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Stock Option and Incentive Plan Stock and Units (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands |  | 12 Months Ended |
--- | --- | --- | ---
Feb. 04, 2020 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Shares issued |  | 155,797,725 | 154,869,198 |  |
Stock based compensation expense |  | $ 43,000 | $ 39,800 | $ 38,000 |
Common Stock, Value, Issued |  | 1,557 | $ 1,548 |  |
Intrinsic Value of stock options |  | $ 400 |  |  |
Vesting Period |  | 4 years |  |  |
Beginning outstanding, shares |  | 351,561 | 395,353 | 540,441 | 540,441
Weighted Average Exercise Price |  | $ 96.97 | $ 96.37 | $ 96.35 | $ 96.35
Restricted Stock [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Shares issued |  | 29,630 | 26,503 | 20,320 |
LTIP Units [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Value Of LTIP Units Issued |  | $ 26,300 | $ 22,100 | $ 22,700 |
Expected life assumed to calculate per unit fair value per LTIP unit (in years) |  | 5 years 8 months 12 days | 5 years 8 months 12 days | 5 years 8 months 12 days |
Risk-free rate |  | 1.47% | 2.68% | 2.63% |
Expected price volatility |  | 18.00% | 27.00% | 27.00% |
Weighted average price (per unit) |  | $ 127.14 | $ 121.50 | $ 110.29 |
Employee Stock |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Common stock available for issuance |  | 250,000 |  |  |
Number of shares issued |  | 7,195 | 5,862 | 6,268 |
Employee discount |  | 85.00% |  |  |
Weighted Average Price of Shares Purchased |  | $ 90.53 | $ 104.11 | $ 107.20 |
stock option |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Intrinsic Value of stock options |  | $ 0 | $ 0 | $ 0 |
Boston Properties Limited Partnership | LTIP Units [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Number of shares issued |  | 207,181 | 181,919 | 205,838 |
Boston Properties Limited Partnership | MYLTIP 2020 [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Number of shares issued |  | 203,278 |  |  |
Boston Properties Limited Partnership | MYLTIP 2019 [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Number of shares issued |  |  | 220,734 |  |
Boston Properties Limited Partnership | MYLTIP 2018 |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Number of shares issued |  |  |  | 342,659 |
Restricted Stock [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Employee and director payment per share (in dollars per share) |  | $ 0.01 |  |  |
Common Stock, Value, Issued |  | $ 4,000 | $ 3,500 | $ 2,400 |
Common Stock Weighted Average Price (in dollars per share) |  | $ 133.81 | $ 131.27 | $ 119.27 |
MYLTIP 2020 [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Vesting Period | 4 years |  |  |  |
LTIP and MYLTIP Units [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Employee and director payment per share (in dollars per share) |  | $ 0.25 |  |  |
Ltips (including vested MYLTIPS) And Restricted Stock [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Unrecognized compensation expenses |  | $ 25,400 |  |  |
Unvested MYLTIP Units [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Unrecognized compensation expenses |  | $ 10,300 |  |  |
Weighted-average period (years) |  | 2 years 1 month 6 days |  |  |

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- Definition The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(ii) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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