XML 151 R101.htm IDEA: XBRL DOCUMENT
v3.19.3.a.u2

Share-Based Payments (Details 5) - Market stock units - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Assumptions used in valuation of market based stock units |  |  |  |
Expected dividend yield | 0.00% |  | 0.00% | 0.00%
Minimum range of risk-free interest rates | 2.50% |  | 1.90% | 0.90%
Maximum range of risk-free interest rates | 2.50% |  | 2.30% | 1.60%
Average stock price on grant date minimum | $ 228.59 |  | $ 279.47 | $ 263.18
Average stock price on grant date maximum | $ 331.18 |  | $ 346.76 | $ 267.88
Weighted average grant date fair value | $ 378.08 | [1] | $ 378.85 | $ 382.59
Minimum |  |  |  |
Assumptions used in valuation of market based stock units |  |  |  |
Range of expected stock price volatility | 31.20% |  | 27.50% | 33.00%
Maximum |  |  |  |
Assumptions used in valuation of market based stock units |  |  |  |
Range of expected stock price volatility | 33.60% |  | 32.40% | 35.60%
[1] | MSUs granted during 2019 include awards granted in conjunction with our annual awards made in February 2019 and MSUs granted in conjunction with the hiring of employees. These grants reflect the target number of shares eligible to be earned at the time of grant. MSUs granted in 2019 also reflect an adjustment based upon the final performance multiplier in relation to shares granted in 2018, 2017 and 2016.

X
- Definition Average stock price on grant date in dollars maximum. + References No definition available. + Details
Name: | biib_AverageStockPriceOnGrantDateInDollarsMaximum
Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Average stock price on grant date in dollars minimum. + References No definition available. + Details
Name: | biib_AverageStockPriceOnGrantDateInDollarsMinimum
Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Valuation of granted stock units. + References No definition available. + Details
Name: | biib_ValuationOfGrantedStockUnitsAbstract
Namespace Prefix: | biib_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + 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 (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Namespace Prefix: | us-gaap_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | duration

X
- Definition The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term. + 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)(iii) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- 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=116856206&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The maximum risk-free interest rate assumption that is used in valuing an option on its own shares. + References No definition available. + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The minimum risk-free interest rate assumption that is used in valuing an option on its own shares. + References No definition available. + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_AwardTypeAxis=biib_MarketStockUnitsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |