Biogen (BIIB) 10-Q 24 Oct 17 2017 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2017	Oct. 20, 2017
Document and Entity Information [Abstract]
Entity Registrant Name	BIOGEN INC.
Entity Central Index Key	875,045
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Type	10-Q
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q3
Amendment Flag	false
Entity Common Stock, Shares Outstanding		211,476,936

Condensed Consolidated Statemen

Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenues:
Product, net	$ 2,622.5	$ 2,539.6	$ 7,642.3	$ 7,315
Revenues from anti-CD20 therapeutic programs	406.5	317.6	1,144.2	996.3
Other	48.8	98.6	180.4	265.5
Total revenues	3,077.8	2,955.8	8,966.9	8,576.8
Cost and expenses:
Cost of sales, excluding amortization of acquired intangible assets	370	416.9	1,120.8	1,100.2
Research and development	446.4	529	1,666	1,439.4
Selling, general and administrative	433.8	462.7	1,363.1	1,452.4
Amortization of acquired intangible assets	108.9	99.7	674.9	281.4
Acquired in-process research and development	0	0	120	0
Collaboration profit (loss) sharing	35.2	4.7	82.5	(0.9)
(Gain) loss on fair value remeasurement of contingent consideration	30	5.9	61.2	18.8
Restructuring charges	0	11.6	0	21.3
Total cost and expenses	1,424.3	1,530.5	5,088.5	4,312.6
Income from operations	1,653.5	1,425.3	3,878.4	4,264.2
Other income (expense), net	(43.6)	(58.1)	(149.4)	(169.4)
Income before income tax expense and equity in loss of investee, net of tax	1,609.9	1,367.2	3,729	4,094.8
Income tax expense	383.8	337	892.6	1,047
Equity in loss of investee, net of tax	0	0	0	0
Net income	1,226.1	1,030.2	2,836.4	3,047.8
Net income (loss) attributable to noncontrolling interests, net of tax	0	(2.7)	(0.1)	(5.8)
Net income attributable to Biogen Inc.	$ 1,226.1	$ 1,032.9	$ 2,836.5	$ 3,053.6
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$ 5.80	$ 4.72	$ 13.32	$ 13.95
Diluted earnings per share attributable to Biogen Inc.	$ 5.79	$ 4.71	$ 13.30	$ 13.92
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	211.4	218.9	213	219
Diluted earnings per share attributable to Biogen Inc.	211.8	219.4	213.3	219.4

Condensed Consolidated Stateme3

Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Inc.	$ 1,226.1	$ 1,032.9	$ 2,836.5	$ 3,053.6
Other comprehensive income:
Unrealized gains (losses) on securities available for sale, net of tax	1	(5.4)	6.6	1.6
Unrealized gains (losses) on cash flow hedges, net of tax	(35.5)	1.3	(162.3)	(17)
Unrealized gains (losses) on pension benefit obligation, net of tax	0	0.4	(0.5)	1.3
Currency translation adjustment	43.9	(14.4)	146.7	(62.8)
Total other comprehensive income (loss), net of tax	9.4	(18.1)	(9.5)	(76.9)
Comprehensive income attributable to Biogen Inc.	1,235.5	1,014.8	2,827	2,976.7
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	0	(2.6)	(0.1)	(5.7)
Comprehensive income	$ 1,235.5	$ 1,012.2	$ 2,826.9	$ 2,971

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 1,548.1	$ 2,326.5
Marketable securities	1,960.2	2,568.6
Accounts receivable, net	1,567.5	1,441.6
Due from anti-CD20 therapeutic programs	518	300.6
Inventory	1,007.2	1,001.6
Other current assets	967	1,093.3
Total current assets	7,568	8,732.2
Marketable securities	3,062	2,829.4
Property, plant and equipment, net	2,995.9	2,501.8
Intangible assets, net	4,019.4	3,808.3
Goodwill	4,127.5	3,669.3
Investments and other assets	1,300.4	1,335.8
Total assets	23,073.2	22,876.8
Current liabilities:
Current portion of notes payable and other financing arrangements	573.2	4.7
Taxes payable	121.3	231.9
Accounts payable	298.7	279.8
Accrued expenses and other	2,455.2	2,903.5
Total current liabilities	3,448.4	3,419.9
Notes payable and other financing arrangements	5,938.3	6,512.7
Deferred tax liability	120.7	93.1
Other long-term liabilities	716.9	722.5
Total liabilities	10,224.3	10,748.2
Commitments and contingencies
Biogen Idec Inc. shareholders' equity
Preferred stock, par value $0.001 per share	0	0
Common stock, par value $0.0005 per share	0.1	0.1
Additional paid-in capital	59.2	0
Accumulated other comprehensive loss	(329.4)	(319.9)
Retained earnings	16,107.7	15,071.6
Treasury stock, at cost	(2,977.1)	(2,611.7)
Total Biogen Inc. shareholders’ equity	12,860.5	12,140.1
Noncontrolling interests	(11.6)	(11.5)
Total equity	12,848.9	12,128.6
Total liabilities and equity	$ 23,073.2	$ 22,876.8

Condensed Consolidated Balance5

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Sep. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Common stock, par value	$ 0.0005	$ 0.0005

Condensed Consolidated Stateme6

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Cash flows from operating activities:
Net income	$ 2,836.4	$ 3,047.8
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation, amortization and acquired in-process research and development	992	505.3
Share-based compensation	97.4	117.8
Deferred income taxes	(39.7)	(56.8)
Other	126.9	45.3
Changes in operating assets and liabilities, net:
Accounts receivable	(225.8)	(238.1)
Inventory	(170.3)	(155.1)
Accrued expenses and other current liabilities	(504.5)	(175.5)
Income tax assets and liabilities	170.5	(147.4)
Other changes in operating assets and liabilities, net	(250.2)	62
Net cash flows provided by operating activities	3,032.7	3,005.3
Cash flows from investing activities:
Proceeds from sales and maturities of marketable securities	4,472.6	5,185.8
Purchases of marketable securities	(4,093.9)	(5,631.7)
Contingent consideration related to Fumapharm AG acquisition	900	900
Acquired in-process research and development	(120)	0
Purchases of property, plant and equipment	(636.8)	(434)
Acquisitions of intangible assets	(910.4)	(110.4)
Other	(5.1)	(12.8)
Net cash flows used in investing activities	(2,193.6)	(1,903.1)
Cash flows from financing activities:
Purchases of treasury stock	(1,365.4)	(348.9)
Payments related to issuance of stock for share-based compensation arrangements, net	(10.7)	(12.4)
Net cash contribution to Bioverativ Inc.	302.7	0
Repayments of Bank Debt	(3.2)	(2.7)
Other	10.1	37.9
Net cash flows used in financing activities	(1,671.9)	(326.1)
Net (decrease) increase in cash and cash equivalents	(832.8)	776.1
Effect of exchange rate changes on cash and cash equivalents	54.4	0.7
Cash and cash equivalents, beginning of the period	2,326.5	1,308
Cash and cash equivalents, end of the period	$ 1,548.1	$ 2,084.8

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Summary of significant accounting policies	Summary of Significant Accounting Policies Business Overview Biogen is a global biopharmaceutical company focused on discovering, developing, manufacturing and delivering therapies to people living with serious neurological and neurodegenerative diseases. Our marketed products include TECFIDERA, AVONEX, PLEGRIDY, TYSABRI, ZINBRYTA and FAMPYRA for multiple sclerosis (MS), SPINRAZA for the treatment of spinal muscular atrophy (SMA) and FUMADERM for the treatment of severe plaque psoriasis. We also have certain business and financial rights with respect to RITUXAN for the treatment of non-Hodgkin's lymphoma, chronic lymphocytic leukemia (CLL) and other conditions, GAZYVA indicated for the treatment of CLL and follicular lymphoma, OCREVUS indicated for the treatment of primary progressive MS and relapsing MS, and other potential anti-CD20 therapies under a collaboration agreement with Genentech, Inc., a wholly-owned member of the Roche Group. We support our drug discovery and development efforts through the commitment of significant resources to discovery, research and development programs and business development opportunities, particularly within areas of our scientific, manufacturing and technical capabilities. We intend to invest in the future across our core growth areas of MS and neuroimmunology, Alzheimer's disease (AD) and dementia, Parkinson's disease and movement disorders, and neuromuscular diseases including SMA and amyotrophic lateral sclerosis (ALS). Further, we see opportunities to invest in emerging growth areas such as pain, ophthalmology, neuropsychiatry and acute neurology. In addition, we are employing innovative technologies to discover potential treatments for rare and genetic disorders, including new ways of treating diseases through gene therapy. Our innovative drug development and commercialization activities are complemented by our biosimilar therapies that expand access to medicines and reduce the cost burden for healthcare systems. We are leveraging our manufacturing capabilities and know-how to develop, manufacture and market biosimilars through Samsung Bioepis, our joint venture with Samsung BioLogics Co. Ltd. (Samsung Biologics). Under our commercial agreement, we market and sell BENEPALI, an etanercept biosimilar referencing ENBREL, and FLIXABI, an infliximab biosimilar referencing REMICADE, in the European Union (E.U.). Hemophilia Spin-Off On February 1, 2017, we completed the spin-off of our hemophilia business, Bioverativ Inc. (Bioverativ), as an independent, publicly traded company. Our consolidated results of operations and financial position included in these unaudited condensed consolidated financial statements reflect the financial results of our hemophilia business for all periods through January 31, 2017. For additional information related to the spin-off of our hemophilia business, please read Note 3, Hemophilia Spin-Off, to these condensed consolidated financial statements. 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, 2016 ( 2016 Form 10-K). Our accounting policies are described in the “ Notes to Consolidated Financial Statements ” in our 2016 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 nine months ended September 30, 2017 are not necessarily indicative of the operating results for the full year or for any other subsequent interim period. We operate as one operating segment, focused on discovering, developing, manufacturing and delivering therapies to people living with serious neurological and neurodegenerative diseases. 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, 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 ongoing basis we evaluate our estimates, judgments and methodologies. We base our estimates on historical experience and on various other assumptions that we believe to be reasonable, the results of which form the basis for making judgments about the carrying values of assets, liabilities and equity and the amount of revenues and expenses. Actual results may differ from these estimates under different assumptions or conditions. 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 we adopt as of the specified effective date. Unless otherwise discussed, we do not believe that the impact of recently issued standards that are not yet effective will have a material impact on our financial position or results of operations upon adoption. We adopted the following new standards effective January 1, 2017: • Accounting Standards Update (ASU) No. 2016-06, Derivatives and Hedging (Topic 815): Contingent Put and Call Options in Debt Instruments; • ASU No. 2016-07, Investments - Equity Method and Joint Ventures (Topic 323): Simplifying the Transition to the Equity Method of Accounting; and • ASU No. 2016-09, Compensation - Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting. The adoption of these standards did not have a material impact on our financial position, results of operations or statement of cash flows; however, the adoption of ASU No. 2016-09 resulted in the reclassification of certain prior year amounts in our condensed consolidated statements of cash flows to conform to our current year presentation. Specifically, amounts previously disclosed in net cash flows used in financing activities related to our excess tax benefit from share-based compensation have been reclassified to net cash flows provided by operating activities and amounts related to cash paid when withholding shares for tax withholding purposes, previously disclosed in net cash flows provided by operating activities, have been reclassified to net cash flows used in financing activities. For additional information related to these standards, please read Note 1, Summary of Significant Accounting Policies: New Accounting Pronouncements , to our consolidated financial statements included in our 2016 Form 10-K. In May 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 FASB has subsequently issued amendments to ASU No. 2014-09 that have the same effective date and transition date of January 1, 2018. We expect to adopt these standards using the modified retrospective method. We have performed a review of the new standards as compared to our current accounting policies and our review of customer contracts and collaborative relationships remains in process. As of September 30, 2017 , we have not identified any accounting changes that would materially impact the amount of reported revenues with respect to our product revenues and revenues from anti-CD20 therapeutic programs. During the fourth quarter of 2017 we plan to finalize our assessments over the impact that these standards may have on our results of operations, financial position and disclosures. In January 2016 the FASB issued ASU No. 2016-01, Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities. The new standard amends certain aspects of accounting and disclosure requirements of financial instruments, including the requirement that equity investments with readily determinable fair values be measured at fair value with changes in fair value recognized in a company's results of operations. The new standard does not apply to investments accounted for under the equity method of accounting or those that result in consolidation of the investee. Equity investments that do not have readily determinable fair values may be measured at fair value or at cost minus impairment adjusted for changes in observable prices. A financial liability that is measured at fair value in accordance with the fair value option is required to be presented separately in other comprehensive income for the portion of the total change in the fair value resulting from change in the instrument-specific credit risk. In addition, a valuation allowance should be evaluated on deferred tax assets related to available-for-sale debt securities in combination with other deferred tax assets. The new standard will be effective for us on January 1, 2018. Based on our current investment holdings, the adoption of this standard is not expected to have a material impact on our financial position or results of operations; however, it will result in the reclassification of certain investments. In February 2016 the FASB issued ASU No. 2016-02, Leases (Topic 842). The new standard requires that all lessees recognize the assets and liabilities that arise from leases on their balance sheet and disclose qualitative and quantitative information about their leasing arrangements. The new standard will be effective for us on January 1, 2019. We are currently evaluating the impact that this standard may have on our results of operations, financial position and disclosures. The adoption of this standard is not expected to have a material impact on our net financial position, but may materially impact the reported amount of total assets and total liabilities. In June 2016 the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. The new standard changes the impairment model for most financial assets and certain other instruments. Under the new standard, entities holding financial assets and net investment in leases that are not accounted for at fair value through net income are to be presented at the net amount expected to be collected. An allowance for credit losses will be a valuation account that will be deducted from the amortized cost basis of the financial asset to present the net carrying value at the amount expected to be collected on the financial asset. The new standard will be effective for us on January 1, 2020. The adoption of this standard is not expected to have a material impact on our financial position or results of operations. In August 2016 the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments. The new standard clarifies certain aspects of the statement of cash flows, including the classification of debt prepayment or debt extinguishment costs, settlement of zero-coupon debt instruments or other debt instruments with coupon interest rates that are insignificant in relation to the effective interest rate of the borrowing, contingent consideration payments made after a business combination, proceeds from the settlement of insurance claims, proceeds from the settlement of corporate-owned life insurance policies, distributions received from equity method investees and beneficial interests in securitization transactions. The new standard also clarifies that an entity should determine each separately identifiable source or use within the cash receipts and cash payments on the basis of the nature of the underlying cash flows. In situations in which cash receipts and payments have aspects of more than one class of cash flows and cannot be separated by source or use, the appropriate classification should depend on the activity that is likely to be the predominant source or use of cash flows for the item. The new standard will be effective for us on January 1, 2018. The adoption of this standard is not expected to have a material impact on our statements of cash flows upon adoption. In October 2016 the FASB issued ASU No. 2016-16, Income Taxes (Topic 740): Intra-Entity Transfer of Assets Other Than Inventory. The new standard eliminates the deferral of the tax effects of intra-entity transfers of an asset other than inventory. Under the new standard, entities should recognize the income tax consequences on an intra-entity transfer of an asset other than inventory when the transfer occurs. The new standard will be effective for us on January 1, 2018. We expect to adopt this standard using the modified retrospective method applied through a cumulative-effect adjustment directly to retained earnings as of the beginning of the period of adoption. The adoption of this standard is expected to have a material impact on our net financial position; however, the final effect of the adoption of this standard will depend on the nature and amount of future transactions. In January 2017 the FASB issued ASU No. 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business. The new standard clarifies the definition of a business and provides a screen to determine when an integrated set of assets and activities is not a business. The screen requires that when substantially all of the fair value of the gross assets acquired (or disposed of) is concentrated in a single identifiable asset or a group of similar identifiable assets, the set is not a business. The new standard will be effective for us on January 1, 2018; however, we have adopted this standard as of January 1, 2017, with prospective application to any business development transaction. In January 2017 the FASB issued ASU No. 2017-04, Intangibles - Goodwill and Other (Topic 350): Simplifying the Test of Goodwill Impairment. The new standard eliminates Step 2 from the goodwill impairment test. Under the amendments in ASU No. 2017-04, an entity should recognize an impairment charge for the amount by which the carrying amount of a reporting unit exceeds that reporting unit’s fair value; however, the loss recognized should not exceed the total amount of goodwill allocated to that reporting unit. The new standard will be effective for us on January 1, 2020; however, early adoption is permitted. We intend to early adopt this standard as of October 31, 2017, during our annual review of goodwill. The adoption of this standard is not expected to have a material impact on our financial position or results of operations. In March 2017 the FASB issued ASU No. 2017-07, Compensation - Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost. The new standard will require that an employer disaggregate the service cost component from the other components of net benefit cost. The new standard also provides explicit guidance on how to present the service cost component and the other components of net benefit cost in the income statement and allow only the service cost component of net benefit cost to be eligible for capitalization. The other components of the net periodic benefit cost must be presented separately from the line items that include service cost and outside of any subtotal of operating income on the condensed consolidated statements of income. The new standard will be effective for us on January 1, 2018. The adoption of this standard is not expected to have a material impact on our financial position or results of operations. In March 2017 the FASB issued ASU No. 2017-08, Receivables - Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities. The new standard amends the amortization period for certain purchased callable debt securities held at a premium by shortening the amortization period for the premium to the earliest call date. The new standard will be effective for us on January 1, 2019. We are currently evaluating the potential impact that this standard may have on our financial position and results of operations. In May 2017 the FASB issued ASU No. 2017-09, Compensation - Stock Compensation (Topic 718): Scope of Modification Accounting. The new standard provides guidance about which changes to the terms or conditions of a share-based payment award require an entity to apply modification accounting in Topic 718. The new standard will be effective for us on January 1, 2018; however, early adoption is permitted. The adoption of this standard is not expected to have a material impact on our financial position or results of operations. In August 2017 the FASB issued ASU No. 2017-12, Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities. The new standard provides guidance to better align an entity’s risk management activities and financial reporting for hedging relationships through changes to both the designation and measurement guidance for qualifying hedging relationships and the presentation of hedge results. The new standard expands and refines hedge accounting for both non-financial and financial risk components and aligns the recognition and presentation of the effects of the hedging instrument and the hedged item in the financial statements. The new standard will be effective for us on January 1, 2019; however, early adoption is permitted. The adoption of this standard is not expected to have a material impact on our financial position or results of operations.

Acquisitions

Acquisitions	9 Months Ended
	Sep. 30, 2017
Business Combinations [Abstract]
Acquisitions	Acquisitions On May 15, 2017, we completed an asset purchase of the Phase 3 candidate, CIRARA (now known as BIIB093), from Remedy Pharmaceuticals Inc. (Remedy). The target indication for BIIB093 is large hemispheric infarction (LHI), a severe form of ischemic stroke where brain swelling (cerebral edema) often leads to a disproportionately large share of stroke-related morbidity and mortality. The U.S. Food and Drug Administration (FDA) recently granted BIIB093 Orphan Drug designation for severe cerebral edema in patients with acute ischemic stroke. The FDA has also granted BIIB093 Fast Track designation. Under the terms of the purchase agreement, we will have responsibility for the future development and commercialization of BIIB093 and have agreed to pay Remedy certain development and sales based milestone payments, which are substantially payable upon or after regulatory approval, as well as royalties on future commercial sales. Remedy will share in the cost of development for the target indication for BIIB093 in LHI stroke. We accounted for this transaction as an asset acquisition as we did not acquire any employees from Remedy nor did we acquire any significant processes required in the development of BIIB093. Upon closing of the transaction, we made a $120.0 million upfront payment, which was recorded as acquired in-process research and development in our condensed consolidated statements of income as BIIB093 has not yet reached technological feasibility.

Hemophilia Spin Off

Hemophilia Spin Off	9 Months Ended
	Sep. 30, 2017
Discontinued Operations and Disposal Groups [Abstract]
Hemophilia Spin Off	Hemophilia Spin-Off On February 1, 2017, we completed the spin-off of our hemophilia business, Bioverativ, as an independent, publicly traded company trading under the symbol "BIVV" on the Nasdaq Global Select Market. The spin-off was accomplished through the distribution of all the then outstanding shares of common stock of Bioverativ to Biogen shareholders, who received one share of Bioverativ common stock for every two shares of Biogen common stock they owned. The separation and distribution was structured to be tax-free for shareholders for federal income tax purposes. Bioverativ assumed all of our rights and obligations under our collaboration agreement with Swedish Orphan Biovitrum AB and our collaboration and license agreement with Sangamo Biosciences Inc. In connection with the distribution, Biogen and Bioverativ entered into a separation agreement and various other agreements (including a transition services agreement, a tax matters agreement, a manufacturing and supply agreement, an employee matters agreement, an intellectual property matters agreement and certain other commercial agreements). These agreements govern the separation and distribution and the relationship between the two companies going forward. They also provide for the performance of services by each company for the benefit of the other for a period of time. In addition, under the terms of the separation agreement, Bioverativ is obligated to indemnify us for liabilities that may exist relating to its business activities, whether incurred prior to or after the distribution, including any pending or future litigation. The services under these agreements generally commenced on February 1, 2017 (the distribution date), and are expected to terminate within 12 months of the distribution date, with the exception of the manufacturing and supply agreement, which has an initial term of five years , with a five year extension at Bioverativ's sole discretion and a further five year extension with Bioverativ's and our consent. In connection with the distribution we made a net cash contribution to Bioverativ, during the first quarter of 2017, totaling $302.7 million . The following table summarizes the assets and liabilities that were charged against equity as a result of the spin-off of our hemophilia business: (In millions) Assets Cash $ 302.7 Accounts receivable 144.7 Inventory 116.1 Property, plant and equipment, net 20.2 Intangible assets, net 56.8 Goodwill 314.1 Other, net 53.7 Assets transferred, net $ 1,008.3 Liabilities Accrued expenses and other current liabilities $ 87.8 Other long-term liabilities 67.7 Liabilities transferred, net $ 155.5 Under the manufacturing and supply agreement, we manufacture and supply certain products and materials to Bioverativ. For the three and nine months ended September 30, 2017 , we recognized $5.0 million and $12.1 million , respectively, in revenues in relation to these services, which is reflected as a component of other revenues in our condensed consolidated statements of income. We also recorded $4.5 million and $11.1 million as cost of sales in relation to these services during the three and nine months ended September 30, 2017 , respectively. Pursuant to the terms of our agreements with Bioverativ, upon completion of the spin-off, we distributed ALPROLIX and ELOCTATE on behalf of Bioverativ until Bioverativ obtained appropriate regulatory authorizations in certain countries, including a Biologics License Application transfer in the United States (U.S.), which was received in September 2017. Accordingly, commencing October 2017, we ceased distribution of ALPROLIX and ELOCTATE on behalf of Bioverativ under this arrangement. Under this arrangement, we distributed these products as an agent of Bioverativ and also provided related cash management services in connection with sales transactions, including the collection of receivables and the remittance of applicable discounts and allowances. Our consolidated financial position and results of operations did not reflect recognition of activity or balances related to these transactions. Amounts earned under the non-manufacturing and supply related transaction service agreements are recorded as a reduction of costs and expenses in their respective expense line items, primarily in selling, general and administrative expenses, in our condensed consolidated statements of income. For the three and nine months ended September 30, 2017 , these amounts were not significant. Hemophilia related product revenues reflected in our condensed consolidated statements of income for the nine months ended September 30, 2017 , totaled $74.4 million , as compared to $217.0 million and $604.7 million for the three and nine months ended September 30, 2016 , respectively. Results for the nine months ended September 30, 2017 , only reflect hemophilia-related product revenues through January 31, 2017. Patents Prior to the spin-off of our hemophilia business, we were awarded various methods of treatment and composition of matter patents related to ELOCTATE and ALPROLIX. Upon completion of the spin-off, these patents were transferred to the patent portfolio of Bioverativ.

Reserves for Discounts and Allo

Reserves for Discounts and Allowances	9 Months Ended
	Sep. 30, 2017
Sales Discounts, Returns and Allowances, Goods [Abstract]
Reserves for discounts and allowances	Reserves for Discounts and Allowances An analysis of the change in reserves for discounts and allowances is summarized as follows: (In millions) Discounts Contractual Adjustments Returns Total Balance, as of December 31, 2016 $ 71.6 $ 482.7 $ 51.2 $ 605.5 Current provisions relating to sales in current year 422.0 1,681.2 20.0 2,123.2 Adjustments relating to prior years (0.2 ) 11.2 (8.6 ) 2.4 Payments/credits relating to sales in current year (320.1 ) (1,227.1 ) (0.1 ) (1,547.3 ) Payments/credits relating to sales in prior years (70.4 ) (431.6 ) (17.2 ) (519.2 ) Balance, as of September 30, 2017 $ 102.9 $ 516.4 $ 45.3 $ 664.6 The total reserves above, which are included in our condensed consolidated balance sheets, are summarized as follows: (In millions) As of As of Reduction of accounts receivable $ 197.4 $ 166.9 Component of accrued expenses and other 467.2 438.6 Total reserves $ 664.6 $ 605.5

Inventory

Inventory	9 Months Ended
	Sep. 30, 2017
Inventory Disclosure [Abstract]
Inventory	Inventory The components of inventory are summarized as follows: (In millions) As of As of Raw materials $ 208.1 $ 170.4 Work in process 647.6 698.7 Finished goods 175.5 170.3 Total inventory $ 1,031.2 $ 1,039.4 Balance Sheet Classification: Inventory $ 1,007.2 $ 1,001.6 Investments and other assets 24.0 37.8 Total inventory $ 1,031.2 $ 1,039.4 Long-term inventory, which primarily consists of work in process, is included in investments and other assets in our condensed consolidated balance sheets. Balances in the table above as of September 30, 2017 , also reflect the elimination of certain amounts transferred to Bioverativ in connection with the completion of the spin-off of our hemophilia business. Balances transferred to Bioverativ related to work in process and finished goods inventory totaled $84.5 million and $31.6 million , respectively.

Intangible Assets and Goodwill

Intangible Assets and Goodwill	9 Months Ended
	Sep. 30, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible Assets and Goodwill	Intangible Assets and Goodwill Intangible Assets Intangible assets, net of accumulated amortization, impairment charges and adjustments, are summarized as follows: As of September 30, 2017 As of December 31, 2016 (In millions) Estimated Life Cost Accumulated Amortization Net Cost Accumulated Amortization Net Out-licensed patents 13-23 years $ 543.3 $ (532.7 ) $ 10.6 $ 543.3 $ (523.6 ) $ 19.7 Developed technology 15-23 years 3,005.3 (2,675.8 ) 329.5 3,005.3 (2,634.3 ) 371.0 In-process research and development Indefinite until commercialization 680.6 — 680.6 648.0 — 648.0 Trademarks and tradenames Indefinite 64.0 — 64.0 64.0 — 64.0 Acquired and in-licensed rights and patents 4-18 years 4,002.6 (1,067.9 ) 2,934.7 3,481.7 (776.1 ) 2,705.6 Total intangible assets $ 8,295.8 $ (4,276.4 ) $ 4,019.4 $ 7,742.3 $ (3,934.0 ) $ 3,808.3 Balances in the table above as of September 30, 2017 , reflect the elimination of certain amounts transferred to Bioverativ in connection with the completion of the spin-off of our hemophilia business. For additional information relating to the spin-off of our hemophilia business, please read Note 3, Hemophilia Spin-Off, to these condensed consolidated financial statements. In-process research and development balances include adjustments related to foreign currency exchange rate fluctuations. For the three and nine months ended September 30, 2017 , amortization of acquired intangible assets totaled $108.9 million and $674.9 million , respectively, as compared to $99.7 million and $281.4 million , respectively, in the prior year comparative periods. Amortization of acquired intangible assets for the three and nine months ended September 30, 2017 , includes $30.4 million and $413.4 million , respectively, of amortization and impairment charges related to our U.S. and rest of world licenses to Forward Pharma A/S' (Forward Pharma) intellectual property related to TECFIDERA, as further discussed below. Developed Technology Developed technology primarily relates to our AVONEX product, which was recorded in connection with the merger of Biogen, Inc. and IDEC Pharmaceuticals Corporation in 2003. The net book value of this asset as of September 30, 2017 , was $322.5 million . Acquired and In-licensed Rights and Patents Acquired and in-licensed rights and patents primarily relate to our acquisition of all remaining rights to TYSABRI from Elan Corporation plc. The net book value of this asset as of September 30, 2017 , was $2,293.9 million . The increase in acquired and in-licensed rights and patents during the nine months ended September 30, 2017 , reflects the $50.0 million and $40.0 million milestone payments due to Ionis Pharmaceuticals, Inc., which became payable upon the approval of SPINRAZA for the treatment of SMA in the E.U. and Japan, respectively, the $25.0 million milestone payment due to Samsung Bioepis, which became payable upon the approval of IMRALDI, an adalimumab biosimilar referencing HUMIRA, in the E.U. and net amounts related to our TECFIDERA license rights, as described below. TECFIDERA License Rights In January 2017 we entered into a settlement and license agreement among Biogen Swiss Manufacturing GmbH, Biogen International Holding Ltd., Forward Pharma and certain related parties, which was effective as of February 1, 2017. Pursuant to the agreement, we obtained U.S. and rest of world licenses to Forward Pharma's intellectual property, including Forward Pharma's intellectual property related to TECFIDERA. In exchange, we agreed to pay Forward Pharma $1.25 billion in cash. During the fourth quarter of 2016, we recognized a pre-tax charge of $454.8 million related to this agreement, representing the fair value of our license to Forward Pharma’s intellectual property for the period April 2014, when we started selling TECFIDERA, through December 31, 2016. For additional information related to this agreement, please read Note 21, Commitments and Contingencies , to our consolidated financial statements included in our 2016 Form 10-K. We paid the $1.25 billion in February 2017 and recognized an intangible asset of $795.2 million . The asset represented the fair value of the U.S. and rest of world licenses to Forward Pharma’s intellectual property related to TECFIDERA revenues for the period January 2017, the month in which we entered into this agreement, through December 2020, the last month before royalty payments could first commence pursuant to this agreement. We have two intellectual property disputes with Forward Pharma, one in the U.S. and one in the E.U., concerning intellectual property related to TECFIDERA. In March 2017 the U.S. intellectual property dispute was decided in our favor. Forward Pharma has appealed to the U.S. Court of Appeals for the Federal Circuit and the appeal is pending. For additional information related to these disputes, please read Note 19, Litigation , to these condensed consolidated financial statements. As we prevailed in the U.S. proceeding in March 2017, we evaluated the recoverability of the U.S. asset acquired from Forward Pharma and recorded an impairment charge to adjust the carrying value of the acquired U.S. asset to fair value reflecting the impact of the developments in the U.S. legal dispute. We also continue to amortize the remaining net book value of the U.S. and rest of world intangible assets in our condensed consolidated statements of income utilizing an economic consumption model. Estimated Future Amortization of Intangible Assets Our amortization expense is based on the economic consumption of intangible assets. Our most significant intangible assets are related to our TECFIDERA, AVONEX and TYSABRI products. Annually, during our long-range planning cycle, we perform an analysis of anticipated lifetime revenues of TECFIDERA, AVONEX and TYSABRI. This analysis is also updated whenever we determine events or changes in circumstances would significantly affect the anticipated lifetime revenues of either product. Our most recent long range planning cycle was completed in the third quarter of 2017. Based upon the above, the estimated future amortization of acquired intangible assets is expected to be as follows: (In millions) As of 2017 (remaining three months) $ 106.8 2018 421.3 2019 404.0 2020 389.6 2021 256.6 2022 244.7 Goodwill The following table provides a roll forward of the changes in our goodwill balance: (In millions) As of Goodwill, beginning of period $ 3,669.3 Elimination of goodwill allocated to our hemophilia business (314.1 ) Increase to goodwill 762.4 Other 9.9 Goodwill, end of period $ 4,127.5 The elimination of goodwill represents an allocation based upon the relative enterprise fair value of the hemophilia business as of the distribution date. For additional information relating to the spin-off of our hemophilia business, please read Note 3, Hemophilia Spin-Off, to these condensed consolidated financial statements. The increase in goodwill during the nine months ended September 30, 2017 , was related to $900.0 million in contingent milestones achieved (exclusive of $137.6 million in tax benefits) and payable to the former shareholders of Fumapharm AG or holders of their rights. Other includes changes in foreign currency exchange rates. For additional information related to future contingent payments to the former shareholders of Fumapharm AG or holders of their rights, please read Note 21, Commitments and Contingencies, to our consolidated financial statements included in our 2016 Form 10-K. As of September 30, 2017 , we had no accumulated impairment losses related to goodwill.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements The tables below present information about our assets and liabilities that are regularly measured and carried at fair value and indicate the level within the fair value hierarchy of the valuation techniques we utilized to determine such fair value: As of September 30, 2017 (In millions) Total Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets: Cash equivalents $ 1,184.3 $ — $ 1,184.3 $ — Marketable debt securities: Corporate debt securities 2,584.7 — 2,584.7 — Government securities 1,870.0 — 1,870.0 — Mortgage and other asset backed securities 567.5 — 567.5 — Marketable equity securities 11.8 11.8 — — Derivative contracts 1.6 — 1.6 — Plan assets for deferred compensation 30.1 — 30.1 — Total $ 6,250.0 $ 11.8 $ 6,238.2 $ — Liabilities: Derivative contracts $ 106.2 $ — $ 106.2 $ — Contingent consideration obligations 522.1 — — 522.1 Total $ 628.3 $ — $ 106.2 $ 522.1 As of December 31, 2016 (In millions) Total Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets: Cash equivalents $ 2,039.6 $ — $ 2,039.6 $ — Marketable debt securities: Corporate debt securities 2,663.8 — 2,663.8 — Government securities 2,172.5 — 2,172.5 — Mortgage and other asset backed securities 561.7 — 561.7 — Marketable equity securities 24.9 24.9 — — Derivative contracts 61.0 — 61.0 — Plan assets for deferred compensation 34.5 — 34.5 — Total $ 7,558.0 $ 24.9 $ 7,533.1 $ — Liabilities: Derivative contracts $ 13.6 $ — $ 13.6 $ — Contingent consideration obligations 467.6 — — 467.6 Total $ 481.2 $ — $ 13.6 $ 467.6 There have been no material impairments of our assets measured and carried at fair value during the three and nine months ended September 30, 2017 . In addition, there were no changes in valuation techniques or inputs utilized or transfers between fair value measurement levels during the three and nine months ended September 30, 2017 . The fair values of Level 2 instruments classified as cash equivalents and marketable debt securities were determined through third party pricing services. For a description of our validation procedures related to prices provided by third party pricing services, please read Note 1, Summary of Significant Accounting Policies: Fair Value Measurements, to our consolidated financial statements included in our 2016 Form 10-K. Debt Instruments The fair and carrying values of our debt instruments, which are Level 2 liabilities, are summarized as follows: As of September 30, 2017 As of December 31, 2016 (In millions) Fair Value Carrying Value Fair Value Carrying Value Notes payable to Fumedica AG $ 3.2 $ 3.3 $ 6.1 $ 6.0 6.875% Senior Notes due March 1, 2018 561.7 553.1 583.7 558.5 2.900% Senior Notes due September 15, 2020 1,536.2 1,486.5 1,521.5 1,485.3 3.625% Senior Notes due September 15, 2022 1,049.4 994.0 1,026.6 993.2 4.050% Senior Notes due September 15, 2025 1,867.0 1,735.9 1,796.0 1,734.8 5.200% Senior Notes due September 15, 2045 2,030.8 1,721.9 1,874.5 1,721.5 Total $ 7,048.3 $ 6,494.7 $ 6,808.4 $ 6,499.3 The fair value of our notes payable to Fumedica AG was estimated using market observable inputs, including current interest and foreign currency exchange rates. The fair values of each of our series of Senior Notes were determined through market, observable and corroborated sources. For additional information related to our debt instruments, please read Note 11, Indebtedness, to our consolidated financial statements included in our 2016 Form 10-K. Contingent Consideration Obligations The following table provides a roll forward of the fair values of our contingent consideration obligations that includes Level 3 measurements: For the Three Months For the Nine Months (In millions) 2017 2016 2017 2016 Fair value, beginning of period $ 492.1 $ 515.7 $ 467.6 $ 506.0 Additions — — — — Changes in fair value 30.0 5.9 61.2 18.8 Payments — — (6.7 ) (3.2 ) Fair value, end of period $ 522.1 $ 521.6 $ 522.1 $ 521.6 As of September 30, 2017 and December 31, 2016 , approximately $280.0 million and $246.8 million , respectively, of our contingent consideration obligations valued using Level 3 measurements were reflected as components of other long-term liabilities in our condensed consolidated balance sheets with the remaining balances reflected as a component of accrued expenses and other.

Financial Instruments

Financial Instruments	9 Months Ended
	Sep. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Financial Instruments	Financial Instruments The following table summarizes our financial assets with maturities of less than 90 days from the date of purchase included in cash and cash equivalents on the accompanying condensed consolidated balance sheets: (In millions) As of As of Commercial paper $ 126.4 $ 31.0 Overnight reverse repurchase agreements 42.0 — Money market funds 871.1 741.7 Short-term debt securities 144.8 1,266.9 Total $ 1,184.3 $ 2,039.6 The carrying values of our commercial paper, including accrued interest, overnight reverse repurchase agreements, money market funds and short-term debt securities approximate fair value due to their short-term maturities. The following tables summarize our marketable debt and equity securities, classified as available-for-sale: As of September 30, 2017 (In millions) Fair Value Gross Unrealized Gains Gross Unrealized Losses Amortized Cost Corporate debt securities Current $ 1,042.9 $ 0.2 $ (0.4 ) $ 1,043.1 Non-current 1,541.8 2.2 (2.1 ) 1,541.7 Government securities Current 916.0 — (0.8 ) 916.8 Non-current 954.0 0.4 (3.4 ) 957.0 Mortgage and other asset backed securities Current 1.3 — — 1.3 Non-current 566.2 1.3 (1.4 ) 566.3 Total marketable debt securities $ 5,022.2 $ 4.1 $ (8.1 ) $ 5,026.2 Marketable equity securities, non-current $ 11.8 $ — $ (2.6 ) $ 14.4 As of December 31, 2016 (In millions) Fair Value Gross Unrealized Gains Gross Unrealized Losses Amortized Cost Corporate debt securities Current $ 1,408.6 $ 0.2 $ (0.6 ) $ 1,409.0 Non-current 1,255.2 1.2 (4.7 ) 1,258.7 Government securities Current 1,156.0 0.2 (0.3 ) 1,156.1 Non-current 1,016.5 0.5 (3.4 ) 1,019.4 Mortgage and other asset backed securities Current 4.0 — — 4.0 Non-current 557.7 0.8 (2.2 ) 559.1 Total marketable debt securities $ 5,398.0 $ 2.9 $ (11.2 ) $ 5,406.3 Marketable equity securities, non-current $ 24.9 $ 0.7 $ (9.3 ) $ 33.5 Summary of Contractual Maturities: Available-for-Sale Securities The estimated fair value and amortized cost of our marketable debt securities available-for-sale by contractual maturity are summarized as follows: As of September 30, 2017 As of December 31, 2016 (In millions) Estimated Fair Value Amortized Cost Estimated Fair Value Amortized Cost Due in one year or less $ 1,960.2 $ 1,961.2 $ 2,568.6 $ 2,569.1 Due after one year through five years 2,792.9 2,795.7 2,552.6 2,559.7 Due after five years 269.1 269.3 276.8 277.5 Total available-for-sale securities $ 5,022.2 $ 5,026.2 $ 5,398.0 $ 5,406.3 The average maturity of our marketable debt securities available-for-sale as of September 30, 2017 and December 31, 2016 , was approximately 17 months and 12 months , respectively. Proceeds from Marketable Debt Securities The proceeds from maturities and sales of marketable debt securities and resulting realized gains and losses are summarized as follows: For the Three Months For the Nine Months (In millions) 2017 2016 2017 2016 Proceeds from maturities and sales $ 888.1 $ 2,362.2 $ 4,472.6 $ 5,185.8 Realized gains $ 0.3 $ 1.1 $ 2.7 $ 2.1 Realized losses $ (1.2 ) $ (1.1 ) $ (4.4 ) $ (2.7 ) Strategic Investments As of September 30, 2017 and December 31, 2016 , our strategic investment portfolio was comprised of investments totaling $84.9 million and $99.9 million , respectively, which are included in investments and other assets in our condensed consolidated balance sheets. Our strategic investment portfolio includes investments in equity securities of certain biotechnology companies and investments in venture capital funds where the underlying investments are in equity securities of biotechnology companies.

Derivative Instruments

Derivative Instruments	9 Months Ended
	Sep. 30, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Instruments	Derivative Instruments Foreign Currency Forward Contracts - Hedging Instruments Due to the global nature of our operations, portions of our revenues and operating expenses are recorded in currencies other than the U.S. dollar. The value of revenues and operating expenses measured in U.S. dollars is therefore subject to changes in foreign currency exchange rates. In order to mitigate these changes we use foreign currency forward contracts to lock in exchange rates associated with a portion of our forecasted international revenues and operating expenses. Foreign currency forward contracts in effect as of September 30, 2017 and December 31, 2016 , had durations of 1 to 21 months and 1 to 18 months , respectively. These contracts have been designated as cash flow hedges, and, accordingly, to the extent effective, any unrealized gains or losses on these foreign currency forward contracts are reported in accumulated other comprehensive income (loss) (referred to as AOCI in the tables below). Realized gains and losses for the effective portion of such contracts are recognized in revenue when the sale of product in the currency being hedged is recognized and in operating expenses when the expense in the currency being hedged is recorded. To the extent ineffective, hedge transaction gains and losses are reported in other income (expense), net. The notional value of foreign currency forward contracts that were entered into to hedge forecasted revenues and operating expenses is summarized as follows: Notional Amount Foreign Currency: (In millions) As of As of Euro $ 1,573.4 $ 871.7 British pound 39.7 — Canadian dollar 19.7 — Swiss franc 5.1 — Total foreign currency forward contracts $ 1,637.9 $ 871.7 The portion of the fair value of these foreign currency forward contracts that was included in accumulated other comprehensive income (loss) in total equity reflected net losses of $113.0 million and net gains of $49.8 million as of September 30, 2017 and December 31, 2016 , respectively. We expect all contracts outstanding as of September 30, 2017 , to be settled over the next 21 months with any amounts in accumulated other comprehensive income (loss) to be reported as an adjustment to revenue or operating expense. We consider the impact of our and our counterparties’ credit risk on the fair value of the contracts as well as the ability of each party to execute its contractual obligations. As of September 30, 2017 and December 31, 2016 , credit risk did not change the fair value of our foreign currency forward contracts. The following tables summarize the effect of foreign currency forward contracts designated as hedging instruments on our condensed consolidated statements of income: For the Three Months Ended September 30, Net Gains/(Losses) Reclassified from AOCI into Operating Income (Effective Portion) Net Gains/(Losses) Recognized into Net Income (Ineffective Portion) Location 2017 2016 Location 2017 2016 Revenue $ (18.8 ) $ (5.2 ) Other income (expense) $ 0.7 $ (0.6 ) Operating expenses $ 0.5 $ (0.2 ) Other income (expense) $ 0.2 $ — For the Nine Months Ended September 30, Net Gains/(Losses) Reclassified from AOCI into Operating Income (Effective Portion) Net Gains/(Losses) Recognized into Net Income (Ineffective Portion) Location 2017 2016 Location 2017 2016 Revenue $ (15.1 ) $ (0.7 ) Other income (expense) $ 6.7 $ 3.4 Operating expenses $ 0.7 $ (0.4 ) Other income (expense) $ (0.1 ) $ (0.3 ) Interest Rate Contracts - Hedging Instruments We have entered into interest rate swap contracts on certain borrowing transactions to manage our exposure to interest rate changes. In connection with the issuance of our 2.90% Senior Notes due September 15, 2020, we entered into interest rate swaps with an aggregate notional amount of $675.0 million , which expire on September 15, 2020. The interest rate swap contracts are designated as hedges of the fair value changes in the 2.90% Senior Notes due September 15, 2020 attributable to changes in interest rates. Since the specific terms and notional amount of the swaps match the debt being hedged, it is assumed to be a highly effective hedge and all changes in the fair value of the swaps are recorded as a component of our 2.90% Senior Notes due September 15, 2020 with no net impact recorded in income. Any net interest payments made or received on the interest rate swap contracts are recognized as a component of interest expense in our condensed consolidated statements of income. Foreign Currency Forward Contracts - Other Derivatives We also enter into other foreign currency forward contracts, usually with durations of one month or less, to mitigate the foreign currency risk related to certain balance sheet positions. We have not elected hedge accounting for these transactions. The aggregate notional amount of these outstanding foreign currency contracts was $366.2 million and $902.1 million as of September 30, 2017 and December 31, 2016 , respectively. Net gains of $1.2 million and $5.7 million related to these contracts were recognized as a component of other income (expense), net for the three and nine months ended September 30, 2017 , respectively, as compared to net losses of $0.4 million and $16.6 million , respectively, in the prior year comparative periods. Summary of Derivatives While certain of our derivatives are subject to netting arrangements with our counterparties, we do not offset derivative assets and liabilities in our condensed consolidated balance sheets. The following table summarizes the fair value and presentation in our condensed consolidated balance sheets of our outstanding derivatives, including those designated as hedging instruments: Fair Value (In millions) Balance Sheet Location As of September 30, 2017 Hedging Instruments: Asset derivatives Investments and other assets $ 0.3 Liability derivatives Accrued expenses and other $ 80.8 Other long-term liabilities $ 24.2 Other Derivatives: Asset derivatives Other current assets $ 1.3 Liability derivatives Accrued expenses and other $ 1.2 Fair Value (In millions) Balance Sheet Location As of December 31, 2016 Hedging Instruments: Asset derivatives Other current assets $ 50.4 Investments and other assets $ 6.6 Liability derivatives Other long-term liabilities $ 4.6 Other Derivatives: Asset derivatives Other current assets $ 4.0 Liability derivatives Accrued expenses and other $ 9.0

Property, Plant and Equipment P

Property, Plant and Equipment Property, Plant and Equipment	9 Months Ended
	Sep. 30, 2017
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	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 $1,580.6 million and $1,439.3 million as of September 30, 2017 and December 31, 2016 , respectively. Solothurn, Switzerland Facility During the first quarter of 2016 we purchased land in Solothurn, Switzerland for 64.4 million Swiss Francs (approximately $62.5 million ), where we are building a biologics manufacturing facility. As of September 30, 2017 and December 31, 2016 , we had $968.8 million and $481.5 million , respectively, capitalized as construction in progress related to the construction of this facility. As of September 30, 2017 , we had contractual commitments of approximately $235.0 million for the construction of this facility.

Equity

Equity	9 Months Ended
	Sep. 30, 2017
Equity [Abstract]
Equity	Equity Total equity as of September 30, 2017 , increased $720.3 million compared to December 31, 2016 . This increase was primarily driven by net income attributable to Biogen Inc. of $2.8 billion , partially offset by share repurchases totaling approximately $1.4 billion , as described below, and an adjustment to retained earnings of $852.8 million reflecting the spin-off of our hemophilia business, as described in Note 3, Hemophilia Spin-Off, to these condensed consolidated financial statements. Share Repurchases In July 2016 our Board of Directors authorized a program to repurchase up to $5.0 billion of our common stock. This authorization does not have an expiration date. All share repurchases under this authorization will be retired. Under this program, we repurchased and retired 3.7 million shares of common stock at a cost of $1.0 billion during the nine months ended September 30, 2017 . We did not repurchase any shares of common stock under this program during the three months ended September 30, 2017 . During the three and nine months ended September 30, 2016 , we repurchased and retired 1.1 million shares of common stock at a cost of $348.9 million . As of September 30, 2017 , approximately $3.0 billion remains available to repurchase shares under this authorization. In February 2011 our Board of Directors authorized a program to repurchase up to 20.0 million shares of common stock, which was completed as of March 31, 2017. During the nine months ended September 30, 2017 , we repurchased 1.3 million shares of common stock at a cost of $365.4 million under this program. We did not repurchase any shares of common stock under this program during the three and nine months ended September 30, 2016 . Noncontrolling Interests The following table reconciles equity (deficit) attributable to noncontrolling interests (NCI): For the Three Months For the Nine Months (In millions) 2017 2016 2017 2016 NCI, beginning of period $ (11.6 ) $ (0.1 ) $ (11.5 ) $ 2.1 Net income (loss) attributable to NCI, net of tax — (2.7 ) (0.1 ) (5.8 ) Fair value of net assets and liabilities acquired and assigned to NCI — — — 0.9 Translation adjustment and other — 0.1 — 0.1 NCI, end of period $ (11.6 ) $ (2.7 ) $ (11.6 ) $ (2.7 )

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss)	9 Months Ended
	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss) The following table summarizes the changes in accumulated other comprehensive income (loss), net of tax by component: (In millions) Unrealized Gains (Losses) on Securities Available for Sale, Net of Tax Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax Unfunded Status of Postretirement Benefit Plans, Net of Tax Translation Adjustments Total Balance, as of December 31, 2016 $ (10.8 ) $ 57.8 $ (32.7 ) $ (334.2 ) $ (319.9 ) Other comprehensive income (loss) before reclassifications 5.5 (176.5 ) (0.5 ) 146.7 (24.8 ) Amounts reclassified from accumulated other comprehensive income (loss) 1.1 14.2 — — 15.3 Net current period other comprehensive income (loss) 6.6 (162.3 ) (0.5 ) 146.7 (9.5 ) Balance, as of September 30, 2017 $ (4.2 ) $ (104.5 ) $ (33.2 ) $ (187.5 ) $ (329.4 ) (In millions) Unrealized Gains (Losses) on Securities Available for Sale, Net of Tax Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax Unfunded Status of Postretirement Benefit Plans, Net of Tax Translation Adjustments Total Balance, as of December 31, 2015 $ (0.8 ) $ 10.2 $ (37.8 ) $ (195.6 ) $ (224.0 ) Other comprehensive income (loss) before reclassifications 1.2 (17.9 ) 1.3 (62.8 ) (78.2 ) Amounts reclassified from accumulated other comprehensive income (loss) 0.4 0.9 — — 1.3 Net current period other comprehensive income (loss) 1.6 (17.0 ) 1.3 (62.8 ) (76.9 ) Balance, as of September 30, 2016 $ 0.8 $ (6.8 ) $ (36.5 ) $ (258.4 ) $ (300.9 ) The following table summarizes the amounts reclassified from accumulated other comprehensive income: (In millions) Income Statement Location Amounts Reclassified from Accumulated Other Comprehensive Income For the Three Months For the Nine Months 2017 2016 2017 2016 Gains (losses) on securities available for sale Other income (expense) $ (0.9 ) $ — $ (1.7 ) $ (0.6 ) Income tax benefit (expense) 0.3 — 0.6 0.2 Gains (losses) on cash flow hedges Revenues (18.8 ) (5.2 ) (15.1 ) (0.7 ) Operating expenses 0.5 (0.2 ) 0.7 (0.4 ) Other income (expense) 0.1 0.1 0.2 0.2 Income tax benefit (expense) — — — — Total reclassifications, net of tax $ (18.8 ) $ (5.3 ) $ (15.3 ) $ (1.3 )

Earnings per Share

Earnings per Share	9 Months Ended
	Sep. 30, 2017
Earnings Per Share [Abstract]
Earnings per Share	Earnings per Share Basic and diluted earnings per share are calculated as follows: For the Three Months For the Nine Months (In millions) 2017 2016 2017 2016 Numerator: Net income attributable to Biogen Inc. $ 1,226.1 $ 1,032.9 $ 2,836.5 $ 3,053.6 Denominator: Weighted-average number of common shares outstanding 211.4 218.9 213.0 219.0 Effect of dilutive securities: Stock options and employee stock purchase plan 0.1 0.1 — 0.1 Time-vested restricted stock units 0.2 0.2 0.2 0.2 Market stock units 0.1 0.2 0.1 0.1 Dilutive potential common shares 0.4 0.5 0.3 0.4 Shares used in calculating diluted earnings per share 211.8 219.4 213.3 219.4 Amounts excluded from the calculation of net income per diluted share because their effects were anti-dilutive were insignificant. The adjustments related to the spin-off of our hemophilia business did not have a material impact on the potentially dilutive securities to be considered in the calculation of diluted earnings per share of common stock.

Share-based Payments

Share-based Payments	9 Months Ended
	Sep. 30, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-based Payments	Share-based Payments Share-based Compensation Expense The following table summarizes share-based compensation expense included in our condensed consolidated statements of income: For the Three Months For the Nine Months (In millions) 2017 2016 2017 2016 Research and development $ 19.5 $ 22.7 $ 55.7 $ 65.6 Selling, general and administrative 23.6 31.2 71.9 94.6 Restructuring charges — — — (1.8 ) Subtotal 43.1 53.9 127.6 158.4 Capitalized share-based compensation costs (2.5 ) (3.2 ) (7.6 ) (10.7 ) Share-based compensation expense included in total cost and expenses 40.6 50.7 120.0 147.7 Income tax effect (10.9 ) (14.6 ) (31.8 ) (42.5 ) Share-based compensation expense included in net income attributable to Biogen Inc. $ 29.7 $ 36.1 $ 88.2 $ 105.2 The following table summarizes share-based compensation expense associated with each of our share-based compensation programs: For the Three Months For the Nine Months (In millions) 2017 2016 2017 2016 Market stock units $ 4.5 $ 5.7 $ 17.1 $ 27.0 Time-vested restricted stock units 26.7 28.5 81.2 92.5 Cash settled performance units 7.0 7.9 13.0 12.7 Performance units 3.2 9.1 8.9 17.3 Employee stock purchase plan 1.7 2.7 7.4 8.9 Subtotal 43.1 53.9 127.6 158.4 Capitalized share-based compensation costs (2.5 ) (3.2 ) (7.6 ) (10.7 ) Share-based compensation expense included in total cost and expenses $ 40.6 $ 50.7 $ 120.0 $ 147.7 We estimate the fair value of our obligations associated with our performance units and cash settled performance units at the end of each reporting period through expected settlement. Cumulative adjustments to these obligations are recorded each quarter to reflect changes in the stock price and estimated outcome of the performance-related conditions. Spin-off Related Equity Adjustments Pursuant to an employee matters agreement entered into in connection with the spin-off of our hemophilia business and the provisions of our existing share-based compensation arrangements, we made certain adjustments to the number and terms of our outstanding stock options, restricted stock units, cash settled performance units and other share-based awards to preserve the intrinsic value of the awards immediately before and after the spin-off. For purposes of the vesting of these equity awards, continued employment or service with Biogen or with Bioverativ was treated as continued employment for purposes of both Biogen’s and Bioverativ’s equity awards with the outstanding awards continuing to vest over their respective original vesting periods. Outstanding unvested awards for employees transferring to Bioverativ were converted to unvested Bioverativ awards. Adjustments to the number of our share-based compensation awards were made using an adjustment ratio based upon the weighted-average closing price of our common stock for the 10 calendar days prior to the effective date of the spin-off and the volume weighted-average prices for the 10 calendar days of our common stock following the effective date of the spin-off. For stock options, the exercise prices of the awards were modified to maintain the pre-spin intrinsic value of the awards in relation to the post-spin stock price of Biogen. The difference between the fair value of the awards based upon the adjustment ratio and the opening price on the distribution date was not material. 2017 Omnibus Equity Plan In June 2017 our shareholders approved the Biogen Inc. 2017 Omnibus Equity Plan (2017 Omnibus Equity Plan) for share-based awards to our employees. Awards granted from the 2017 Omnibus Equity Plan may include stock options, shares of restricted stock, restricted stock units, performance shares, stock appreciation rights and other awards in such amounts and with such terms and conditions as may be determined by a committee of our Board of Directors, subject to the provisions of the plan. Shares of common stock available for grant under the 2017 Omnibus Equity Plan consist of 8.0 million shares reserved for this purpose, plus shares of common stock that remained available for grant under our 2008 Omnibus Equity Plan as of June 7, 2017 or that could again become available for grant if outstanding awards under the 2008 Omnibus Equity Plan as of June 7, 2017 are cancelled, surrendered or terminated in whole or in part. The 2017 Omnibus Equity Plan provides that awards other than stock options and stock appreciation rights will be counted against the total number of shares available under the plan in a 1.5-to-1 ratio. We have not made any awards pursuant to the 2008 Omnibus Equity Plan since our shareholders approved the 2017 Omnibus Equity Plan, and do not intend to make any awards pursuant to the 2008 Omnibus Equity Plan in the future, except that unused shares under the 2008 Omnibus Equity Plan have been carried over for use under the 2017 Omnibus Equity Plan.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes A reconciliation between the U.S. federal statutory tax rate and our effective tax rate is summarized as follows: For the Three Months For the Nine Months 2017 2016 2017 2016 Statutory rate 35.0 % 35.0 % 35.0 % 35.0 % State taxes 0.7 1.1 0.6 1.0 Taxes on foreign earnings (11.4 ) (10.1 ) (11.4 ) (9.4 ) Credits and net operating loss utilization (0.7 ) (1.4 ) (0.8 ) (1.3 ) Purchased intangible assets 1.2 1.2 1.3 1.1 Manufacturing deduction (2.0 ) (1.8 ) (2.1 ) (1.7 ) Other permanent items 0.6 0.2 0.7 0.5 Other 0.4 0.5 0.6 0.4 Effective tax rate 23.8 % 24.7 % 23.9 % 25.6 % For the three and nine months ended September 30, 2017 , compared to the same periods in 2016 , the decrease in our effective tax rate was primarily due to a lower relative percentage of our earnings being recognized in the U.S., a high tax jurisdiction, along with a higher deduction for U.S. manufacturing activities. The geographic split of our earnings was affected by milestone and upfront payments in the current year and the spin-off of our hemophilia business, partially offset by growth from the U.S. launch of SPINRAZA and increases in our revenues from anti-CD20 therapeutic programs in the U.S. In addition, in 2017 we are earning a lower benefit from the orphan drug credit due to the FDA's approval of SPINRAZA. Accounting for Uncertainty in Income Taxes We and our subsidiaries are routinely examined by various taxing authorities. We file income tax returns in various U.S. states and in U.S. federal and other foreign jurisdictions. With few exceptions, we are no longer subject to U.S. federal tax examination for years before 2013 or state, local or non-U.S. income tax examinations for years before 2010. We made payments totaling approximately $60.0 million to the Danish Tax Authority (SKAT) for assessments received for fiscal 2009, 2011 and 2013 regarding withholding taxes and the treatment of certain intercompany transactions involving a Danish affiliate and another of our affiliates. We continue to dispute the assessments for all of these periods and believe that the positions taken in our historical filings are valid.

Other Consolidated Financial St

Other Consolidated Financial Statement Detail	9 Months Ended
	Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Other Consolidated Financial Statement Detail	Other Consolidated Financial Statement Detail Other Income (Expense), Net Components of other income (expense), net, are summarized as follows: For the Three Months For the Nine Months (In millions) 2017 2016 2017 2016 Interest income $ 20.6 $ 16.4 $ 54.2 $ 43.0 Interest expense (61.8 ) (66.0 ) (188.8 ) (195.2 ) Gain (loss) on investments, net (4.0 ) 0.8 (15.0 ) (0.3 ) Foreign exchange gains (losses), net 6.7 (4.9 ) 8.4 (2.6 ) Other, net (5.1 ) (4.4 ) (8.2 ) (14.3 ) Total other income (expense), net $ (43.6 ) $ (58.1 ) $ (149.4 ) $ (169.4 ) Other Current Assets Other current assets include prepaid taxes totaling approximately $714.4 million and $817.0 million as of September 30, 2017 and December 31, 2016 , respectively. Accrued Expenses and Other Accrued expenses and other consists of the following: (In millions) As of As of Current portion of contingent consideration obligations and milestones $ 611.5 $ 580.8 Revenue-related reserves for discounts and allowances 467.2 438.6 Employee compensation and benefits 241.9 282.9 Collaboration expenses 180.1 130.9 Royalties and licensing fees 190.8 195.8 Construction in progress 157.6 134.0 Accrued TECFIDERA litigation settlement and license charges — 454.8 Other 606.1 685.7 Total accrued expenses and other $ 2,455.2 $ 2,903.5 Pricing of TYSABRI in Italy - AIFA In the first quarter of 2017, we reached an agreement with the Price and Reimbursement Committee of the Italian National Medicines Agency (Agenzia Italiana del Farmaco, or AIFA) resolving all of AIFA's claims relating to sales of TYSABRI in excess of the reimbursement limit for prior periods. As a result, in the first quarter of 2017 we recognized EUR 41.8 million (approximately $45.0 million ) in revenues for sales which were previously deferred. These amounts were previously accrued for and included in the table above in Other as of December 31, 2016 . For additional information regarding our agreement with AIFA relating to sales of TYSABRI in Italy, please read Note 20, Litigation, to our consolidated financial statements included in our 2016 Form 10-K.

Investments in Variable Interes

Investments in Variable Interest Entities	9 Months Ended
	Sep. 30, 2017
Investments in Variable Interest Entities [Abstract]
Investments in Variable Interest Entities	Investments in Variable Interest Entities Consolidated Variable Interest Entities Our condensed consolidated financial statements include the financial results of variable interest entities in which we are the primary beneficiary. Neurimmune SubOne AG In 2007 we entered into a collaboration agreement with Neurimmune SubOne AG (Neurimmune), a subsidiary of Neurimmune AG, for the development and commercialization of antibodies for the treatment of AD. Neurimmune conducts research to identify potential therapeutic antibodies and we are responsible for the development, manufacturing and commercialization of all products. We consolidate the results of Neurimmune as we determined that we are the primary beneficiary of Neurimmune because we have the power through the collaboration to direct the activities that most significantly impact the entity’s economic performance and are required to fund 100% of the research and development costs incurred in support of the collaboration agreement. Amounts incurred by Neurimmune for research and development expenses in support of the collaboration and reimbursed by us were immaterial for the three and nine months ended September 30, 2017 and 2016 . The assets and liabilities of Neurimmune are not significant to our financial position or results of operations as it is a research and development organization. We have provided no financing to Neurimmune other than previously contractually required amounts. Unconsolidated Variable Interest Entities We have relationships with other variable interest entities that we do not consolidate as we lack the power to direct the activities that significantly impact the economic success of these entities. These relationships include investments in certain biotechnology companies and research collaboration agreements. As of September 30, 2017 and December 31, 2016 , the total carrying value of our investments in biotechnology companies totaled $49.7 million and $47.4 million , respectively. Our maximum exposure to loss related to these variable interest entities is limited to the carrying value of our investments. We have also entered into research collaboration agreements with certain variable interest entities where we are required to fund certain development activities. These development activities are included in research and development expense in our condensed consolidated statements of income as they are incurred. We have provided no financing to these variable interest entities other than previously contractually required amounts. For additional information related to our investments in Neurimmune and other variable interest entities, please read Note 18, Investments in Variable Interest Entities, to our consolidated financial statements included in our 2016 Form 10-K.

Collaborative and Other Relatio

Collaborative and Other Relationships	9 Months Ended
	Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaborative and Other Relationships	Collaborative and Other Relationships Bristol-Myers Squibb Company On June 1, 2017, we finalized an agreement with Bristol-Myers Squibb Company (BMS) to exclusively license BMS-986168 (now known as BIIB092), a Phase 2-ready experimental medicine with potential in AD and progressive supranuclear palsy (PSP), and made an upfront payment of $300.0 million to BMS. BIIB092 is an antibody targeting tau, the protein that forms the deposits, or tangles, in the brain associated with AD and other neurodegenerative tauopathies such as PSP. PSP is a rare condition that affects movement, speech, vision and cognitive function. Under the agreement, we received worldwide rights to BIIB092 and are responsible for the full development and global commercialization of BIIB092 in AD and PSP. We may pay BMS up to $410.0 million in additional milestone payments, and potential royalties. We also assumed all remaining obligations to the former stockholders of iPierian, Inc. (iPierian) related to BMS’s acquisition of iPierian in 2014. In June 2017 we triggered a $60.0 million developmental milestone payable to the former stockholders of iPierian upon dosing of the first patient in the Phase 2 PSP study for BIIB092 and we may pay the former stockholders of iPierian up to $490.0 million in remaining milestone payments, and potential royalties. Both the $300.0 million upfront payment and the $60.0 million developmental milestone payment were recognized as research and development expense in our condensed consolidated statements of income for the nine months ended September 30, 2017 . AbbVie Inc. We have a collaboration agreement with AbbVie Inc. (AbbVie) aimed at advancing the development and commercialization of ZINBRYTA in MS, which was approved for the treatment of relapsing forms of MS in the U.S. in May 2016 and in the E.U. in July 2016. Under the agreement, we and AbbVie conduct ZINBRYTA co-promotion activities in the U.S., E.U. and Canadian territories (Collaboration Territory), where development and commercialization costs and profits are shared equally. Outside of the Collaboration Territory, we are solely responsible for development and commercialization of ZINBRYTA and will pay a tiered royalty to AbbVie as a percentage of net sales in the low to high teens. In July 2017 the European Medicines Agency (EMA) announced that it had provisionally restricted the use of ZINBRYTA to adult patients with highly active relapsing disease despite a full and adequate course of treatment with at least one disease modifying therapy (DMT) or with rapidly evolving severe relapsing MS who are unsuitable for treatment with other DMTs. These restrictions followed the initiation of an EMA review (referred to as an Article 20 Procedure) of ZINBRYTA following the report of a case of fatal fulminant liver failure, as well as four cases of serious liver injury. The outcome of the EMA review process may affect the market for ZINBRYTA, which may result in the impairment of all or a portion of certain assets related to ZINBRYTA. As of September 30, 2017 , our condensed consolidated balance sheet includes ZINBRYTA related assets, primarily related to inventory, prepaid tax and intangible assets, totaling approximately $200 million . Offsetting these amounts, we also have an unrecorded tax benefit related to certain ZINBRYTA assets totaling approximately $100 million as of September 30, 2017 . Co-promotion Profits and Losses In the U.S., AbbVie recognizes revenues on sales to third parties and we recognize our 50% share of the co-promotion profits or losses as a component of other revenues from collaborative and other relationships in our condensed consolidated statements of income. The collaboration began selling ZINBRYTA in the U.S. in the third quarter of 2016. During the three and nine months ended September 30, 2017 , we recognized a net reduction in revenue of $2.8 million and $12.6 million , respectively, to reflect our share of an overall net loss within the collaboration, as compared to $13.5 million in each of the prior year comparative periods. In the E.U. and Canada, we recognize revenues on sales to third parties in product revenues, net and record the related cost of revenues and sales and marketing expenses to their respective line items in our condensed consolidated statements of income as revenues are recognized and related costs are incurred. We also reimburse AbbVie for their 50% share of the co-promotion profits or losses in the E.U. and Canada, which is recognized in collaboration profit (loss) sharing in our condensed consolidated statements of income. We began to recognize product revenues on sales of ZINBRYTA in the E.U. in the third quarter of 2016. For the three and nine months ended September 30, 2017 , we recognized net expense of $0.7 million and $2.0 million , respectively, to reflect AbbVie's 50% sharing of the net collaboration profits in the E.U. and Canada, as compared to net income recognized of $2.7 million to reflect AbbVie's 50% sharing of the net collaboration losses in the E.U. and Canada in the prior year comparative periods. Ionis Pharmaceuticals, Inc. SPINRAZA (nusinersen) In January 2012 we entered into an exclusive worldwide option and collaboration agreement with Ionis Pharmaceuticals, Inc. (Ionis) under which both companies develop and commercialize the antisense investigational drug candidate, SPINRAZA, for the treatment of SMA. Under the terms of the agreement, during the third quarter of 2016, we exercised our option to develop and commercialize SPINRAZA and paid Ionis a $75.0 million license fee in connection with the option exercise. This amount was recognized as research and development expense in our condensed consolidated statements of income. SPINRAZA was approved for use in the U.S., E.U. and Japan in December 2016, June 2017 and July 2017, respectively. These approvals triggered milestone payments to Ionis totaling $150.0 million during 2017, which were capitalized in intangible assets, net in our condensed consolidated balance sheets. For the three and nine months ended September 30, 2017 , we recognized product revenues totaling $197.6 million and $438.8 million , respectively, on our sales of SPINRAZA in the U.S. and $73.3 million and $82.4 million , respectively, on our sales of SPINRAZA outside of the U.S. Pursuant to our agreement with Ionis, we make royalty payments to Ionis on annual worldwide net sales of SPINRAZA using a tiered royalty rate between 11% and 15% , which are recognized in royalty cost of sales within our condensed consolidated statements of income. Royalty cost of sales related to sales of SPINRAZA for the three and nine months ended September 30, 2017 totaled $34.0 million and $64.9 million , respectively. Samsung Bioepis Joint Venture Agreement In February 2012 we entered into a joint venture agreement with Samsung Biologics, establishing an entity, Samsung Bioepis, to develop, manufacture and market biosimilar pharmaceuticals. As of September 30, 2017 , our ownership interest was approximately 5% , which reflects the effect of additional equity financings in which we did not participate. We maintain an option to purchase additional stock in Samsung Bioepis that would allow us to increase our ownership percentage up to 49.9% . The exercise of this option is within our control. We recognize our share of the results of operations related to our investment in Samsung Bioepis one quarter in arrears when the results of the entity become available, which is reflected as equity in loss of investee, net of tax in our condensed consolidated statements of income. During 2015, as our share of losses exceeded the carrying value of our investment, we suspended recognizing additional losses and will continue to do so unless we commit to providing additional funding. Commercial Agreement We began to recognize revenue on sales of BENEPALI and FLIXABI in the E.U. in the first and third quarters of 2016, respectively. We reflect revenues on sales of BENEPALI and FLIXABI to third parties in product revenues, net in our condensed consolidated statements of income and record the related cost of revenues and sales and marketing expenses in our condensed consolidated statements of income to their respective line items when these costs are incurred. In August 2017 the European Commission granted a marketing authorization in the E.U. for IMRALDI, an adalimumab biosimilar referencing HUMIRA, triggering an additional $25.0 million milestone payment due to Samsung Bioepis, which was capitalized in intangible assets, net in our condensed consolidated balance sheets as of September 30, 2017 . We share 50% of the profit or loss in accordance with the cost sharing provision of the commercial agreement with Samsung Bioepis, which is recognized in collaboration profit (loss) sharing in our condensed consolidated statements of income. For the three and nine months ended September 30, 2017 , we recognized net expense of $34.5 million and $80.5 million , respectively, to reflect Samsung Bioepis's 50% sharing of the net collaboration profits, as compared to net expense recognized of $7.4 million and $1.8 million to reflect sharing of the net collaboration profits in the prior year comparative periods. Other Services Simultaneous with the formation of Samsung Bioepis, we also entered into a license agreement, a technical development services agreement and a manufacturing agreement with Samsung Bioepis. For the three and nine months ended September 30, 2017 , we recognized $8.8 million and $23.7 million , respectively, in connection with these services as a component of other revenues from collaborative and other relationships in our condensed consolidated statements of income, as compared to $0.4 million and $17.3 million , respectively, in the prior year comparative periods. For additional information related to these and our other significant collaboration arrangements, please read Note 19, Collaborative and Other Relationships, to our consolidated financial statements included in our 2016 Form 10-K.

Litigation

Litigation	9 Months Ended
	Sep. 30, 2017
Loss Contingency, Information about Litigation Matters [Abstract]
Litigation	Litigation We are currently involved in various claims and legal proceedings, including the matters described below. For information as to our accounting policies relating to claims and legal proceedings, including use of estimates and contingencies, please read Note 1, Summary of Significant Accounting Policies, to our consolidated financial statements included in our 2016 Form 10-K. With respect to some loss contingencies, an estimate of the possible loss or range of loss cannot be made until management has further information, including, for example, (i) which claims, if any, will survive dispositive motion practice; (ii) information to be obtained through discovery; (iii) information as to the parties' damages claims and supporting evidence; (iv) the parties’ legal theories; and (v) the parties' settlement positions. The claims and legal proceedings in which we are involved also include challenges to the scope, validity or enforceability of the patents relating to our products, pipeline or processes, and challenges to the scope, validity or enforceability of the patents held by others. These include claims by third parties that we infringe their patents. An adverse outcome in any of these proceedings could result in one or more of the following and have a material impact on our business or consolidated results of operations and financial position: (i) loss of patent protection; (ii) inability to continue to engage in certain activities; and (iii) payment of significant damages, royalties, penalties and/or license fees to third parties. Loss Contingencies Qui Tam Litigation On July 6, 2015, a qui tam action filed on behalf of the United States and certain states was unsealed by the U.S. District Court for the District of Massachusetts. The action alleges sales and promotional activities in violation of the federal False Claims Act and state law counterparts, and seeks single and treble damages, civil penalties, interest, attorneys’ fees and costs. Our motion to dismiss is pending. The United States has not made an intervention decision. An estimate of the possible loss or range of loss cannot be made at this time. Securities Litigation We and certain current and former officers are defendants in an action filed by a shareholder on October 20, 2016 in the U.S. District Court for the District of Massachusetts alleging violations of federal securities laws under 15 U.S.C §78j(b) and §78t(a) and 17 C.F.R. §240.10b-5 and seeking a declaration of the action as a class action and an award of damages, interest and attorneys' fees. An estimate of the possible loss or range of loss cannot be made at this time. Other Matters Abbreviated New Drug Application Litigation relating to TECFIDERA In June, July and September 2017 we filed patent infringement actions pursuant to the Hatch-Waxman Act against parties who filed Abbreviated New Drug Applications (ANDAs) for generic versions of TECFIDERA. We have sued the following parties in the United States District Court for the District of Delaware: Amneal Pharmaceuticals LLC, Aurobindo Pharma U.S.A., Inc., Hetero USA, Inc., Impax Laboratories, Inc., Prinston Pharmaceuticals, Inc., Slayback Pharma LLC, Teva Pharmaceuticals USA, Inc., Alkem Laboratories Ltd., Cipla Limited, Glenmark Pharmaceuticals Pvt. Ltd., Lupin Atlantis Holdings SA, Macleods Pharmaceuticals, Ltd., MSN Laboratories Pvt. Ltd., Pharmathen S.A., Shipla Medicare Limited, Sun Pharma Global FZE, Torren Pharmaceuticals, Ltd., TWI Pharmaceuticals, Inc., Windlas Healthcare Pvt., Ltd., Accord Healthcare Inc., Par Pharmaceuticals, Sandoz Inc., Sawai (USA), Inc. and Zydus Pharmaceuticals (USA), Inc. We have also filed actions against Stason Pharmaceuticals, Inc. in the United States District Court for the Central District of California, Zydus Pharmaceuticals (USA), Inc. in the United States District Court for the District of New Jersey, Accord Healthcare Inc. in the Middle District of North Carolina, Par Pharmaceuticals in the Southern District of New York, Sandoz, Inc. in the District of Colorado and Mylan Pharmaceuticals in the United States District Court for the District of West Virginia. No trial has been scheduled in any of these matters. Interference Proceeding with Forward Pharma In April 2015 the U.S. Patent and Trademark Office (USPTO) declared an interference between Forward Pharma’s pending U.S. Patent Application No. 11/576,871 and our U.S. Patent No. 8,399,514 (the '514 patent). The '514 patent includes claims covering the treatment of MS with 480 mg of dimethyl fumarate as provided for in our TECFIDERA label. In March 2017 the USPTO ruled against Forward Pharma. Forward Pharma has appealed to the U.S. Court of Appeals for the Federal Circuit and the appeal is pending. For additional information regarding this matter, please read Note 6, Intangibles Assets and Goodwill, to these condensed consolidated financial statements. European Patent Office Oppositions In June 2016 the European Patent Office (EPO) issued a written decision confirming its earlier revocation of our European patent number 2 137 537 (the '537 patent), which we have appealed. The '537 patent includes claims covering the treatment of MS with 480 mg of dimethyl fumarate as provided for in our TECFIDERA label. The EPO has scheduled a hearing for January 2018 on Biogen’s and others’ oppositions to Forward Pharma’s European Patent No. 2 801 355, which was issued in May 2015 and expires in October 2025. The settlement and license agreement that we entered with Forward Pharma in January 2017 did not resolve the issues pending in this proceeding and we and Forward Pharma intend to permit the EPO and the Technical Board of Appeal and the Enlarged Board of Appeal, as applicable, to make a final determination. For additional information regarding this matter, please read Note 6, Intangibles Assets and Goodwill, to these condensed consolidated financial statements. Patent Revocation Matter Swiss Pharma International AG filed actions in the District Court of The Hague (on January 11, 2016) and the German Patents Court (on March 3, 2016) to invalidate the Dutch and German counterparts of our European Patent Number 1 485 127 (“Administration of agents to treat inflammation”), which was issued in June 2011 and concerns administration of natalizumab (TYSABRI) to treat MS. The patent expires in February 2023. In July 2017 the District Court of The Hague ruled that the Dutch counterpart of the patent is invalid and we have appealed. A hearing has been scheduled in the German action for early 2018. '755 Patent Litigation On May 28, 2010, Biogen MA Inc. (formerly Biogen Idec MA Inc.) filed a complaint in the U.S. District Court for the District of New Jersey alleging infringement by Bayer Healthcare Pharmaceuticals Inc. (Bayer) (manufacturer, marketer and seller of BETASERON and manufacturer of EXTAVIA), EMD Serono, Inc. (EMD Serono) (manufacturer, marketer and seller of REBIF), Pfizer Inc. (Pfizer) (co-marketer of REBIF) and Novartis Pharmaceuticals Corp. (Novartis) (marketer and seller of EXTAVIA) of our U.S. Patent No. 7,588,755 ('755 Patent), which claims the use of interferon beta for immunomodulation or treating a viral condition, viral disease, cancers or tumors. The complaint seeks monetary damages, including lost profits and royalties. Bayer had previously filed a complaint against us in the same court, on May 27, 2010, seeking a declaratory judgment that it does not infringe the '755 Patent and that the patent is invalid, and seeking monetary relief in the form of attorneys' fees, costs and expenses. The court has consolidated the two lawsuits, and we refer to the two actions as the “Consolidated '755 Patent Actions.” Bayer, Pfizer, Novartis and EMD Serono have all filed counterclaims in the Consolidated '755 Patent Actions seeking declaratory judgments of patent invalidity and non-infringement, and seeking monetary relief in the form of costs and attorneys' fees, and EMD Serono and Bayer have each filed a counterclaim seeking a declaratory judgment that the '755 Patent is unenforceable based on alleged inequitable conduct. Bayer has also amended its complaint to seek such a declaration. Trial has been set for the first quarter of 2018. Government Matters We have learned that state and federal governmental authorities are investigating our sales and promotional practices and have received related subpoenas. We are cooperating with the government. We have received subpoenas and other requests from the federal government for documents and information relating to our relationship with non-profit organizations that provide assistance to patients taking drugs sold by Biogen and Biogen's co-pay assistance programs. We are cooperating with the government. On July 1, 2016, we received civil investigative demands from the federal government for documents and information relating to our treatment of certain service agreements with wholesalers when calculating and reporting Average Manufacturer Prices in connection with the Medicaid Drug Rebate Program. We are cooperating with the government. On December 29, 2016, we received a civil investigative demand from the federal government for documents and information relating to our relationships with entities providing clinical education and reimbursement support services. We regard this matter as closed. In July 2017 we learned that the Prosecution Office of Milan is investigating our interactions with certain healthcare providers in Italy. We are cooperating with the government. Product Liability and Other Legal Proceedings We are also involved in product liability claims and other legal proceedings generally incidental to our normal business activities. While the outcome of any of these proceedings cannot be accurately predicted, we do not believe the ultimate resolution of any of these existing matters would have a material adverse effect on our business or financial condition.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2017
Subsequent Events [Abstract]
Subsequent Events	20. Subsequent Events Eisai Co., Ltd. BAN2401 and E2609 Collaboration In March 2014, we entered into a collaboration agreement (Eisai Collaboration Agreement) with Eisai Co., Ltd. (Eisai) to jointly develop and commercialize two Eisai product candidates for the treatment of AD, BAN2401, a monoclonal antibody that targets amyloid-beta aggregates, and E2609, a BACE inhibitor. Under the Eisai Collaboration Agreement, Eisai had an option to jointly develop and commercialize aducanumab, our anti-amyloid beta antibody candidate for AD (Aducanumab Option). On October 23, 2017, Eisai exercised its Aducanumab Option and we entered into a new collaboration agreement (Aducanumab Agreement) for the joint development and commercialization of aducanumab. Under the Aducanumab Agreement, the two companies will co-promote aducanumab with a region-based profit split. We will receive a 55% share of the potential profits (losses) in the U.S., a 68.5% share of the potential profits (losses) in the E.U. and a 20% share of the potential profits (losses) in Japan and Asia, excluding China and South Korea. The companies will continue to share equally in the potential profits (losses) in rest of world markets. Under the Aducanumab Agreement, we will continue to lead the ongoing Phase 3 development of aducanumab and will remain responsible for 100% of development costs for aducanumab incurred in support of the agreement until April 2018. Eisai will then reimburse us for 15% of development expenses for the period April 2018 through December 2018, and 45% thereafter. In addition, both companies will continue to jointly develop BAN2401 and E2609; however, we are no longer required to pay Eisai any milestone payments for products containing BAN2401 and we are no longer entitled to any potential development and commercial milestone payments from Eisai in relation to aducanumab. We and Eisai have also agreed to co-promote AVONEX, TYSABRI and TECFIDERA in Japan in certain settings and Eisai will distribute AVONEX, TYSABRI, TECFIDERA and PLEGRIDY in India and other Asia-Pacific markets (excluding China). For additional information related to our collaboration arrangement with Eisai, please read Note 19, Collaborative and Other Relationships , to our consolidated financial statements included in our 2016 Form 10-K. Neurimmune On October 23, 2017, we amended the terms of our existing collaboration agreement with Neurimmune. Pursuant to the amended agreement, we agreed to make a one-time $150.0 million payment to Neurimmune in exchange for a 15% reduction in royalty rates payable on potential commercial sales of aducanumab. Our royalty rates payable on potential commercial sales of aducanumab will now range from the high single digits to low-teens. This payment will be recognized in the fourth quarter of 2017. Under the amended agreement, we also have an option that will expire in April 2018 to further reduce our royalty rate payable on potential commercial sales of aducanumab by an additional 5% in exchange for a one-time $50.0 million payment to Neurimmune. Under the terms of the Aducanumab Agreement, Eisai may elect to share in the benefit and cost of the royalty reduction. For additional information relating to our collaboration agreement with Neurimmune, please read Note 17, Investments in Variable Interest Entities , to these condensed consolidated financial statements.

Summary of Significant Accoun27

Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Overview	Biogen is a global biopharmaceutical company focused on discovering, developing, manufacturing and delivering therapies to people living with serious neurological and neurodegenerative diseases. Our marketed products include TECFIDERA, AVONEX, PLEGRIDY, TYSABRI, ZINBRYTA and FAMPYRA for multiple sclerosis (MS), SPINRAZA for the treatment of spinal muscular atrophy (SMA) and FUMADERM for the treatment of severe plaque psoriasis. We also have certain business and financial rights with respect to RITUXAN for the treatment of non-Hodgkin's lymphoma, chronic lymphocytic leukemia (CLL) and other conditions, GAZYVA indicated for the treatment of CLL and follicular lymphoma, OCREVUS indicated for the treatment of primary progressive MS and relapsing MS, and other potential anti-CD20 therapies under a collaboration agreement with Genentech, Inc., a wholly-owned member of the Roche Group. We support our drug discovery and development efforts through the commitment of significant resources to discovery, research and development programs and business development opportunities, particularly within areas of our scientific, manufacturing and technical capabilities. We intend to invest in the future across our core growth areas of MS and neuroimmunology, Alzheimer's disease (AD) and dementia, Parkinson's disease and movement disorders, and neuromuscular diseases including SMA and amyotrophic lateral sclerosis (ALS). Further, we see opportunities to invest in emerging growth areas such as pain, ophthalmology, neuropsychiatry and acute neurology. In addition, we are employing innovative technologies to discover potential treatments for rare and genetic disorders, including new ways of treating diseases through gene therapy. Our innovative drug development and commercialization activities are complemented by our biosimilar therapies that expand access to medicines and reduce the cost burden for healthcare systems. We are leveraging our manufacturing capabilities and know-how to develop, manufacture and market biosimilars through Samsung Bioepis, our joint venture with Samsung BioLogics Co. Ltd. (Samsung Biologics). Under our commercial agreement, we market and sell BENEPALI, an etanercept biosimilar referencing ENBREL, and FLIXABI, an infliximab biosimilar referencing REMICADE, in the European Union (E.U.). Hemophilia Spin-Off On February 1, 2017, we completed the spin-off of our hemophilia business, Bioverativ Inc. (Bioverativ), as an independent, publicly traded company. Our consolidated results of operations and financial position included in these unaudited condensed consolidated financial statements reflect the financial results of our hemophilia business for all periods through January 31, 2017. For additional information related to the spin-off of our hemophilia business, please read Note 3, Hemophilia Spin-Off, to these condensed consolidated financial statements.
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, 2016 ( 2016 Form 10-K). Our accounting policies are described in the “ Notes to Consolidated Financial Statements ” in our 2016 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 nine months ended September 30, 2017 are not necessarily indicative of the operating results for the full year or for any other subsequent interim period. We operate as one operating segment, focused on discovering, developing, manufacturing and delivering therapies to people living with serious neurological and neurodegenerative diseases.
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, 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 ongoing basis we evaluate our estimates, judgments and methodologies. We base our estimates on historical experience and on various other assumptions that we believe to be reasonable, the results of which form the basis for making judgments about the carrying values of assets, liabilities and equity and the amount of revenues and expenses. Actual results may differ from these estimates under different assumptions or conditions.
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 we adopt as of the specified effective date. Unless otherwise discussed, we do not believe that the impact of recently issued standards that are not yet effective will have a material impact on our financial position or results of operations upon adoption. We adopted the following new standards effective January 1, 2017: • Accounting Standards Update (ASU) No. 2016-06, Derivatives and Hedging (Topic 815): Contingent Put and Call Options in Debt Instruments; • ASU No. 2016-07, Investments - Equity Method and Joint Ventures (Topic 323): Simplifying the Transition to the Equity Method of Accounting; and • ASU No. 2016-09, Compensation - Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting. The adoption of these standards did not have a material impact on our financial position, results of operations or statement of cash flows; however, the adoption of ASU No. 2016-09 resulted in the reclassification of certain prior year amounts in our condensed consolidated statements of cash flows to conform to our current year presentation. Specifically, amounts previously disclosed in net cash flows used in financing activities related to our excess tax benefit from share-based compensation have been reclassified to net cash flows provided by operating activities and amounts related to cash paid when withholding shares for tax withholding purposes, previously disclosed in net cash flows provided by operating activities, have been reclassified to net cash flows used in financing activities. For additional information related to these standards, please read Note 1, Summary of Significant Accounting Policies: New Accounting Pronouncements , to our consolidated financial statements included in our 2016 Form 10-K. In May 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 FASB has subsequently issued amendments to ASU No. 2014-09 that have the same effective date and transition date of January 1, 2018. We expect to adopt these standards using the modified retrospective method. We have performed a review of the new standards as compared to our current accounting policies and our review of customer contracts and collaborative relationships remains in process. As of September 30, 2017 , we have not identified any accounting changes that would materially impact the amount of reported revenues with respect to our product revenues and revenues from anti-CD20 therapeutic programs. During the fourth quarter of 2017 we plan to finalize our assessments over the impact that these standards may have on our results of operations, financial position and disclosures. In January 2016 the FASB issued ASU No. 2016-01, Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities. The new standard amends certain aspects of accounting and disclosure requirements of financial instruments, including the requirement that equity investments with readily determinable fair values be measured at fair value with changes in fair value recognized in a company's results of operations. The new standard does not apply to investments accounted for under the equity method of accounting or those that result in consolidation of the investee. Equity investments that do not have readily determinable fair values may be measured at fair value or at cost minus impairment adjusted for changes in observable prices. A financial liability that is measured at fair value in accordance with the fair value option is required to be presented separately in other comprehensive income for the portion of the total change in the fair value resulting from change in the instrument-specific credit risk. In addition, a valuation allowance should be evaluated on deferred tax assets related to available-for-sale debt securities in combination with other deferred tax assets. The new standard will be effective for us on January 1, 2018. Based on our current investment holdings, the adoption of this standard is not expected to have a material impact on our financial position or results of operations; however, it will result in the reclassification of certain investments. In February 2016 the FASB issued ASU No. 2016-02, Leases (Topic 842). The new standard requires that all lessees recognize the assets and liabilities that arise from leases on their balance sheet and disclose qualitative and quantitative information about their leasing arrangements. The new standard will be effective for us on January 1, 2019. We are currently evaluating the impact that this standard may have on our results of operations, financial position and disclosures. The adoption of this standard is not expected to have a material impact on our net financial position, but may materially impact the reported amount of total assets and total liabilities. In June 2016 the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. The new standard changes the impairment model for most financial assets and certain other instruments. Under the new standard, entities holding financial assets and net investment in leases that are not accounted for at fair value through net income are to be presented at the net amount expected to be collected. An allowance for credit losses will be a valuation account that will be deducted from the amortized cost basis of the financial asset to present the net carrying value at the amount expected to be collected on the financial asset. The new standard will be effective for us on January 1, 2020. The adoption of this standard is not expected to have a material impact on our financial position or results of operations. In August 2016 the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments. The new standard clarifies certain aspects of the statement of cash flows, including the classification of debt prepayment or debt extinguishment costs, settlement of zero-coupon debt instruments or other debt instruments with coupon interest rates that are insignificant in relation to the effective interest rate of the borrowing, contingent consideration payments made after a business combination, proceeds from the settlement of insurance claims, proceeds from the settlement of corporate-owned life insurance policies, distributions received from equity method investees and beneficial interests in securitization transactions. The new standard also clarifies that an entity should determine each separately identifiable source or use within the cash receipts and cash payments on the basis of the nature of the underlying cash flows. In situations in which cash receipts and payments have aspects of more than one class of cash flows and cannot be separated by source or use, the appropriate classification should depend on the activity that is likely to be the predominant source or use of cash flows for the item. The new standard will be effective for us on January 1, 2018. The adoption of this standard is not expected to have a material impact on our statements of cash flows upon adoption. In October 2016 the FASB issued ASU No. 2016-16, Income Taxes (Topic 740): Intra-Entity Transfer of Assets Other Than Inventory. The new standard eliminates the deferral of the tax effects of intra-entity transfers of an asset other than inventory. Under the new standard, entities should recognize the income tax consequences on an intra-entity transfer of an asset other than inventory when the transfer occurs. The new standard will be effective for us on January 1, 2018. We expect to adopt this standard using the modified retrospective method applied through a cumulative-effect adjustment directly to retained earnings as of the beginning of the period of adoption. The adoption of this standard is expected to have a material impact on our net financial position; however, the final effect of the adoption of this standard will depend on the nature and amount of future transactions. In January 2017 the FASB issued ASU No. 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business. The new standard clarifies the definition of a business and provides a screen to determine when an integrated set of assets and activities is not a business. The screen requires that when substantially all of the fair value of the gross assets acquired (or disposed of) is concentrated in a single identifiable asset or a group of similar identifiable assets, the set is not a business. The new standard will be effective for us on January 1, 2018; however, we have adopted this standard as of January 1, 2017, with prospective application to any business development transaction. In January 2017 the FASB issued ASU No. 2017-04, Intangibles - Goodwill and Other (Topic 350): Simplifying the Test of Goodwill Impairment. The new standard eliminates Step 2 from the goodwill impairment test. Under the amendments in ASU No. 2017-04, an entity should recognize an impairment charge for the amount by which the carrying amount of a reporting unit exceeds that reporting unit’s fair value; however, the loss recognized should not exceed the total amount of goodwill allocated to that reporting unit. The new standard will be effective for us on January 1, 2020; however, early adoption is permitted. We intend to early adopt this standard as of October 31, 2017, during our annual review of goodwill. The adoption of this standard is not expected to have a material impact on our financial position or results of operations. In March 2017 the FASB issued ASU No. 2017-07, Compensation - Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost. The new standard will require that an employer disaggregate the service cost component from the other components of net benefit cost. The new standard also provides explicit guidance on how to present the service cost component and the other components of net benefit cost in the income statement and allow only the service cost component of net benefit cost to be eligible for capitalization. The other components of the net periodic benefit cost must be presented separately from the line items that include service cost and outside of any subtotal of operating income on the condensed consolidated statements of income. The new standard will be effective for us on January 1, 2018. The adoption of this standard is not expected to have a material impact on our financial position or results of operations. In March 2017 the FASB issued ASU No. 2017-08, Receivables - Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities. The new standard amends the amortization period for certain purchased callable debt securities held at a premium by shortening the amortization period for the premium to the earliest call date. The new standard will be effective for us on January 1, 2019. We are currently evaluating the potential impact that this standard may have on our financial position and results of operations. In May 2017 the FASB issued ASU No. 2017-09, Compensation - Stock Compensation (Topic 718): Scope of Modification Accounting. The new standard provides guidance about which changes to the terms or conditions of a share-based payment award require an entity to apply modification accounting in Topic 718. The new standard will be effective for us on January 1, 2018; however, early adoption is permitted. The adoption of this standard is not expected to have a material impact on our financial position or results of operations. In August 2017 the FASB issued ASU No. 2017-12, Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities. The new standard provides guidance to better align an entity’s risk management activities and financial reporting for hedging relationships through changes to both the designation and measurement guidance for qualifying hedging relationships and the presentation of hedge results. The new standard expands and refines hedge accounting for both non-financial and financial risk components and aligns the recognition and presentation of the effects of the hedging instrument and the hedged item in the financial statements. The new standard will be effective for us on January 1, 2019; however, early adoption is permitted. The adoption of this standard is not expected to have a material impact on our financial position or results of operations.

Hemophilia Spin Off (Tables)

Hemophilia Spin Off (Tables)	9 Months Ended
	Sep. 30, 2017
Discontinued Operations and Disposal Groups [Abstract]
Assets and liabilities charged against equity	The following table summarizes the assets and liabilities that were charged against equity as a result of the spin-off of our hemophilia business: (In millions) Assets Cash $ 302.7 Accounts receivable 144.7 Inventory 116.1 Property, plant and equipment, net 20.2 Intangible assets, net 56.8 Goodwill 314.1 Other, net 53.7 Assets transferred, net $ 1,008.3 Liabilities Accrued expenses and other current liabilities $ 87.8 Other long-term liabilities 67.7 Liabilities transferred, net $ 155.5

Reserves for Discounts and Al29

Reserves for Discounts and Allowances (Tables)	9 Months Ended
	Sep. 30, 2017
Sales Discounts, Returns and Allowances, Goods [Abstract]
Analysis of the changes in reserves	An analysis of the change in reserves for discounts and allowances is summarized as follows: (In millions) Discounts Contractual Adjustments Returns Total Balance, as of December 31, 2016 $ 71.6 $ 482.7 $ 51.2 $ 605.5 Current provisions relating to sales in current year 422.0 1,681.2 20.0 2,123.2 Adjustments relating to prior years (0.2 ) 11.2 (8.6 ) 2.4 Payments/credits relating to sales in current year (320.1 ) (1,227.1 ) (0.1 ) (1,547.3 ) Payments/credits relating to sales in prior years (70.4 ) (431.6 ) (17.2 ) (519.2 ) Balance, as of September 30, 2017 $ 102.9 $ 516.4 $ 45.3 $ 664.6
Total reserves included in condensed consolidated balance sheets	The total reserves above, which are included in our condensed consolidated balance sheets, are summarized as follows: (In millions) As of As of Reduction of accounts receivable $ 197.4 $ 166.9 Component of accrued expenses and other 467.2 438.6 Total reserves $ 664.6 $ 605.5

Inventory (Tables)

Inventory (Tables)	9 Months Ended
	Sep. 30, 2017
Inventory Disclosure [Abstract]
Components of inventory	The components of inventory are summarized as follows: (In millions) As of As of Raw materials $ 208.1 $ 170.4 Work in process 647.6 698.7 Finished goods 175.5 170.3 Total inventory $ 1,031.2 $ 1,039.4 Balance Sheet Classification: Inventory $ 1,007.2 $ 1,001.6 Investments and other assets 24.0 37.8 Total inventory $ 1,031.2 $ 1,039.4

Intangible Assets and Goodwill

Intangible Assets and Goodwill (Tables)	9 Months Ended
	Sep. 30, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible assets	Intangible assets, net of accumulated amortization, impairment charges and adjustments, are summarized as follows: As of September 30, 2017 As of December 31, 2016 (In millions) Estimated Life Cost Accumulated Amortization Net Cost Accumulated Amortization Net Out-licensed patents 13-23 years $ 543.3 $ (532.7 ) $ 10.6 $ 543.3 $ (523.6 ) $ 19.7 Developed technology 15-23 years 3,005.3 (2,675.8 ) 329.5 3,005.3 (2,634.3 ) 371.0 In-process research and development Indefinite until commercialization 680.6 — 680.6 648.0 — 648.0 Trademarks and tradenames Indefinite 64.0 — 64.0 64.0 — 64.0 Acquired and in-licensed rights and patents 4-18 years 4,002.6 (1,067.9 ) 2,934.7 3,481.7 (776.1 ) 2,705.6 Total intangible assets $ 8,295.8 $ (4,276.4 ) $ 4,019.4 $ 7,742.3 $ (3,934.0 ) $ 3,808.3
Estimated future amortization for acquired intangible assets	Based upon the above, the estimated future amortization of acquired intangible assets is expected to be as follows: (In millions) As of 2017 (remaining three months) $ 106.8 2018 421.3 2019 404.0 2020 389.6 2021 256.6 2022 244.7
Summary of roll forward of the changes in goodwill	The following table provides a roll forward of the changes in our goodwill balance: (In millions) As of Goodwill, beginning of period $ 3,669.3 Elimination of goodwill allocated to our hemophilia business (314.1 ) Increase to goodwill 762.4 Other 9.9 Goodwill, end of period $ 4,127.5

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Summary of assets and liabilities recorded at fair value	The tables below present information about our assets and liabilities that are regularly measured and carried at fair value and indicate the level within the fair value hierarchy of the valuation techniques we utilized to determine such fair value: As of September 30, 2017 (In millions) Total Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets: Cash equivalents $ 1,184.3 $ — $ 1,184.3 $ — Marketable debt securities: Corporate debt securities 2,584.7 — 2,584.7 — Government securities 1,870.0 — 1,870.0 — Mortgage and other asset backed securities 567.5 — 567.5 — Marketable equity securities 11.8 11.8 — — Derivative contracts 1.6 — 1.6 — Plan assets for deferred compensation 30.1 — 30.1 — Total $ 6,250.0 $ 11.8 $ 6,238.2 $ — Liabilities: Derivative contracts $ 106.2 $ — $ 106.2 $ — Contingent consideration obligations 522.1 — — 522.1 Total $ 628.3 $ — $ 106.2 $ 522.1 As of December 31, 2016 (In millions) Total Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets: Cash equivalents $ 2,039.6 $ — $ 2,039.6 $ — Marketable debt securities: Corporate debt securities 2,663.8 — 2,663.8 — Government securities 2,172.5 — 2,172.5 — Mortgage and other asset backed securities 561.7 — 561.7 — Marketable equity securities 24.9 24.9 — — Derivative contracts 61.0 — 61.0 — Plan assets for deferred compensation 34.5 — 34.5 — Total $ 7,558.0 $ 24.9 $ 7,533.1 $ — Liabilities: Derivative contracts $ 13.6 $ — $ 13.6 $ — Contingent consideration obligations 467.6 — — 467.6 Total $ 481.2 $ — $ 13.6 $ 467.6
Summary of fair and carrying value of debt instruments	The fair and carrying values of our debt instruments, which are Level 2 liabilities, are summarized as follows: As of September 30, 2017 As of December 31, 2016 (In millions) Fair Value Carrying Value Fair Value Carrying Value Notes payable to Fumedica AG $ 3.2 $ 3.3 $ 6.1 $ 6.0 6.875% Senior Notes due March 1, 2018 561.7 553.1 583.7 558.5 2.900% Senior Notes due September 15, 2020 1,536.2 1,486.5 1,521.5 1,485.3 3.625% Senior Notes due September 15, 2022 1,049.4 994.0 1,026.6 993.2 4.050% Senior Notes due September 15, 2025 1,867.0 1,735.9 1,796.0 1,734.8 5.200% Senior Notes due September 15, 2045 2,030.8 1,721.9 1,874.5 1,721.5 Total $ 7,048.3 $ 6,494.7 $ 6,808.4 $ 6,499.3
Fair value of contingent consideration obligations	The following table provides a roll forward of the fair values of our contingent consideration obligations that includes Level 3 measurements: For the Three Months For the Nine Months (In millions) 2017 2016 2017 2016 Fair value, beginning of period $ 492.1 $ 515.7 $ 467.6 $ 506.0 Additions — — — — Changes in fair value 30.0 5.9 61.2 18.8 Payments — — (6.7 ) (3.2 ) Fair value, end of period $ 522.1 $ 521.6 $ 522.1 $ 521.6

Financial Instruments (Tables)

Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Summary of financial assets with maturities of less than 90 days included within cash and cash equivalents	The following table summarizes our financial assets with maturities of less than 90 days from the date of purchase included in cash and cash equivalents on the accompanying condensed consolidated balance sheets: (In millions) As of As of Commercial paper $ 126.4 $ 31.0 Overnight reverse repurchase agreements 42.0 — Money market funds 871.1 741.7 Short-term debt securities 144.8 1,266.9 Total $ 1,184.3 $ 2,039.6
Marketable debt and equity securities	The following tables summarize our marketable debt and equity securities, classified as available-for-sale: As of September 30, 2017 (In millions) Fair Value Gross Unrealized Gains Gross Unrealized Losses Amortized Cost Corporate debt securities Current $ 1,042.9 $ 0.2 $ (0.4 ) $ 1,043.1 Non-current 1,541.8 2.2 (2.1 ) 1,541.7 Government securities Current 916.0 — (0.8 ) 916.8 Non-current 954.0 0.4 (3.4 ) 957.0 Mortgage and other asset backed securities Current 1.3 — — 1.3 Non-current 566.2 1.3 (1.4 ) 566.3 Total marketable debt securities $ 5,022.2 $ 4.1 $ (8.1 ) $ 5,026.2 Marketable equity securities, non-current $ 11.8 $ — $ (2.6 ) $ 14.4 As of December 31, 2016 (In millions) Fair Value Gross Unrealized Gains Gross Unrealized Losses Amortized Cost Corporate debt securities Current $ 1,408.6 $ 0.2 $ (0.6 ) $ 1,409.0 Non-current 1,255.2 1.2 (4.7 ) 1,258.7 Government securities Current 1,156.0 0.2 (0.3 ) 1,156.1 Non-current 1,016.5 0.5 (3.4 ) 1,019.4 Mortgage and other asset backed securities Current 4.0 — — 4.0 Non-current 557.7 0.8 (2.2 ) 559.1 Total marketable debt securities $ 5,398.0 $ 2.9 $ (11.2 ) $ 5,406.3 Marketable equity securities, non-current $ 24.9 $ 0.7 $ (9.3 ) $ 33.5
Summary of contractual maturities: available-for-sale securities	The estimated fair value and amortized cost of our marketable debt securities available-for-sale by contractual maturity are summarized as follows: As of September 30, 2017 As of December 31, 2016 (In millions) Estimated Fair Value Amortized Cost Estimated Fair Value Amortized Cost Due in one year or less $ 1,960.2 $ 1,961.2 $ 2,568.6 $ 2,569.1 Due after one year through five years 2,792.9 2,795.7 2,552.6 2,559.7 Due after five years 269.1 269.3 276.8 277.5 Total available-for-sale securities $ 5,022.2 $ 5,026.2 $ 5,398.0 $ 5,406.3
Proceeds from marketable debt securities	The proceeds from maturities and sales of marketable debt securities and resulting realized gains and losses are summarized as follows: For the Three Months For the Nine Months (In millions) 2017 2016 2017 2016 Proceeds from maturities and sales $ 888.1 $ 2,362.2 $ 4,472.6 $ 5,185.8 Realized gains $ 0.3 $ 1.1 $ 2.7 $ 2.1 Realized losses $ (1.2 ) $ (1.1 ) $ (4.4 ) $ (2.7 )

Derivative Instruments (Tables)

Derivative Instruments (Tables)	9 Months Ended
	Sep. 30, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Foreign currency forward contracts that were entered into to hedge forecasted revenue	The notional value of foreign currency forward contracts that were entered into to hedge forecasted revenues and operating expenses is summarized as follows: Notional Amount Foreign Currency: (In millions) As of As of Euro $ 1,573.4 $ 871.7 British pound 39.7 — Canadian dollar 19.7 — Swiss franc 5.1 — Total foreign currency forward contracts $ 1,637.9 $ 871.7
Summary of the effect of derivatives designated as hedging instruments on the condensed consolidated statements of income	The following tables summarize the effect of foreign currency forward contracts designated as hedging instruments on our condensed consolidated statements of income: For the Three Months Ended September 30, Net Gains/(Losses) Reclassified from AOCI into Operating Income (Effective Portion) Net Gains/(Losses) Recognized into Net Income (Ineffective Portion) Location 2017 2016 Location 2017 2016 Revenue $ (18.8 ) $ (5.2 ) Other income (expense) $ 0.7 $ (0.6 ) Operating expenses $ 0.5 $ (0.2 ) Other income (expense) $ 0.2 $ — For the Nine Months Ended September 30, Net Gains/(Losses) Reclassified from AOCI into Operating Income (Effective Portion) Net Gains/(Losses) Recognized into Net Income (Ineffective Portion) Location 2017 2016 Location 2017 2016 Revenue $ (15.1 ) $ (0.7 ) Other income (expense) $ 6.7 $ 3.4 Operating expenses $ 0.7 $ (0.4 ) Other income (expense) $ (0.1 ) $ (0.3 )
Summary of fair value and presentation of derivatives	The following table summarizes the fair value and presentation in our condensed consolidated balance sheets of our outstanding derivatives, including those designated as hedging instruments: Fair Value (In millions) Balance Sheet Location As of September 30, 2017 Hedging Instruments: Asset derivatives Investments and other assets $ 0.3 Liability derivatives Accrued expenses and other $ 80.8 Other long-term liabilities $ 24.2 Other Derivatives: Asset derivatives Other current assets $ 1.3 Liability derivatives Accrued expenses and other $ 1.2 Fair Value (In millions) Balance Sheet Location As of December 31, 2016 Hedging Instruments: Asset derivatives Other current assets $ 50.4 Investments and other assets $ 6.6 Liability derivatives Other long-term liabilities $ 4.6 Other Derivatives: Asset derivatives Other current assets $ 4.0 Liability derivatives Accrued expenses and other $ 9.0

Equity (Tables)

Equity (Tables)	9 Months Ended
	Sep. 30, 2017
Equity [Abstract]
Reconciliation of equity attributable to noncontrolling interests	The following table reconciles equity (deficit) attributable to noncontrolling interests (NCI): For the Three Months For the Nine Months (In millions) 2017 2016 2017 2016 NCI, beginning of period $ (11.6 ) $ (0.1 ) $ (11.5 ) $ 2.1 Net income (loss) attributable to NCI, net of tax — (2.7 ) (0.1 ) (5.8 ) Fair value of net assets and liabilities acquired and assigned to NCI — — — 0.9 Translation adjustment and other — 0.1 — 0.1 NCI, end of period $ (11.6 ) $ (2.7 ) $ (11.6 ) $ (2.7 )

Accumulated Other Comprehensi36

Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Tables)	9 Months Ended
	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Schedule of accumulated other comprehensive income (loss)	The following table summarizes the changes in accumulated other comprehensive income (loss), net of tax by component: (In millions) Unrealized Gains (Losses) on Securities Available for Sale, Net of Tax Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax Unfunded Status of Postretirement Benefit Plans, Net of Tax Translation Adjustments Total Balance, as of December 31, 2016 $ (10.8 ) $ 57.8 $ (32.7 ) $ (334.2 ) $ (319.9 ) Other comprehensive income (loss) before reclassifications 5.5 (176.5 ) (0.5 ) 146.7 (24.8 ) Amounts reclassified from accumulated other comprehensive income (loss) 1.1 14.2 — — 15.3 Net current period other comprehensive income (loss) 6.6 (162.3 ) (0.5 ) 146.7 (9.5 ) Balance, as of September 30, 2017 $ (4.2 ) $ (104.5 ) $ (33.2 ) $ (187.5 ) $ (329.4 ) (In millions) Unrealized Gains (Losses) on Securities Available for Sale, Net of Tax Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax Unfunded Status of Postretirement Benefit Plans, Net of Tax Translation Adjustments Total Balance, as of December 31, 2015 $ (0.8 ) $ 10.2 $ (37.8 ) $ (195.6 ) $ (224.0 ) Other comprehensive income (loss) before reclassifications 1.2 (17.9 ) 1.3 (62.8 ) (78.2 ) Amounts reclassified from accumulated other comprehensive income (loss) 0.4 0.9 — — 1.3 Net current period other comprehensive income (loss) 1.6 (17.0 ) 1.3 (62.8 ) (76.9 ) Balance, as of September 30, 2016 $ 0.8 $ (6.8 ) $ (36.5 ) $ (258.4 ) $ (300.9 )
Reclassification out of accumulated other comprehensive income	The following table summarizes the amounts reclassified from accumulated other comprehensive income: (In millions) Income Statement Location Amounts Reclassified from Accumulated Other Comprehensive Income For the Three Months For the Nine Months 2017 2016 2017 2016 Gains (losses) on securities available for sale Other income (expense) $ (0.9 ) $ — $ (1.7 ) $ (0.6 ) Income tax benefit (expense) 0.3 — 0.6 0.2 Gains (losses) on cash flow hedges Revenues (18.8 ) (5.2 ) (15.1 ) (0.7 ) Operating expenses 0.5 (0.2 ) 0.7 (0.4 ) Other income (expense) 0.1 0.1 0.2 0.2 Income tax benefit (expense) — — — — Total reclassifications, net of tax $ (18.8 ) $ (5.3 ) $ (15.3 ) $ (1.3 )

Earnings per Share (Tables)

Earnings per Share (Tables)	9 Months Ended
	Sep. 30, 2017
Earnings Per Share [Abstract]
Basic and diluted earnings per share	Basic and diluted earnings per share are calculated as follows: For the Three Months For the Nine Months (In millions) 2017 2016 2017 2016 Numerator: Net income attributable to Biogen Inc. $ 1,226.1 $ 1,032.9 $ 2,836.5 $ 3,053.6 Denominator: Weighted-average number of common shares outstanding 211.4 218.9 213.0 219.0 Effect of dilutive securities: Stock options and employee stock purchase plan 0.1 0.1 — 0.1 Time-vested restricted stock units 0.2 0.2 0.2 0.2 Market stock units 0.1 0.2 0.1 0.1 Dilutive potential common shares 0.4 0.5 0.3 0.4 Shares used in calculating diluted earnings per share 211.8 219.4 213.3 219.4

Share-Based Payments (Tables)

Share-Based Payments (Tables)	9 Months Ended
	Sep. 30, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-based compensation expense included in condensed consolidated statements of income	The following table summarizes share-based compensation expense included in our condensed consolidated statements of income: For the Three Months For the Nine Months (In millions) 2017 2016 2017 2016 Research and development $ 19.5 $ 22.7 $ 55.7 $ 65.6 Selling, general and administrative 23.6 31.2 71.9 94.6 Restructuring charges — — — (1.8 ) Subtotal 43.1 53.9 127.6 158.4 Capitalized share-based compensation costs (2.5 ) (3.2 ) (7.6 ) (10.7 ) Share-based compensation expense included in total cost and expenses 40.6 50.7 120.0 147.7 Income tax effect (10.9 ) (14.6 ) (31.8 ) (42.5 ) Share-based compensation expense included in net income attributable to Biogen Inc. $ 29.7 $ 36.1 $ 88.2 $ 105.2
Summary of share-based compensation expense associated with each of our share-based compensating programs	The following table summarizes share-based compensation expense associated with each of our share-based compensation programs: For the Three Months For the Nine Months (In millions) 2017 2016 2017 2016 Market stock units $ 4.5 $ 5.7 $ 17.1 $ 27.0 Time-vested restricted stock units 26.7 28.5 81.2 92.5 Cash settled performance units 7.0 7.9 13.0 12.7 Performance units 3.2 9.1 8.9 17.3 Employee stock purchase plan 1.7 2.7 7.4 8.9 Subtotal 43.1 53.9 127.6 158.4 Capitalized share-based compensation costs (2.5 ) (3.2 ) (7.6 ) (10.7 ) Share-based compensation expense included in total cost and expenses $ 40.6 $ 50.7 $ 120.0 $ 147.7

Income Taxes (Tables)

Income Taxes (Tables)	9 Months Ended
	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Reconciliation between the U.S. federal statutory tax rate and effective tax rate	A reconciliation between the U.S. federal statutory tax rate and our effective tax rate is summarized as follows: For the Three Months For the Nine Months 2017 2016 2017 2016 Statutory rate 35.0 % 35.0 % 35.0 % 35.0 % State taxes 0.7 1.1 0.6 1.0 Taxes on foreign earnings (11.4 ) (10.1 ) (11.4 ) (9.4 ) Credits and net operating loss utilization (0.7 ) (1.4 ) (0.8 ) (1.3 ) Purchased intangible assets 1.2 1.2 1.3 1.1 Manufacturing deduction (2.0 ) (1.8 ) (2.1 ) (1.7 ) Other permanent items 0.6 0.2 0.7 0.5 Other 0.4 0.5 0.6 0.4 Effective tax rate 23.8 % 24.7 % 23.9 % 25.6 %

Other Consolidated Financial 40

Other Consolidated Financial Statement Detail (Tables)	9 Months Ended
	Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Other income (expense), net	Components of other income (expense), net, are summarized as follows: For the Three Months For the Nine Months (In millions) 2017 2016 2017 2016 Interest income $ 20.6 $ 16.4 $ 54.2 $ 43.0 Interest expense (61.8 ) (66.0 ) (188.8 ) (195.2 ) Gain (loss) on investments, net (4.0 ) 0.8 (15.0 ) (0.3 ) Foreign exchange gains (losses), net 6.7 (4.9 ) 8.4 (2.6 ) Other, net (5.1 ) (4.4 ) (8.2 ) (14.3 ) Total other income (expense), net $ (43.6 ) $ (58.1 ) $ (149.4 ) $ (169.4 )
Accrued expenses and other	Accrued expenses and other consists of the following: (In millions) As of As of Current portion of contingent consideration obligations and milestones $ 611.5 $ 580.8 Revenue-related reserves for discounts and allowances 467.2 438.6 Employee compensation and benefits 241.9 282.9 Collaboration expenses 180.1 130.9 Royalties and licensing fees 190.8 195.8 Construction in progress 157.6 134.0 Accrued TECFIDERA litigation settlement and license charges — 454.8 Other 606.1 685.7 Total accrued expenses and other $ 2,455.2 $ 2,903.5

Summary of Significant Accoun41

Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Details)	9 Months Ended
	Sep. 30, 2017segment
Accounting Policies [Abstract]
Number of reportable segments	1
Interest in subsidiary (less than given percentage)	100.00%

Acquisitions (Details Textual)

Acquisitions (Details Textual) $ in Millions	3 Months Ended
	Jun. 30, 2017USD ($)
Remedy Pharmaceutical [Member]
Business Acquisition [Line Items]
Acquired in-process research and development	$ 120

Hemophilia Spin Off (Details)

Hemophilia Spin Off (Details) - Bioverativ $ in Millions	Feb. 01, 2017USD ($)
Assets and Liabilities Distributed to Bioverativ [Line Items]
Cash	$ 302.7
Accounts receivable	144.7
Inventory	116.1
Property, plant and equipment, net	20.2
Intangible assets, net	56.8
Goodwill	314.1
Other, net	53.7
Assets transferred, net	1,008.3
Accrued expenses and other current liabilities	87.8
Other long-term liabilities	67.7
Liabilities transferred, net	$ 155.5

Hemophilia Spin Off (Details Te

Hemophilia Spin Off (Details Textual) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Feb. 01, 2017
Assets and Liabilities Distributed to Bioverativ [Line Items]
Other	$ 48.8		$ 98.6	$ 180.4	$ 265.5
Cost of sales	370		416.9	1,120.8	1,100.2
Product, net	2,622.5		2,539.6	7,642.3	7,315
Hemophilia products
Assets and Liabilities Distributed to Bioverativ [Line Items]
Product, net			$ 217	74.4	$ 604.7
Bioverativ
Assets and Liabilities Distributed to Bioverativ [Line Items]
Term of Bioverativ transaction services agreement		12 months
Term of Bioverativ manufacturing and supply agreement		5 years
Cash						$ 302.7
Other	5			12.1
Cost of sales	$ 4.5			$ 11.1

Reserves for Discounts and Al45

Reserves for Discounts and Allowances (Details) $ in Millions	9 Months Ended
	Sep. 30, 2017USD ($)
Analysis of the amount of, and change in, reserves
Balance as of December 31, 2016	$ 605.5
Current provisions relating to sales in current year	2,123.2
Adjustments relating to prior years	2.4
Payments/credits relating to sales in current year	(1,547.3)
Payments/credits relating to sales in prior years	(519.2)
Balance as of September 30, 2017	664.6
Discounts
Analysis of the amount of, and change in, reserves
Balance as of December 31, 2016	71.6
Current provisions relating to sales in current year	422
Adjustments relating to prior years	(0.2)
Payments/credits relating to sales in current year	(320.1)
Payments/credits relating to sales in prior years	(70.4)
Balance as of September 30, 2017	102.9
Contractual Adjustments
Analysis of the amount of, and change in, reserves
Balance as of December 31, 2016	482.7
Current provisions relating to sales in current year	1,681.2
Adjustments relating to prior years	11.2
Payments/credits relating to sales in current year	(1,227.1)
Payments/credits relating to sales in prior years	(431.6)
Balance as of September 30, 2017	516.4
Returns
Analysis of the amount of, and change in, reserves
Balance as of December 31, 2016	51.2
Current provisions relating to sales in current year	20
Adjustments relating to prior years	(8.6)
Payments/credits relating to sales in current year	(0.1)
Payments/credits relating to sales in prior years	(17.2)
Balance as of September 30, 2017	$ 45.3

Reserves for Discounts and Al46

Reserves for Discounts and Allowances (Details 1) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Summary of total product revenue reserves included in consolidated balance sheets
Total reserves	$ 664.6	$ 605.5
Reduction of accounts receivable
Summary of total product revenue reserves included in consolidated balance sheets
Total reserves	197.4	166.9
Component of accrued expenses and other
Summary of total product revenue reserves included in consolidated balance sheets
Total reserves	$ 467.2	$ 438.6

Inventory (Details)

Inventory (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Components of inventories
Raw materials	$ 208.1	$ 170.4
Work in process	647.6	698.7
Finished goods	175.5	170.3
Total inventory	1,031.2	1,039.4
Inventory, current	1,007.2	1,001.6
Inventory, Noncurrent	$ 24	$ 37.8

Inventory Inventory (Details Te

Inventory Inventory (Details Textual) - Bioverativ $ in Millions	Feb. 01, 2017USD ($)
Inventory [Line Items]
Inventory	$ 116.1
Work-in-process
Inventory [Line Items]
Inventory	84.5
Finished goods
Inventory [Line Items]
Inventory	$ 31.6

Intangible Assets and Goodwil49

Intangible Assets and Goodwill (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Intangible assets
Amortization of acquired intangible assets	$ 108.9			$ 99.7	$ 674.9	$ 281.4
Total intangible assets, gross	8,295.8				8,295.8		$ 7,742.3
Accumulated Amortization	(4,276.4)				(4,276.4)		(3,934)
Intangible assets, net	4,019.4				4,019.4		3,808.3
Payment made to Forward Pharma			$ 1,250
TECFIDERA intellectual property charges							454.8
Intangible Assets and Goodwill (Textual) [Abstract]
Accumulated impairment losses related to goodwill	0				0
Expected future amortization expense, 2017 (remaining three months)	106.8				106.8
Expected future amortization expense, 2018	421.3				421.3
Expected future amortization expense, 2019	404				404
Expected future amortization expense, 2020	389.6				389.6
Expected future amortization expense, 2021	256.6				256.6
Expected future amortization expense, 2022	244.7				244.7
TECFIDERA
Intangible assets
Amortization of acquired intangible assets	30.4				413.4
Increase in acquired and in-licensed rights and patents			$ 795.2
Out-licensed patents
Intangible assets
Cost	543.3				543.3		543.3
Accumulated Amortization	(532.7)				(532.7)		(523.6)
Net	10.6				$ 10.6		19.7
Out-licensed patents | Minimum
Intangible assets
Estimated life, (in years)					13 years
Out-licensed patents | Maximum
Intangible assets
Estimated life, (in years)					23 years
Developed technology
Intangible assets
Cost	3,005.3				$ 3,005.3		3,005.3
Accumulated Amortization	(2,675.8)				(2,675.8)		(2,634.3)
Net	329.5				$ 329.5		371
Developed technology | Minimum
Intangible assets
Estimated life, (in years)					15 years
Developed technology | Maximum
Intangible assets
Estimated life, (in years)					23 years
Developed technology | AVONEX
Intangible assets
Net	322.5				$ 322.5
Acquired and in-licensed rights and patents
Intangible assets
Cost	4,002.6				4,002.6		3,481.7
Accumulated Amortization	(1,067.9)				(1,067.9)		(776.1)
Net	2,934.7				$ 2,934.7		2,705.6
Acquired and in-licensed rights and patents | Minimum
Intangible assets
Estimated life, (in years)					4 years
Acquired and in-licensed rights and patents | Maximum
Intangible assets
Estimated life, (in years)					18 years
Acquired and in-licensed rights and patents | SPINRAZA
Intangible assets
Increase in acquired and in-licensed rights and patents	40	$ 50			$ 150
Acquired and in-licensed rights and patents | Samsung Biosimilar Agreement [Member]
Intangible assets
Increase in acquired and in-licensed rights and patents	25
In Process Research and Development [Member]
Intangible assets
Indefinite lived intangible assets useful life					Indefinite until commercialization
Cost and Net	680.6				$ 680.6		648
Accumulated Amortization	0				$ 0		0
Trademarks and Trade Names [Member]
Intangible assets
Indefinite lived intangible assets useful life					Indefinite
Cost and Net	64				$ 64		64
Accumulated Amortization	0				0		$ 0
TYSABRI product | Acquired and in-licensed rights and patents
Intangible assets
Net	$ 2,293.9				$ 2,293.9

Intangible Assets and Goodwil50

Intangible Assets and Goodwill (Details 1) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Feb. 01, 2017
Goodwill [Line Items]
Increase to goodwill			$ 762.4
Income tax benefit (expense)	$ 383.8	$ 337	892.6	$ 1,047
Summary of roll forward of the changes in goodwill
Goodwill, beginning of period			3,669.3
Other			9.9
Goodwill, end of period	$ 4,127.5		4,127.5
TECFIDERA | Fumapharm AG
Goodwill [Line Items]
Increase to goodwill			900
Income tax benefit (expense)			$ 137.6
Bioverativ
Goodwill [Line Items]
Elimination of goodwill allocated to our hemophilia business					$ (314.1)

Fair Value Measurements (Detail

Fair Value Measurements (Details) - USD ($) $ in Millions	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Assets:
Marketable debt securities	$ 5,022.2		$ 5,398
Fair Value, Measurements, Recurring
Assets:
Cash equivalents	1,184.3		2,039.6
Marketable equity securities	11.8		24.9
Derivative contracts	1.6		61
Plan assets for deferred compensation	30.1		34.5
Total	6,250		7,558
Liabilities:
Derivative contracts	106.2		13.6
Contingent consideration obligations	522.1	$ 492.1	467.6	$ 521.6	$ 515.7	$ 506
Total	628.3		481.2
Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	2,584.7		2,663.8
Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	1,870		2,172.5
Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	567.5		561.7
Quoted Prices in Active Markets, Level 1 | Fair Value, Measurements, Recurring
Assets:
Cash equivalents	0		0
Marketable equity securities	11.8		24.9
Derivative contracts	0		0
Plan assets for deferred compensation	0		0
Total	11.8		24.9
Liabilities:
Derivative contracts	0		0
Contingent consideration obligations	0		0
Total	0		0
Quoted Prices in Active Markets, Level 1 | Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	0		0
Quoted Prices in Active Markets, Level 1 | Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	0		0
Quoted Prices in Active Markets, Level 1 | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	0		0
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Assets:
Cash equivalents	1,184.3		2,039.6
Marketable equity securities	0		0
Derivative contracts	1.6		61
Plan assets for deferred compensation	30.1		34.5
Total	6,238.2		7,533.1
Liabilities:
Derivative contracts	106.2		13.6
Contingent consideration obligations	0		0
Total	106.2		13.6
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	2,584.7		2,663.8
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	1,870		2,172.5
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	567.5		561.7
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Assets:
Cash equivalents	0		0
Marketable equity securities	0		0
Derivative contracts	0		0
Plan assets for deferred compensation	0		0
Total	0		0
Liabilities:
Derivative contracts	0		0
Contingent consideration obligations	522.1		467.6
Total	522.1		467.6
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	0		0
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	0		0
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	$ 0		$ 0

Fair Value Measurements (Deta52

Fair Value Measurements (Details 1) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Notes payable, carrying value	$ 6,494.7	$ 6,499.3
Debt instruments, fair value	7,048.3	6,808.4
Notes Payable to Fumedica
Debt Instrument [Line Items]
Notes payable, fair value	3.2	6.1
Notes payable, carrying value	3.3	6
6.875% Senior Notes due 2018
Debt Instrument [Line Items]
Notes payable, fair value	561.7	583.7
Notes payable, carrying value	553.1	558.5
2.900% Senior Notes due 2020
Debt Instrument [Line Items]
Notes payable, fair value	1,536.2	1,521.5
Notes payable, carrying value	1,486.5	1,485.3
3.625% Senior Notes due 2022
Debt Instrument [Line Items]
Notes payable, fair value	1,049.4	1,026.6
Notes payable, carrying value	994	993.2
4.050% Senior Notes due 2025
Debt Instrument [Line Items]
Notes payable, fair value	1,867	1,796
Notes payable, carrying value	1,735.9	1,734.8
5.200% Senior Notes due 2045
Debt Instrument [Line Items]
Notes payable, fair value	2,030.8	1,874.5
Notes payable, carrying value	$ 1,721.9	$ 1,721.5

Fair Value Measurements (Deta53

Fair Value Measurements (Details 2) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Business Acquisition, Contingent Consideration [Line Items]
Additions	$ 0	$ 0	$ 0	$ 0
Changes in fair value	30	5.9	61.2	18.8
Payments	0	0	(6.7)	(3.2)
Fair Value, Measurements, Recurring
Business Acquisition, Contingent Consideration [Line Items]
Fair value, beginning of period	492.1	515.7	467.6	506
Fair value, end of period	$ 522.1	$ 521.6	$ 522.1	$ 521.6

Fair Value Measurements (Deta54

Fair Value Measurements (Details Textual) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Business Acquisition, Contingent Consideration [Line Items]
Asset impairment charges	$ 0
Changes in valuation techniques	0
Contingent consideration obligations	$ 280	$ 246.8
6.875% Senior Notes due 2018
Business Acquisition, Contingent Consideration [Line Items]
Senior Notes interest rate	6.875%
2.900% Senior Notes due 2020
Business Acquisition, Contingent Consideration [Line Items]
Senior Notes interest rate	2.90%
3.625% Senior Notes due 2022
Business Acquisition, Contingent Consideration [Line Items]
Senior Notes interest rate	3.625%
4.050% Senior Notes due 2025
Business Acquisition, Contingent Consideration [Line Items]
Senior Notes interest rate	4.05%
5.200% Senior Notes due 2045
Business Acquisition, Contingent Consideration [Line Items]
Senior Notes interest rate	5.20%

Financial Instruments (Details)

Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$ 1,184.3	$ 2,039.6
Commercial paper
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	126.4	31
Overnight reverse repurchase agreements
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	42	0
Money market funds
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	871.1	741.7
Short-term debt securities
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$ 144.8	$ 1,266.9

Financial Instruments (Details

Financial Instruments (Details 1) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Marketable Debt and Equity Securities
Fair Value	$ 5,022.2	$ 5,398
Gross Unrealized Gains	4.1	2.9
Gross Unrealized Losses	(8.1)	(11.2)
Amortized Cost	5,026.2	5,406.3
Corporate debt securities Current
Marketable Debt and Equity Securities
Fair Value	1,042.9	1,408.6
Gross Unrealized Gains	0.2	0.2
Gross Unrealized Losses	(0.4)	(0.6)
Amortized Cost	1,043.1	1,409
Corporate debt securities Non-current
Marketable Debt and Equity Securities
Fair Value	1,541.8	1,255.2
Gross Unrealized Gains	2.2	1.2
Gross Unrealized Losses	(2.1)	(4.7)
Amortized Cost	1,541.7	1,258.7
Government securities Current
Marketable Debt and Equity Securities
Fair Value	916	1,156
Gross Unrealized Gains	0	0.2
Gross Unrealized Losses	(0.8)	(0.3)
Amortized Cost	916.8	1,156.1
Government securities Non-current
Marketable Debt and Equity Securities
Fair Value	954	1,016.5
Gross Unrealized Gains	0.4	0.5
Gross Unrealized Losses	(3.4)	(3.4)
Amortized Cost	957	1,019.4
Mortgage and other asset backed securities Current
Marketable Debt and Equity Securities
Fair Value	1.3	4
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Amortized Cost	1.3	4
Mortgage and other asset backed securities Non-current
Marketable Debt and Equity Securities
Fair Value	566.2	557.7
Gross Unrealized Gains	1.3	0.8
Gross Unrealized Losses	(1.4)	(2.2)
Amortized Cost	566.3	559.1
Marketable equity securities
Marketable Debt and Equity Securities
Fair Value	11.8	24.9
Gross Unrealized Gains	0	0.7
Gross Unrealized Losses	(2.6)	(9.3)
Amortized Cost	$ 14.4	$ 33.5

Financial Instruments (Detail57

Financial Instruments (Details 2) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Summary of Contractual Maturities: Available-for-Sale Securities
Due in one year or less, estimated fair value	$ 1,960.2	$ 2,568.6
Due in one year or less, amortized cost	1,961.2	2,569.1
Due after one year through five years, Estimated Fair Value	2,792.9	2,552.6
Due after one year through five years, Amortized Cost	2,795.7	2,559.7
Due after five years, estimated fair value	269.1	276.8
Due after five years, amortized cost	269.3	277.5
Total available-for-sale securities, Fair Value	5,022.2	5,398
Total available-for-sale securities, Amortized Cost	$ 5,026.2	$ 5,406.3

Financial Instruments (Detail58

Financial Instruments (Details 3) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Investments, Debt and Equity Securities [Abstract]
Proceeds from maturities and sales	$ 888.1	$ 2,362.2	$ 4,472.6	$ 5,185.8
Realized gains	0.3	1.1	2.7	2.1
Realized losses	$ (1.2)	$ (1.1)	$ (4.4)	$ (2.7)

Financial Instruments (Detail59

Financial Instruments (Details Textual)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Average maturity of marketable securities, months	17 months	12 months

Financial Instruments Financial

Financial Instruments Financial Instruments (Details Textual 2) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Strategic Investments
Business Acquisition [Line Items]
Strategic investment portfolio	$ 84.9	$ 99.9

Derivative Instruments (Details

Derivative Instruments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Derivative Instruments (Textual) [Abstract]
Gain/Loss on fair value of foreign currency forward contracts	$ 113		$ 113		$ (49.8)
Net gains (losses) of other income (expense) related to foreign currency forward contracts	(1.2)	$ 0.4	$ (5.7)	$ 16.6
Minimum
Derivative Instruments (Textual) [Abstract]
Range of durations of foreign currency forward contracts			1 month		1 month
Maximum
Derivative Instruments (Textual) [Abstract]
Range of durations of foreign currency forward contracts			21 months		18 months
Not designated as hedging instrument
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	366.2		$ 366.2		$ 902.1
Other current assets | Designated as hedging instrument
Summary of Derivatives designated as Hedging Instruments
Derivative asset, fair value, net					50.4
Other current assets | Not designated as hedging instrument
Summary of Derivatives designated as Hedging Instruments
Derivative asset, fair value, net	1.3		1.3		4
Investments and other assets | Designated as hedging instrument
Summary of Derivatives designated as Hedging Instruments
Derivative asset, fair value, net	0.3		0.3		6.6
Accrued expenses and other | Designated as hedging instrument
Summary of Derivatives designated as Hedging Instruments
Derivative liability, fair value, net	80.8		80.8
Accrued expenses and other | Not designated as hedging instrument
Summary of Derivatives designated as Hedging Instruments
Derivative liability, fair value, net	1.2		1.2		9
Other long-term liabilities | Designated as hedging instrument
Summary of Derivatives designated as Hedging Instruments
Derivative liability, fair value, net	24.2		24.2		4.6
Foreign exchange contract | Designated as hedging instrument
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	1,637.9		1,637.9		871.7
Interest rate swap | Designated as hedging instrument
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	675		675
Revenue | Foreign exchange contract
Derivative Instruments (Textual) [Abstract]
Net gains (losses) in product revenue for the settlement of certain effective cash flow hedge instruments	(18.8)	(5.2)	(15.1)	(0.7)
Operating expense | Foreign exchange contract
Derivative Instruments (Textual) [Abstract]
Net gains (losses) in product revenue for the settlement of certain effective cash flow hedge instruments	0.5	(0.2)	0.7	(0.4)
Other income (expense) | Cash flows, revenue | Foreign exchange contract
Derivative Instruments (Textual) [Abstract]
Net gains (losses) in earnings of Foreign currency forward contracts due to hedge ineffectiveness	0.7	(0.6)	6.7	3.4
Other income (expense) | Cash flows, operating expenses | Foreign exchange contract
Derivative Instruments (Textual) [Abstract]
Net gains (losses) in earnings of Foreign currency forward contracts due to hedge ineffectiveness	0.2	$ 0	(0.1)	$ (0.3)
Euro | Foreign exchange contract | Designated as hedging instrument
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	1,573.4		1,573.4		871.7
British pound | Foreign exchange contract | Designated as hedging instrument
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	39.7		39.7		0
Canadian dollar | Foreign exchange contract | Designated as hedging instrument
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	19.7		19.7		0
Swiss franc | Foreign exchange contract | Designated as hedging instrument
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	$ 5.1		$ 5.1		$ 0
2.900% Senior Notes due 2020
Derivative Instruments (Textual) [Abstract]
Senior Notes interest rate	2.90%		2.90%

Property, Plant and Equipment62

Property, Plant and Equipment Property, Plant and Equipment (Details) SFr in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2016CHF (SFr)	Mar. 31, 2016USD ($)	Sep. 30, 2017USD ($)	Dec. 31, 2016USD ($)
Property, Plant and Equipment [Line Items]
Accumulated depreciation			$ 1,580.6	$ 1,439.3
Solothurn, Switzerland
Property, Plant and Equipment [Line Items]
Construction in progress			968.8	$ 481.5
Contractual commitments for the construction of the facility			$ 235
Solothurn, Switzerland | Land
Property, Plant and Equipment [Line Items]
Land purchase	SFr 64.4	$ 62.5

Equity (Details)

Equity (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reconciliation of equity attributable to non-controlling interests
NCI, beginning of period	$ (11.6)	$ (0.1)	$ (11.5)	$ 2.1
Net income (loss) attributable to NCI, net of tax	0	(2.7)	(0.1)	(5.8)
Fair value of net assets and liabilities acquired and assigned to NCI	0	0	0	0.9
Translation adjustment and other	0	0.1	0	0.1
NCI, end of period	$ (11.6)	$ (2.7)	$ (11.6)	$ (2.7)

Equity (Details Textual)

Equity (Details Textual) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Mar. 31, 2011
Class of Stock [Line Items]
Payments for repurchase of common stock			$ 1,365.4	$ 348.9
Equity increase during the period			720.3
Net income attributable to Biogen Inc.	$ 1,226.1	$ 1,032.9	2,836.5	3,053.6
Adjustment to retained earnings reflecting spin-off			852.8
2016 Share Repurchase Program [Member]
Class of Stock [Line Items]
Authorized amount of share repurchases under the 2015 Share Repurchase Program	$ 3,000	$ 5,000	$ 3,000	$ 5,000
Repurchase of common stock, shares			3.7	1.1
Payments for repurchase of common stock			$ 1,000	$ 348.9
2011 Share Repurchase Program [Member]
Class of Stock [Line Items]
Common stock shares authorized for repurchased					20
Repurchase of common stock, shares			1.3
Payments for repurchase of common stock			$ 365.4

Accumulated Other Comprehensi65

Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated other comprehensive income [Line Items]
Balance, as of the beginning of the period			$ (319.9)	$ (224)
Other comprehensive income loss before reclassifications			(24.8)	(78.2)
Amounts reclassified from accumulated other comprehensive income loss			15.3	1.3
Net current period other comprehensive income loss	$ 9.4	$ (18.1)	(9.5)	(76.9)
Balance, as of the end of the period	(329.4)	(300.9)	(329.4)	(300.9)
Accumulated Net Unrealized Investment Gain (Loss)
Accumulated other comprehensive income [Line Items]
Balance, as of the beginning of the period			(10.8)	(0.8)
Other comprehensive income loss before reclassifications			5.5	1.2
Amounts reclassified from accumulated other comprehensive income loss			1.1	0.4
Net current period other comprehensive income loss			6.6	1.6
Balance, as of the end of the period	(4.2)	0.8	(4.2)	0.8
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated other comprehensive income [Line Items]
Balance, as of the beginning of the period			57.8	10.2
Other comprehensive income loss before reclassifications			(176.5)	(17.9)
Amounts reclassified from accumulated other comprehensive income loss			14.2	0.9
Net current period other comprehensive income loss			(162.3)	(17)
Balance, as of the end of the period	(104.5)	(6.8)	(104.5)	(6.8)
Accumulated Defined Benefit Plans Adjustment
Accumulated other comprehensive income [Line Items]
Balance, as of the beginning of the period			(32.7)	(37.8)
Other comprehensive income loss before reclassifications			(0.5)	1.3
Amounts reclassified from accumulated other comprehensive income loss			0	0
Net current period other comprehensive income loss			(0.5)	1.3
Balance, as of the end of the period	(33.2)	(36.5)	(33.2)	(36.5)
Accumulated Translation Adjustment
Accumulated other comprehensive income [Line Items]
Balance, as of the beginning of the period			(334.2)	(195.6)
Other comprehensive income loss before reclassifications			146.7	(62.8)
Amounts reclassified from accumulated other comprehensive income loss			0	0
Net current period other comprehensive income loss			146.7	(62.8)
Balance, as of the end of the period	$ (187.5)	$ (258.4)	$ (187.5)	$ (258.4)

Accumulated Other Comprehensi66

Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Details 1) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (expense), net	$ (43.6)	$ (58.1)	$ (149.4)	$ (169.4)
Income tax benefit (expense)	383.8	337	892.6	1,047
Product, net	(2,622.5)	(2,539.6)	(7,642.3)	(7,315)
Net income attributable to Biogen Inc.	1,226.1	1,032.9	2,836.5	3,053.6
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income attributable to Biogen Inc.	(18.8)	(5.3)	(15.3)	(1.3)
Accumulated Net Unrealized Investment Gain (Loss) | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (expense), net	(0.9)	0	(1.7)	(0.6)
Income tax benefit (expense)	0.3	0	0.6	0.2
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (expense), net	0.1	0.1	0.2	0.2
Income tax benefit (expense)	0	0	0	0
Product, net	(18.8)	(5.2)	(15.1)	(0.7)
Operating expenses	$ 0.5	$ 0.2	$ 0.7	$ 0.4

Earnings per Share (Details)

Earnings per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net income attributable to Biogen Inc	$ 1,226.1	$ 1,032.9	$ 2,836.5	$ 3,053.6
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	211.4	218.9	213	219
Effect of dilutive securities:
Dilutive potential common shares	0.4	0.5	0.3	0.4
Shares used in calculating diluted earnings per share	211.8	219.4	213.3	219.4
Stock options and employee stock purchase plan
Effect of dilutive securities:
Stock units	0.1	0.1	0	0.1
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	0.2	0.2	0.2	0.2
Market stock units
Effect of dilutive securities:
Stock units	0.1	0.2	0.1	0.1

Share-Based Payments (Details)

Share-Based Payments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Share-based Compensation Expense included in consolidated statements of income
Subtotal	$ 43.1	$ 53.9	$ 127.6	$ 158.4
Capitalized share-based compensation costs	(2.5)	(3.2)	(7.6)	(10.7)
Share-based compensation expense included in total costs and expenses	40.6	50.7	120	147.7
Income tax effect	(10.9)	(14.6)	(31.8)	(42.5)
Research and development
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	(19.5)	(22.7)	(55.7)	(65.6)
Selling, general and administrative
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	(23.6)	(31.2)	(71.9)	(94.6)
Restructuring charges
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	0	0	0	(1.8)
Total share-based compensation expense, net of tax
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	$ (29.7)	$ (36.1)	$ (88.2)	$ (105.2)

Share-Based Payments (Details 1

Share-Based Payments (Details 1) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2017
Summary of share based compensation expense associated with different programs
Subtotal	$ 43.1	$ 53.9	$ 127.6	$ 158.4
Capitalized share-based compensation costs	(2.5)	(3.2)	(7.6)	(10.7)
Share-based compensation expense included in total costs and expenses	40.6	50.7	120	147.7
Market stock units
Summary of share based compensation expense associated with different programs
Share-based compensation expense	4.5	5.7	17.1	27
Time-vested restricted stock units
Summary of share based compensation expense associated with different programs
Share-based compensation expense	26.7	28.5	81.2	92.5
Cash settled performance shares
Summary of share based compensation expense associated with different programs
Share-based compensation expense	7	7.9	13	12.7
Performance units
Summary of share based compensation expense associated with different programs
Share-based compensation expense	3.2	9.1	8.9	17.3
Employee stock purchase plan
Summary of share based compensation expense associated with different programs
Share-based compensation expense	$ 1.7	$ 2.7	$ 7.4	$ 8.9
2017 Omnibus Equity Plan
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Shares of common stock available for grant under the 2017 Omnibus Equity Plan					8

Income Taxes (Details)

Income Taxes (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reconciliation between the U.S. federal statutory tax rate and effective tax rate
Statutory rate	35.00%	35.00%	35.00%	35.00%
State taxes	0.70%	1.10%	0.60%	1.00%
Taxes on foreign earnings	(11.40%)	(10.10%)	(11.40%)	(9.40%)
Credits and net operating loss utilization	(0.70%)	(1.40%)	(0.80%)	(1.30%)
Purchased intangible assets	1.20%	1.20%	1.30%	1.10%
Manufacturing deduction	(2.00%)	(1.80%)	(2.10%)	(1.70%)
Permanent items	0.60%	0.20%	0.70%	0.50%
Other	0.40%	0.50%	0.60%	0.40%
Effective tax rate	23.80%	24.70%	23.90%	25.60%

Income Taxes (Details Textual)

Income Taxes (Details Textual) $ in Millions	Sep. 30, 2017USD ($)
Foreign tax authority
Income Tax Contingency [Line Items]
Income tax payment, including penalties and interest	$ 60

Other Consolidated Financial 72

Other Consolidated Financial Statement Detail (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Accrued Expenses and Other
Current portion of contingent consideration obligations and milestones	$ 611.5		$ 611.5		$ 580.8
Revenue-related reserves for discounts and allowances	664.6		664.6		605.5
Employee compensation and benefits	241.9		241.9		282.9
Collaboration expenses accrual	180.1		180.1		130.9
Royalties and licensing fees	190.8		190.8		195.8
Construction Payable	157.6		157.6		134
Accrued TECFIDERA litigation settlement and license charges	0		0		454.8
Other	606.1		606.1		685.7
Total accrued expenses and other	2,455.2		2,455.2		2,903.5
Other Income (Expense), Net
Interest income	20.6	$ 16.4	54.2	$ 43
Interest expense	(61.8)	(66)	(188.8)	(195.2)
Gain (loss) on sales of investments, net	(4)	0.8	(15)	(0.3)
Foreign exchange gains (losses), net	6.7	(4.9)	8.4	(2.6)
Other, net	(5.1)	(4.4)	(8.2)	(14.3)
Other income (expense), net	(43.6)	$ (58.1)	(149.4)	$ (169.4)
Component of accrued expenses and other
Accrued Expenses and Other
Revenue-related reserves for discounts and allowances	$ 467.2		$ 467.2		$ 438.6

Other Consolidated Financial 73

Other Consolidated Financial Statement Detail Other Consolidated Financial Statement (Details Textual) € in Millions, $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Mar. 31, 2017EUR (€)	Sep. 30, 2016USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Dec. 31, 2016USD ($)
Product Liability Contingency [Line Items]
Prepaid taxes	$ 714.4				$ 714.4		$ 817
Product, net	$ 2,622.5			$ 2,539.6	$ 7,642.3	$ 7,315
Italy
Product Liability Contingency [Line Items]
Product, net		$ 45	€ 41.8

Investments in Variable Inter74

Investments in Variable Interest Entities (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Variable Interest Entity [Line Items]
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities	$ 49.7	$ 47.4
Neurimmune
Variable Interest Entity [Line Items]
Research and development cost in support of collaboration agreement	100.00%

Collaborative and Other Relat75

Collaborative and Other Relationships Collaborative and Other Relationships - Collaborations (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development	$ 446.4		$ 529	$ 1,666	$ 1,439.4
ZINBRYTA related assets	23,073.2			23,073.2		$ 22,876.8
Other	48.8		98.6	180.4	265.5
Collaboration profit (loss) sharing	35.2		4.7	82.5	(0.9)
Product, net	2,622.5		2,539.6	7,642.3	7,315
ZINBRYTA
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
ZINBRYTA related assets	200			200
Unrecorded tax benefit	100			100
Bristol-Myers Squibb
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development				300
Additional milestone payments	410			410
iPerian
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments	490			490
Developmental Milestone Payment	60			$ 60
AbbVie
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Biogen share of co-promotion profits or losses				50.00%
AbbVie | U.S.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Other	2.8			$ 12.6	13.5
AbbVie | Rest of world
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration profit (loss) sharing	0.7			2	2.7
Ionis Pharmaceuticals [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License Fee			75
Ionis Pharmaceuticals [Member] | SPINRAZA
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Direct Operating Cost, Royalty Expense	34			64.9
Ionis Pharmaceuticals [Member] | SPINRAZA | U.S.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Product, net	197.6			438.8
Ionis Pharmaceuticals [Member] | SPINRAZA | Rest of world
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Product, net			$ 73.3		$ 82.4
Acquired and in-licensed rights and patents | SPINRAZA
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Increase in acquired and in-licensed rights and patents	$ 40	$ 50		$ 150
Minimum | Ionis Pharmaceuticals [Member] | SPINRAZA
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty payments per the collaboration				11.00%
Maximum | Ionis Pharmaceuticals [Member] | SPINRAZA
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty payments per the collaboration				15.00%

Collaborative and Other Relat76

Collaborative and Other Relationships Collaborative and Other Relationships - Equity Method Investments (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Feb. 29, 2012	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of Equity Method Investments [Line Items]
Collaboration profit (loss) sharing		$ 35.2	$ 4.7	$ 82.5	$ (0.9)
Revenue related to technical development and technology transfer services		$ 48.8	98.6	$ 180.4	265.5
Samsung Biosimilar Agreement [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of stake in entity minimum		5.40%		5.40%
Percentage of stake in entity maximum	49.90%
Expected profit share with Samsung Bioepis				50.00%
Collaboration profit (loss) sharing		$ 34.5	7.4	$ 80.5	1.8
Revenue related to technical development and technology transfer services		$ 8.8	$ 0.4	$ 23.7	$ 17.3

Subsequent Events - Eisai (Deta

Subsequent Events - Eisai (Details) - Subsequent Event - Eisai	1 Months Ended		9 Months Ended
	Jan. 31, 2019	Oct. 31, 2017	Dec. 31, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of future development costs related to Eisai	45.00%	100.00%	15.00%
U.S.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Biogen share of co-promotion profits or losses		55.00%
E.U.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Biogen share of co-promotion profits or losses		68.50%
JAPAN
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Biogen share of co-promotion profits or losses		20.00%

Subsequent Events - Neurimmune

Subsequent Events - Neurimmune (Details) - Subsequent Event - Neurimmune $ in Millions	1 Months Ended
	Oct. 31, 2017USD ($)
Noncontrolling Interest [Line Items]
Payments to Neurimmune	$ 150
Reduction in royalty rate payable on commercial sales	15.00%
Additional reduction in royalty rate payable on commercial sales	5.00%
Additional potential payment to Neurimmune	$ 50