Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Overview	Biogen Idec is a global biotechnology company focused on discovering, developing, manufacturing and marketing therapies for the treatment of neurodegenerative diseases, hematologic conditions and autoimmune disorders. We also collaborate on the development and commercialization of RITUXAN for the treatment of non-Hodgkin's lymphoma, chronic lymphocytic leukemia and other conditions and share profits and losses for GAZYVA for the treatment of chronic lymphocytic leukemia.
Basis of presentation	In the opinion of management, the accompanying unaudited condensed consolidated financial statements include all adjustments, consisting of normal recurring accruals, necessary for a fair presentation of our financial statements for interim periods in accordance with accounting principles generally accepted in the United States (U.S. GAAP). The information included in this quarterly report on Form 10-Q should be read in conjunction with our consolidated financial statements and the accompanying notes included in our Annual Report on Form 10-K for the year ended December 31, 2013 (2013 Form 10-K). Our accounting policies are described in the “Notes to Consolidated Financial Statements” in our 2013 Form 10-K and updated, as necessary, in this Form 10-Q. The year-end condensed consolidated balance sheet data presented for comparative purposes was derived from our audited financial statements, but does not include all disclosures required by U.S. GAAP. The results of operations for the three and six months ended June 30, 2014 are not necessarily indicative of the operating results for the full year or for any other subsequent interim period.
Consolidation	Our condensed consolidated financial statements reflect our financial statements, those of our wholly-owned subsidiaries and those of certain variable interest entities where we are the primary beneficiary. For consolidated entities where we own or are exposed to less than 100% of the economics, we record net income (loss) attributable to noncontrolling interests in our condensed consolidated statements of income equal to the percentage of the economic or ownership interest retained in such entities by the respective noncontrolling parties. Intercompany balances and transactions are eliminated in consolidation. In determining whether we are the primary beneficiary of an entity and therefore required to consolidate, we apply a qualitative approach that determines whether we have both (1) the power to direct the economically significant activities of the entity and (2) the obligation to absorb losses of, or the right to receive benefits from, the entity that could potentially be significant to that entity. These considerations impact the way we account for our existing collaborative relationships and other arrangements. We continuously assess whether we are the primary beneficiary of a variable interest entity as changes to existing relationships or future transactions may result in us consolidating or deconsolidating one or more of our collaborators or partners.
Use of estimates	The preparation of our condensed consolidated financial statements requires us to make estimates, judgments, and assumptions that may affect the reported amounts of assets, liabilities, equity, revenues and expenses, and related disclosure of contingent assets and liabilities. On an on-going basis, we evaluate our estimates and judgments and methodologies. We base our estimates on historical experience and on various other assumptions that are believed to be reasonable, the results of which form the basis for making judgments about the carrying values of assets and liabilities. Actual results may differ from these estimates under different assumptions or conditions.
Property, plant and equipment	Property, plant and equipment are recorded at historical cost, net of accumulated depreciation. Accumulated depreciation on property, plant and equipment was $1,210.6 million and $1,118.3 million as of June 30, 2014 and December 31, 2013, respectively.
Accounting for share-based compensation	During the six months ended June 30, 2014, under our share-based compensation program we began to grant awards for performance-vested restricted stock units, which can be settled in cash or shares of our common stock (PUs) at the sole discretion of the Compensation and Management Development Committee of the Board of Directors. We have classified these awards as a liability as, historically, similar awards have been settled in cash. We record the estimated fair value of PUs as compensation expense over the requisite service period, which is generally the vesting period. Where awards are made with non-substantive vesting periods (for instance, where a portion of the award vests upon retirement eligibility), we estimate and recognize expense, net of forfeitures, over the period from the grant date to the date on which the employee is retirement eligible. We apply an accelerated attribution method to recognize share based compensation expense when accounting for our PUs and the fair value of the liability is remeasured at the end of each reporting period through expected settlement. Compensation expense associated with PUs is based upon the share price and the number of units expected to be earned after assessing the probability that certain performance criteria will be met and the associated targeted payout level that is forecasted will be achieved, net of estimated forfeitures. Cumulative adjustments are recorded each quarter to reflect changes in the share price and estimated outcome of the performance-related conditions until the date results are determined and settled.
New accounting pronouncements	From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (FASB) or other standard setting bodies that are adopted by the Company as of the specified effective date. Unless otherwise discussed, we believe that the impact of recently issued standards that are not yet effective will not have a material impact on our financial position or results of operations upon adoption. During the quarter ended June 30, 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606), which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. The new standard will be effective for us on January 1, 2017. We are currently evaluating the potential impact that Topic 606 may have on our financial position and results of operations. Also during the quarter ended June 30, 2014, the FASB issued ASU No. 2014-11, Transfers and Servicing (Topic 860): Repurchase-to-Maturity Transactions, Repurchase Financings, and Disclosure. The new standard expands secured borrowing accounting for repurchase-to-maturity transactions and repurchase financings and sets forth new disclosure requirements for repurchase agreements, securities lending transactions, and repurchase-to-maturity transactions that are accounted for as secured borrowings. The new standard will be effective for us on April 1, 2015. The adoption of this standard is not expected to have an impact on our financial position or results of operations

Disclosure of accounting policy regarding (1) the principles it follows in consolidating or combining the separate financial statements, including the principles followed in determining the inclusion or exclusion of subsidiaries or other entities in the consolidated or combined financial statements and (2) its treatment of interests (for example, common stock, a partnership interest or other means of exerting influence) in other entities, for example consolidation or use of the equity or cost methods of accounting. The accounting policy may also address the accounting treatment for intercompany accounts and transactions, noncontrolling interest, and the income statement treatment in consolidation for issuances of stock by a subsidiary.

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Disclosure of accounting policy for long-lived, physical assets used in the normal conduct of business and not intended for resale. Includes, but is not limited to, basis of assets, depreciation and depletion methods used, including composite deprecation, estimated useful lives, capitalization policy, accounting treatment for costs incurred for repairs and maintenance, capitalized interest and the method it is calculated, disposals and impairments.

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