting attribution method, each portion of the award that vests at a different date is accounted for as a separate award and recognized over the period appropriate to that portion so that the compensation cost for each portion should be recognized in full by the time that portion vests. The Company recognizes forfeitures as they occur on its awards of stock-based compensation. Dividends paid on both vested and unvested shares of restricted stock are charged directly to Dividends in Excess of Earnings in Boston Properties, Inc.’s Consolidated Balance Sheets and Partners’ Capital in Boston Properties Limited Partnership’s Consolidated Balance Sheets. Aggregate stock-based compensation expense associated with restricted stock, non-qualified stock options, LTIP Units, 2012 OPP Units, 2013 MYLTIP Units, 2014 MYLTIP Units, 2015 MYLTIP Units, 2016 MYLTIP Units, 2017 MYLTIP Units and 2018 MYLTIP Units was approximately $14.2 million and $10.3 million for the three months ended March 31, 2018 and 2017, respectively. At March 31, 2018, there was (1) an aggregate of approximately $36.5 million of unrecognized compensation expense related to unvested restricted stock, LTIP Units and 2015 MYLTIP Units and (2) an aggregate of approximately $26.0 million of unrecognized compensation expense related to unvested 2016 MYLTIP Units, 2017 MYLTIP Units and 2018 MYLTIP Units that is expected to be recognized over a weighted-average period of approximately 2.8 years.

10. Stock Option and Incentive Plan

On February 6, 2018, Boston Properties, Inc.’s Compensation Committee approved the 2018 MYLTIP awards under Boston Properties, Inc.’s 2012 Plan to certain officers and employees of Boston Properties, Inc. The 2018 MYLTIP awards utilize Boston Properties, Inc.’s TSR over a three-year measurement period, on an annualized, compounded basis, as the performance metric. Earned awards will be based on Boston Properties, Inc.’s TSR relative to (i) the Cohen & Steers Realty Majors Portfolio Index (50% weight) and (ii) the Nareit Office Index adjusted to include Vornado Realty Trust (50% weight). Earned awards will range from zero to a maximum of approximately $32.3 million depending on Boston Properties, Inc.’s TSR relative to the two indices, with a target of approximately $16.2 million and linear interpolation between zero and maximum. Earned awards measured on the basis of relative TSR performance are subject to an absolute TSR component in the form of relatively simple modifiers that (A) reduce the level of earned awards in the event Boston Properties, Inc.’s annualized TSR is less than 0% and (B) cause some awards to be earned in the event Boston Properties, Inc.’s annualized TSR is more than 12% even though on a relative basis alone Boston Properties, Inc.’s TSR would not result in any earned awards.

Earned awards (if any) will vest 50% on February 5, 2021 and 50% on February 5, 2022, based on continued employment. Vesting will be accelerated in the event of a change in control, termination of employment by Boston Properties, Inc. without cause, or termination of employment by the award recipient for good reason, death, disability or retirement. If there is a change of control prior to February 5, 2021, earned awards will be calculated based on TSR performance up to the date of the change of control. The 2018 MYLTIP awards are in the form of LTIP Units issued on the grant date which (i) are subject to forfeiture to the extent awards are not earned and (ii) prior to the performance measurement date are only entitled to one-tenth (10%) of the regular quarterly distributions payable on common partnership units.

Under ASC 718, the 2018 MYLTIP awards have an aggregate value of approximately $13.3 million, which amount will generally be amortized into earnings over the four-year plan period under the graded vesting method.

On February 4, 2018, the measurement period for the Company’s 2015 MYLTIP awards ended and, based on Boston Properties, Inc.’s relative TSR performance, the final awards were determined to be 22.0% of target or an aggregate of approximately $3.6 million (after giving effect to voluntary employee separations). As a result, an aggregate of 337,847 2015 MYLTIP Units that had been previously granted were automatically forfeited.

During the three months ended March 31, 2018, Boston Properties, Inc. issued 18,226 shares of restricted common stock and Boston Properties Limited Partnership issued 195,546 LTIP Units and 342,659 2018 MYLTIP Units to employees under the 2012 Plan. Employees paid $0.01 per share of restricted common stock and $0.25 per LTIP Unit and 2018 MYLTIP Unit. When issued, LTIP Units are not economically equivalent in value to a share of Common Stock, but over time can increase in value to one-for-one parity with Common Stock if there is sufficient appreciation in the value of the Company’s assets. The aggregate value of the LTIP Units is included in noncontrolling interests in the Consolidated Balance Sheets. Grants of restricted stock and LTIP Units to employees vest in four equal annual installments. Restricted stock is measured at fair value on the date of grant based on the number of shares granted and the closing price of Boston Properties, Inc.’s Common Stock on the date of grant as quoted on the New York Stock Exchange. Such value is recognized as an expense ratably over the corresponding employee service period. The shares of restricted stock granted during the three months ended March 31, 2018 were valued at approximately $2.2 million ($118.98 per share weighted-average). The LTIP Units granted were valued at approximately $21.5 million (approximately $109.88 per unit weighted-average fair value) using a Monte Carlo simulation method model. The per unit fair values of the LTIP Units granted were estimated on the dates of grant and for a substantial majority of such units were valued using the following assumptions: an expected life of 5.7 years, a risk-free interest rate of 2.63% and an expected price volatility of 27.0%. Because the 2012 OPP Units, 2013 MYLTIP Units, 2014 MYLTIP Units, 2015 MYLTIP Units, 2016 MYLTIP Units, 2017 MYLTIP Units and 2018 MYLTIP Units are subject to both a service condition and a market condition, the Company recognizes the related compensation expense under the graded vesting attribution method. Under the graded vesting attribution method, each portion of the award that vests at a different date is accounted for as a separate award and recognized over the period appropriate to that portion so that the compensation cost for each portion should be recognized in full by the time that portion vests. The Company recognizes forfeitures as they occur on its awards of stock-based compensation. Dividends paid on both vested and unvested shares of restricted stock are charged directly to Dividends in Excess of Earnings in Boston Properties, Inc.’s Consolidated Balance Sheets and Partners’ Capital in Boston Properties Limited Partnership’s Consolidated Balance Sheets. Aggregate stock-based compensation expense associated with restricted stock, non-qualified stock options, LTIP Units, 2012 OPP Units, 2013 MYLTIP Units, 2014 MYLTIP Units, 2015 MYLTIP Units, 2016 MYLTIP Units, 2017 MYLTIP Units and 2018 MYLTIP Units was approximately $14.2 million and $10.3 million for the three months ended March 31, 2018 and 2017, respectively. At March 31, 2018, there was (1) an aggregate of approximately $36.5 million of unrecognized compensation expense related to unvested restricted stock, LTIP Units and 2015 MYLTIP Units and (2) an aggregate of approximately $26.0 million of unrecognized compensation expense related to unvested 2016 MYLTIP Units, 2017 MYLTIP Units and 2018 MYLTIP Units that is expected to be recognized over a weighted-average period of approximately 2.8 years.