Biogen (BIIB) 10-Q 24 Jul 19 2019 Q2 Quarterly report Financial data

Cover Page

Cover Page - shares	6 Months Ended
	Jun. 30, 2019	Jul. 19, 2019
Cover page.
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2019
Document Transition Report	false
Entity File Number	0-19311
Entity Registrant Name	BIOGEN INC.
Entity Central Index Key	0000875045
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Amendment Flag	false
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	33-0112644
Entity Address, Address Line One	225 Binney Street
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02142
City Area Code	617
Local Phone Number	679-2000
Title of 12(b) Security	Common Stock, $0.0005 par value
Trading Symbol	BIIB
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		184,447,218

Condensed Consolidated Statemen

Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Total revenues	$ 3,616.7	$ 3,356.5	$ 7,106.5	$ 6,487.6
Cost and expenses:
Cost of sales, excluding amortization and impairment of acquired intangible assets	476.3	421	1,078.3	867
Research and development	484.8	981	1,048.5	1,477.7
Selling, general and administrative	587.6	516.2	1,155.3	1,017.5
Amortization and impairment of acquired intangible assets	70.1	107.4	138.3	211.3
Collaboration profit (loss) sharing	63.5	39.2	121.6	81.7
Loss on assets and liabilities held for sale	(2.3)	0	113.2	0
(Gain) loss on fair value remeasurement of contingent consideration	(20)	1.9	(8.5)	(3.7)
Restructuring charges	0.8	1.6	1.2	3.2
Acquired in-process research and development	0	75	0	85
Total cost and expenses	1,660.8	2,143.3	3,647.9	3,739.7
Income from operations	1,955.9	1,213.2	3,458.6	2,747.9
Other income (expense), net	(197.4)	(34.5)	159.9	(75.5)
Income before income tax expense and equity in loss of investee, net of tax	1,758.5	1,178.7	3,618.5	2,672.4
Income tax expense	248.1	263.7	670.6	586.2
Equity in loss of investee, net of tax	16.3	0	45	0
Net income	1,494.1	915	2,902.9	2,086.2
Net income (loss) attributable to noncontrolling interests, net of tax	0	48.4	0	46.7
Net income attributable to Biogen Inc.	$ 1,494.1	$ 866.6	$ 2,902.9	$ 2,039.5
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$ 7.85	$ 4.18	$ 15.01	$ 9.75
Diluted earnings per share attributable to Biogen Inc.	$ 7.85	$ 4.18	$ 14.99	$ 9.73
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	190.3	207.1	193.4	209.2
Diluted earnings per share attributable to Biogen Inc.	190.4	207.3	193.7	209.5
Product, net
Total revenues	$ 2,880.3	$ 2,757.5	$ 5,560.3	$ 5,281
Revenues from anti-CD20 therapeutic programs
Total revenues	576.4	490.4	1,093.8	933.6
Other
Total revenues	$ 160	$ 108.6	$ 452.4	$ 273

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Inc.	$ 1,494.1	$ 866.6	$ 2,902.9	$ 2,039.5
Other comprehensive income:
Unrealized gains (losses) on securities available for sale, net of tax	3.3	0.6	10.2	(1.6)
Unrealized gains (losses) on cash flow hedges, net of tax	(37.9)	132.8	(21)	103.8
Gains (losses) on net investment hedges	11.7	0	25.7	0
Unrealized gains (losses) on pension benefit obligation, net of tax	0.1	0.9	0.7	0.4
Currency translation adjustment	(10.3)	(92)	(28.1)	(47.3)
Total other comprehensive income (loss), net of tax	(33.1)	42.3	(12.5)	55.3
Comprehensive income attributable to Biogen Inc.	1,461	908.9	2,890.4	2,094.8
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	(0.4)	48.4	(0.4)	46.7
Comprehensive income	$ 1,460.6	$ 957.3	$ 2,890	$ 2,141.5

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 1,723.4	$ 1,224.6
Marketable securities	1,228.8	2,313.4
Accounts receivable, net	1,959.6	1,958.5
Due from anti-CD20 therapeutic programs	557.5	526.9
Inventory	776.7	929.9
Assets held for sale	683.4	0
Other current assets	980.4	687.6
Total current assets	7,909.8	7,640.9
Marketable securities	1,309.3	1,375.9
Property, plant and equipment, net	3,077.5	3,601.2
Operating lease assets	434.4	0
Intangible assets, net	3,681.3	3,120
Goodwill	5,749.2	5,706.4
Deferred tax asset	2,820.1	2,153.9
Investments and other assets	1,306	1,690.6
Total assets	26,287.6	25,288.9
Current liabilities:
Taxes payable	427.2	63.5
Accounts payable	377.1	370.5
Liabilities held for sale	88.6	0
Accrued expenses and other	2,318	2,861.2
Total current liabilities	3,210.9	3,295.2
Notes payable	5,948.5	5,936.5
Deferred tax liability	2,400.6	1,636.2
Long-term operating lease liabilities	423	0
Other long-term liabilities	1,355.8	1,389.4
Total liabilities	13,338.8	12,257.3
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	0	0
Accumulated other comprehensive loss	(252.9)	(240.4)
Retained earnings	16,182.8	16,257
Treasury stock, at cost	(2,977.1)	(2,977.1)
Total Biogen Inc. shareholders’ equity	12,952.9	13,039.6
Noncontrolling interests	(4.1)	(8)
Total equity	12,948.8	13,031.6
Total liabilities and equity	$ 26,287.6	$ 25,288.9

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Jun. 30, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Common stock, par value	$ 0.0005	$ 0.0005

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities:
Net income	$ 2,902.9	$ 2,086.2
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation, amortization and impairments	238.1	340.4
Acquired in-process research and development	0	85
Share-based compensation	98	81.7
Contingent consideration	8.5	3.7
Loss on assets and liabilities held for sale	113.2	0
Deferred income taxes	71.6	(57.4)
Unrealized (gain) loss on strategic investments	199.2	(1)
Other	91.1	45.1
Changes in operating assets and liabilities, net:
Accounts receivable	(2.9)	(187.2)
Inventory	(108.7)	40.1
Accrued expenses and other current liabilities	216.1	(13.3)
Income tax assets and liabilities	306.9	183.4
Other changes in operating assets and liabilities, net	(80.3)	8.7
Net cash flows provided by operating activities	3,423.5	2,556.4
Cash flows from investing activities:
Proceeds from sales and maturities of marketable securities	3,255.8	6,802.7
Purchases of marketable securities	(2,075.1)	(4,774.3)
Contingent consideration paid related to Fumapharm AG acquisition	(300)	(900)
Acquisition of Nightstar Therapeutics plc, net of cash acquired	(744.4)	0
Purchases of property, plant and equipment	(314)	(381.5)
Acquired in-process research and development	0	(85)
Acquisitions of intangible assets	0	(3)
Purchase of Ionis Pharmaceuticals, Inc. stock	0	(462.9)
Proceeds from sales of strategic investments	309.7	0
Other	4	(2.1)
Net cash flows provided by investing activities	128	198.1
Cash flows from financing activities:
Purchases of treasury stock	(3,057.3)	(3,000)
Payments related to issuance of stock for share-based compensation arrangements, net	(23.6)	(14.3)
Net distribution to noncontrolling interest	4.3	(38.9)
Other	21.6	(6.9)
Net cash flows used in financing activities	(3,055)	(3,060.1)
Net increase (decrease) in cash and cash equivalents	496.5	(305.6)
Effect of exchange rate changes on cash and cash equivalents	(2.3)	18
Cash and cash equivalents, beginning of the period	1,224.6	1,573.8
Cash and cash equivalents, end of the period	$ 1,723.4	$ 1,250.2

Condensed Consolidated Statem_4

Condensed Consolidated Statement of Equity Statement - USD ($) shares in Millions, $ in Millions	Total	Preferred stock	Common stock	Additional paid-in capital	Accumulated Other Comprehensive Income	Retained earnings	Treasury stock	Noncontrolling interest	Parent	2019 Share Repurchase Program	2019 Share Repurchase ProgramCommon stock	2019 Share Repurchase ProgramAdditional paid-in capital	2019 Share Repurchase ProgramRetained earnings	2019 Share Repurchase ProgramTreasury stock	2018 Share Repurchase Program	2018 Share Repurchase ProgramCommon stock	2018 Share Repurchase ProgramAdditional paid-in capital	2018 Share Repurchase ProgramRetained earnings	2018 Share Repurchase ProgramTreasury stock	2016 Share Repurchase Program	2016 Share Repurchase ProgramCommon stock	2016 Share Repurchase ProgramAdditional paid-in capital	2016 Share Repurchase ProgramRetained earnings	2016 Share Repurchase ProgramTreasury stock	Accounting Standards Update 2016-01Accumulated Other Comprehensive Income	Accounting Standards Update 2016-16Retained earnings
Beginning balance at Dec. 31, 2017	$ 12,598.1	$ 0	$ 0.1	$ 97.8	$ (318.4)	$ 15,810.4	$ (2,977.1)
Beginning balance, shares at Dec. 31, 2017		0	(211.5)				(23.8)
Beginning balance at Dec. 31, 2017	12,612.8
Beginning balance at Dec. 31, 2017								$ (14.7)
Net income attributable to Biogen Inc.	2,039.5					2,039.5
Net income (loss) attributable to noncontrolling interest, net of tax	46.7							46.7
Net income	2,086.2
Other comprehensive income (loss), net of tax	55.3				55.3
Other comprehensive income (loss), net of tax	55							(0.3)
Noncontrolling interest, increase (decrease) other	11.1							11.1	$ 0
Noncontrolling interest, decrease from distributions to noncontrolling interest holders	(50)							50	0
Repurchase of common stock, at cost																				$ (3,000)				$ (3,000)
Repurchase of common stock, at cost, shares																				(10.5)				(10.5)
Retirement of common stock pursuant to Share Repurchase Progams, at cost																				$ 0	$ 0	$ (171.2)	$ (2,828.8)	$ (3,000)
Retirement of common stock pursuant to Share Repurchase Programs, at cost, shares																					(10.5)			(10.5)
Issuance of common stock under stock option and stock purchase plans	24.6		$ 0	24.6
Issuance of common stock under stock option and stock purchase plans, shares			0.1
Issuance of common stock under award plan			0.3
Issuance of common stock under stock award plan	(38.8)		$ 0	(38.8)
Compensation related to share-based payments	87.6			87.6
Adoption of new accounting guidance	479.9																								$ 1.5	$ 478.4
Ending balance at Jun. 30, 2018	12,253.7	$ 0	$ 0.1	0	(261.6)	15,499.5	$ (2,977.1)
Ending balance at Jun. 30, 2018	12,260.9
Ending balance at Jun. 30, 2018								(7.2)
Ending balance, shares at Jun. 30, 2018		0	(201.4)				(23.8)
Beginning balance at Mar. 31, 2018	14,037.5	$ 0	$ 0.1	0	(303.9)	17,334.6	$ (2,977.1)
Beginning balance, shares at Mar. 31, 2018		0	(211)				(23.8)
Beginning balance at Mar. 31, 2018	14,053.7
Beginning balance at Mar. 31, 2018								(16.2)
Net income attributable to Biogen Inc.	866.6					866.6
Net income (loss) attributable to noncontrolling interest, net of tax	48.4							48.4
Net income	915
Other comprehensive income (loss), net of tax	42.3				42.3
Other comprehensive income (loss), net of tax	41.8							(0.5)
Noncontrolling interest, increase (decrease) other	11.1							11.1	0
Noncontrolling interest, decrease from distributions to noncontrolling interest holders	(50)							50	0
Repurchase of common stock, at cost																				$ (2,750)				$ (2,750)
Repurchase of common stock, at cost, shares																				(9.6)				(9.6)
Retirement of common stock pursuant to Share Repurchase Progams, at cost																				$ 0	$ 0	$ (48.3)	$ (2,701.7)	$ (2,750)
Retirement of common stock pursuant to Share Repurchase Programs, at cost, shares																					(9.6)			(9.6)
Issuance of common stock under stock option and stock purchase plans	8.4		$ 0	8.4
Issuance of common stock under stock option and stock purchase plans, shares			0
Issuance of common stock under award plan			0
Issuance of common stock under stock award plan	(1.5)		$ 0	(1.5)
Compensation related to share-based payments	41.4			41.4
Ending balance at Jun. 30, 2018	12,253.7	$ 0	$ 0.1	0	(261.6)	15,499.5	$ (2,977.1)
Ending balance at Jun. 30, 2018	12,260.9
Ending balance at Jun. 30, 2018								(7.2)
Ending balance, shares at Jun. 30, 2018		0	(201.4)				(23.8)
Beginning balance at Dec. 31, 2018	13,031.6	$ 0	$ 0.1	0	(240.4)	16,257	$ (2,977.1)
Beginning balance, shares at Dec. 31, 2018		0	(221)				(23.8)
Beginning balance at Dec. 31, 2018	13,039.6
Beginning balance at Dec. 31, 2018	(8)							(8)
Net income attributable to Biogen Inc.	2,902.9					2,902.9
Net income (loss) attributable to noncontrolling interest, net of tax	0							0
Net income	2,902.9
Other comprehensive income (loss), net of tax	(12.5)				(12.5)
Other comprehensive income (loss), net of tax	(12.9)							(0.4)
Noncontrolling interest, increase (decrease) other	4.3							4.3	0
Repurchase of common stock, at cost										$ (909.9)				$ (909.9)	$ (2,147.4)				$ (2,147.4)
Repurchase of common stock, at cost, shares										(3.9)				(3.9)	(8.9)				(8.9)
Retirement of common stock pursuant to Share Repurchase Progams, at cost										$ 0	$ 0	$ (19.7)	$ (890.2)	$ (909.9)	$ 0	$ 0	$ (110.5)	$ (2,036.9)	$ (2,147.4)
Retirement of common stock pursuant to Share Repurchase Programs, at cost, shares											(3.9)			(3.9)		(8.9)			(8.9)
Issuance of common stock under stock option and stock purchase plans	26.2		$ 0	26.2
Issuance of common stock under stock option and stock purchase plans, shares			0.1
Issuance of common stock under award plan			0.3
Issuance of common stock under stock award plan	(50)		$ 0	0		(50)
Compensation related to share-based payments	104			104
Ending balance at Jun. 30, 2019	12,948.8	$ 0	$ 0.1	0	(252.9)	16,182.8	$ (2,977.1)
Ending balance at Jun. 30, 2019	12,952.9
Ending balance at Jun. 30, 2019	(4.1)							(4.1)
Ending balance, shares at Jun. 30, 2019		0	(208.6)				(23.8)
Beginning balance at Mar. 31, 2019	13,822	$ 0	$ 0.1	0	(219.8)	17,026.7	$ (2,977.1)
Beginning balance, shares at Mar. 31, 2019		0	(219)				(23.8)
Beginning balance at Mar. 31, 2019	13,829.9
Beginning balance at Mar. 31, 2019								(7.9)
Net income attributable to Biogen Inc.	1,494.1					1,494.1
Net income (loss) attributable to noncontrolling interest, net of tax	0							0
Net income	1,494.1
Other comprehensive income (loss), net of tax	(33.1)				(33.1)
Other comprehensive income (loss), net of tax	(33.5)							(0.4)
Noncontrolling interest, increase (decrease) other	4.2							4.2	$ 0
Repurchase of common stock, at cost										(909.9)				$ (909.9)	$ (1,491.6)				$ (1,491.6)
Repurchase of common stock, at cost, shares														(3.9)	(6.5)				(6.5)
Retirement of common stock pursuant to Share Repurchase Progams, at cost										$ 0	$ 0	$ (19.7)	$ (890.2)	$ (909.9)	$ 0	$ 0	$ (44.9)	$ (1,446.7)	$ (1,491.6)
Retirement of common stock pursuant to Share Repurchase Programs, at cost, shares											(3.9)			(3.9)		(6.5)			(6.5)
Issuance of common stock under stock option and stock purchase plans	9.6		$ 0	9.6
Issuance of common stock under stock option and stock purchase plans, shares			0
Issuance of common stock under award plan			0
Issuance of common stock under stock award plan	(1.1)		$ 0	0		(1.1)
Compensation related to share-based payments	55			55
Ending balance at Jun. 30, 2019	12,948.8	$ 0	$ 0.1	$ 0	$ (252.9)	$ 16,182.8	$ (2,977.1)
Ending balance at Jun. 30, 2019	12,952.9
Ending balance at Jun. 30, 2019	$ (4.1)							$ (4.1)
Ending balance, shares at Jun. 30, 2019		0	(208.6)				(23.8)

Summary of Significant Accounti

Summary of Significant Accounting Policies	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Summary of significant accounting policies	Summary of Significant Accounting Policies References in these notes to "Biogen," the "company," "we," "us" and "our" refer to Biogen Inc. and its consolidated subsidiaries. Business Overview Biogen is a global biopharmaceutical company focused on discovering, developing and delivering worldwide innovative therapies for people living with serious neurological and neurodegenerative diseases as well as related therapeutic adjacencies. Our core growth areas include multiple sclerosis (MS) and neuroimmunology, neuromuscular disorders, including spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS), movement disorders, including Parkinson's disease and progressive supranuclear palsy, Alzheimer's disease (AD) and dementia and ophthalmology. We are also focused on discovering, developing and delivering worldwide innovative therapies in our emerging growth areas of immunology, neurocognitive disorders, acute neurology and pain. In addition, we commercialize biosimilars of advanced biologics. We support our drug discovery and development efforts through the commitment of significant resources to discovery, research and development programs and business development opportunities. Our marketed products include TECFIDERA, AVONEX, PLEGRIDY, TYSABRI and FAMPYRA for the treatment of MS, SPINRAZA for the treatment of 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, RITUXAN HYCELA for the treatment of non-Hodgkin's lymphoma and CLL, GAZYVA for the treatment of CLL and follicular lymphoma, OCREVUS for the treatment of primary progressive MS (PPMS) and relapsing MS (RMS) and other potential anti-CD20 therapies pursuant to our collaboration arrangements with Genentech, Inc. (Genentech), a wholly-owned member of the Roche Group. For additional information on our collaboration arrangements with Genentech, please read Note 19, Collaborative and Other Relationships , to our consolidated financial statements included in our Annual Report on Form 10-K for the year ended December 31, 2018 ( 2018 Form 10-K). Our innovative drug development and commercialization activities are complemented by our biosimilar products that expand access to medicines and reduce the cost burden for healthcare systems. Through Samsung Bioepis Co., Ltd. (Samsung Bioepis), our joint venture with Samsung BioLogics Co., Ltd. (Samsung BioLogics), we market and sell BENEPALI, an etanercept biosimilar referencing ENBREL, IMRALDI, an adalimumab biosimilar referencing HUMIRA, and FLIXABI, an infliximab biosimilar referencing REMICADE, in the European Union (E.U.). For additional information on our collaboration arrangement with Samsung Bioepis, please read Note 17, Collaborative and Other Relationships , to these unaudited condensed consolidated financial statements (condensed consolidated financial statements). Basis of Presentation In the opinion of management, our 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 audited consolidated financial statements and the accompanying notes included in our 2018 Form 10-K. Our accounting policies are described in the “ Notes to Consolidated Financial Statements ” in our 2018 Form 10-K and updated, as necessary, in this report. The year-end condensed consolidated balance sheet data presented for comparative purposes was derived from our audited financial statements but does not include all disclosures required by U.S. GAAP. The results of operations for the three and six months ended June 30, 2019 , 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 and delivering worldwide innovative therapies for people living with serious neurological and neurodegenerative diseases as well as related therapeutic adjacencies. 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 are 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. Assets and Liabilities Held For Sale Upon determining that a long-lived asset or disposal group meets the criteria to be classified as held for sale, we cease depreciation and separately present such assets and liabilities of the disposal group in our condensed consolidated balance sheet. We initially measure a long-lived asset or disposal group that is classified as held for sale at the lower of its carrying value or fair value less any costs to sell. Any loss resulting from this measurement is recognized in the period in which the held for sale criteria are met. Conversely, gains are not recognized on the sale of a long-lived asset or disposal group until the date of sale. We assess the fair value of a long-lived asset or disposal group less any costs to sell each reporting period it remains classified as held for sale and recognize any subsequent changes as an adjustment to the carrying value of the asset or disposal group, as long as the remeasured carrying value does not exceed the carrying value less costs to sell of the asset or disposal group at the time it was initially classified as held for sale. 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 below, we do not believe that the adoption of recently issued standards have or may have a material impact on our condensed consolidated financial statements or disclosures. Leases In February 2016 the FASB issued Accounting Standards Update (ASU) No. 2016-02, Leases (Topic 842) , a new standard issued to increase transparency and comparability among organizations related to their leasing activities. This standard established a right-of-use model that requires all lessees to recognize right-of-use assets and lease liabilities on their balance sheet that arise from leases as well as provide disclosures with respect to certain qualitative and quantitative information related to a company's leasing arrangements to meet the objective of allowing users of financial statements to assess the amount, timing and uncertainty of cash flows arising from leases. The FASB subsequently issued the following amendments to ASU 2016-02 that have the same effective date and transition date: ASU No. 2018-01, Leases (Topic 842): Land Easement Practical Expedient for Transition to Topic 842 , ASU No. 2018-10, Codification Improvements to Topic 842, Leases , ASU No. 2018-11, Leases (Topic 842): Targeted Improvements , ASU No. 2018-20, Narrow-Scope Improvement for Lessors , and ASU No. 2019-01, Leases (Topic 842): Codification Improvements . We adopted these amendments with ASU 2016-02 (collectively, the new leasing standards) effective January 1, 2019. We adopted the new leasing standards using the modified retrospective transition approach, as of January 1, 2019, with no restatement of prior periods or cumulative adjustment to retained earnings. Upon adoption, we elected the package of transition practical expedients, which allowed us to carry forward prior conclusions related to whether any expired or existing contracts are or contain leases, the lease classification for any expired or existing leases and initial direct costs for existing leases. We also elected the practical expedient to not reassess certain land easements and made an accounting policy election to not recognize leases with an initial term of 12 months or less within our condensed consolidated balance sheets and to recognize those lease payments on a straight-line basis in our condensed consolidated statements of income over the lease term. Upon adoption of the new leasing standards we recognized an operating lease asset of approximately $463.0 million and a corresponding operating lease liability of approximately $526.0 million , which are included in our condensed consolidated balance sheet. The adoption of the new leasing standards did not have an impact on our condensed consolidated statements of income. We determine if an arrangement is a lease at contract inception. Operating lease assets represent our right to use an underlying asset for the lease term and operating lease liabilities represent our obligation to make lease payments arising from the lease. Operating lease assets and liabilities are recognized at the commencement date of the lease based upon the present value of lease payments over the lease term. When determining the lease term, we include options to extend or terminate the lease when it is reasonably certain that we will exercise that option. We use the implicit rate when readily determinable and use our incremental borrowing rate when the implicit rate is not readily determinable based upon the information available at the commencement date in determining the present value of the lease payments. Our incremental borrowing rate is determined using a secured borrowing rate for the same currency and term as the associated lease. The lease payments used to determine our operating lease assets may include lease incentives, stated rent increases and escalation clauses linked to rates of inflation when determinable and are recognized in our operating lease assets in our condensed consolidated balance sheets. In addition, our contracts contain lease and non-lease components. We separate lease payments for the identified assets from any non-lease payments included in the contract. For certain equipment leases, such as vehicles, we apply a portfolio approach to effectively account for the operating lease assets and liabilities. Our operating leases are reflected in operating lease assets, accrued expenses and other and in long-term operating lease liabilities in our condensed consolidated balance sheets. Lease expense for minimum lease payments is recognized on a straight-line basis over the lease term. We also have real estate lease agreements which are subleased to third parties. Operating leases for which we are the sublessor are included in accrued expenses and other and other long-term liabilities in our condensed consolidated balance sheets. We recognize sublease income on a straight-line basis over the lease term in our condensed consolidated statements of income. For additional information on the adoption of the new leasing standards, please read Note 11, Leases , to these condensed consolidated financial statements. Credit Losses In June 2016 the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments . The FASB has subsequently issued amendments to ASU 2016-13, which have the same effective date and transition date of January 1, 2020. These standards require that credit losses be reported using an expected losses model rather than the incurred losses model that is currently used, and establishes additional disclosures related to credit risks. For available-for-sale debt securities with unrealized losses, these standards now require allowances to be recorded instead of reducing the amortized cost of the investment. These standards limit the amount of credit losses to be recognized for available-for-sale debt securities to the amount by which carrying value exceeds fair value and requires the reversal of previously recognized credit losses if fair value increases. Based on the composition of our investment portfolio and other financial assets, current market conditions and historical credit loss activity, the adoption of these standards is not expected to have a material impact on our consolidated financial position and results of operations and related disclosures. Debt Securities 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 . This standard amends the amortization period for certain purchased callable debt securities held at a premium by shortening the amortization period to the earliest call date. This standard became effective for us on January 1, 2019, and was adopted using a modified retrospective transition approach. The adoption of this standard did not result in a significant adjustment to our marketable debt securities. Fair Value Measurements In August 2018 the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement . This standard modifies certain disclosure requirements on fair value measurements. This standard will become effective for us on January 1, 2020. We do not expect that the adoption of this standard will have a material impact on our disclosures. Derivative Instruments and Hedging Activities In October 2018 the FASB issued ASU No. 2018-16, Derivatives and Hedging (Topic 815): Inclusion of the Secured Overnight Financing Rate (SOFR) Overnight Index Swap (OIS) Rate as a Benchmark Interest Rate for Hedge Accounting Purposes . This standard permits the use of the OIS rate based on the SOFR as a United States (U.S.) benchmark interest rate for hedge accounting purposes under Accounting Standards Codification (ASC) 815, Derivative and Hedging . This standard became effective for us on January 1, 2019, and did not have an impact on our condensed consolidated results of operations or financial position. Collaborative Arrangements In November 2018 the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 . This standard makes targeted improvements for collaborative arrangements as follows: • Clarifies that certain transactions between collaborative arrangement participants should be accounted for as revenue under ASC 606, Revenue from Contracts with Customers , when the collaborative arrangement participant is a customer in the context of a unit of account. In those situations, all the guidance in ASC 606 should be applied, including recognition, measurement, presentation and disclosure requirements; • Adds unit-of-account guidance to ASC 808, Collaborative Arrangements , to align with the guidance in ASC 606 (that is, a distinct good or service) when an entity is assessing whether the collaborative arrangement or a part of the arrangement is within the scope of ASC 606; and • Precludes a company from presenting transactions with collaborative arrangement participants that are not directly related to sales to third parties with revenue recognized under ASC 606 if the collaborative arrangement participant is not a customer. This standard will become effective for us on January 1, 2020; however, early adoption is permitted. A retrospective transition approach is required for either all contracts or only for contracts that are not completed at the date of initial application of ASC 606, with a cumulative adjustment to opening retained earnings, as of January 1, 2018. We are currently evaluating the potential impact that this standard may have on our consolidated financial position, results of operations and related disclosures.

Acquisitions

Acquisitions	6 Months Ended
	Jun. 30, 2019
Business Combinations [Abstract]
Acquisitions	Acquisitions Acquisition of Nightstar Therapeutics plc On June 7, 2019, we completed our acquisition of all of the outstanding shares of Nightstar Therapeutics plc (NST), a clinical-stage gene therapy company focused on adeno-associated virus treatments for inherited retinal disorders. As a result of this acquisition, we added two mid-to late-stage clinical assets, as well as preclinical programs, in ophthalmology. These assets include BIIB111 (formerly known as NSR-REP1), which is in Phase 3 development for the potential treatment of choroideremia, a rare, degenerative, X-linked inherited retinal disorder that leads to blindness and has no approved treatments, and BIIB112 (formerly known as NSR-RPGR), which is in Phase 2/3 development for the potential treatment of X-linked retinitis pigmentosa, which is a rare inherited retinal disease with no approved treatments. Under the terms of this acquisition, we paid NST shareholders $25.50 in cash for each issued and outstanding NST share, which totaled $847.6 million . In addition, we paid $4.6 million in cash for equity compensation, which is attributable to pre-combination services and is reflected as a component of the total purchase price paid. The fair value of equity compensation attributable to the post-combination service period was $26.2 million , of which $18.4 million was recognized as a charge to selling, general and administrative expense with the remaining $7.8 million as a charge to research and development expense in our condensed consolidated statements of income. These amounts were associated with the accelerated vesting of stock options previously granted to NST employees and were fully paid in cash as of June 30, 2019. We funded this acquisition through available cash and accounted for it as an acquisition of a business. The following table summarizes the estimated fair values of the separately identifiable assets acquired and liabilities assumed as of June 7, 2019: (In millions) Cash and cash equivalents $ 107.8 Marketable securities 7.5 In-process research and development intangible assets 700.0 Goodwill 112.6 Deferred tax liability (77.0 ) Other, net 1.3 Total purchase price $ 852.2 Our estimate of the fair value of the in-process research and development (IPR&D) programs acquired was determined through a probability adjusted discounted cash flow analysis utilizing a discount rate of 12.5% . This valuation was primarily driven by the value associated with BIIB111. The fair value associated with BIIB111 was $480.0 million . We have recorded an additional IPR&D asset related to BIIB112 of $220.0 million . Some of the more significant assumptions utilized in our asset valuations included the estimated net cash flows for each year for each asset or product, including net revenues, cost of sales, research and development and other operating expenses, the potential regulatory and commercial success risks, competitive trends impacting the asset and each cash flow stream as well as other factors. These fair value measurements were based on significant inputs not observable in the market and thus represent Level 3 fair value measurements. We have recognized goodwill in relation to the fair value associated with NST workforce's expertise and early research in retinal disorders. We also recognized goodwill in relation to the establishment of a deferred tax liability for the acquired IPR&D intangible assets, which have no tax basis. This deferred tax liability is net of the related impacts on the deferred taxes for global intangible low-taxed income (GILTI). Goodwill that is tax deductible for GILTI purposes is approximately $35.0 million . Pro forma results of operations as a result of this acquisition have not been presented as this acquisition is not material to our condensed consolidated statements of income. Subsequent to the acquisition date, our results of operations include the results of operations of NST. Our preliminary estimate of the fair value of the specifically identifiable assets acquired and liabilities assumed as of the date of acquisition is subject to the finalization of management’s analysis related to certain matters, such as finalizing our assessment of the IPR&D programs acquired and preparing and submitting certain income tax and non-income tax filings and returns. The final determination of these fair values will be completed as additional information becomes available but no later than one year from the acquisition date. Although the final determination may result in asset and liability fair values that are different than the preliminary estimates of these amounts included herein, it is not expected that those differences will be material to our financial position.

Divestitures Divestitures

Divestitures Divestitures	6 Months Ended
	Jun. 30, 2019
Divestitures [Abstract]
Divestitures	Divestitures Proposed Divestiture of Hillerød, Denmark Manufacturing Operations In March 2019 we entered into a share purchase agreement with FUJIFILM Corporation (FUJIFILM) under which FUJIFILM will acquire all of the outstanding shares of our subsidiary that owns our biologics manufacturing operations in Hillerød, Denmark. Upon closing of the proposed transaction, we expect to receive up to $890.0 million in cash, subject to certain working capital adjustments and other contractual terms. As part of the proposed transaction, we have provided FUJIFILM with certain minimum batch production commitment guarantees. There is a risk that the minimum contractual batch production commitments will not be met. Based upon current estimates we expect to incur an adverse commitment obligation of approximately $120.0 million associated with such guarantees. We may adjust this estimate based upon changes in business conditions, which may result in the recognition of additional losses. We are also obligated to indemnify FUJIFILM for liabilities that may exist relating to certain business activities incurred prior to the closing of the proposed transaction. In addition, we may earn certain contingent payments based on future manufacturing activities at the Hillerød facility. For the disposition of a business, our policy is to recognize contingent consideration when the consideration is realizable. We currently believe the probability of earning these payments is remote and therefore we have not included these contingent payments in our estimate of the fair value of the operations. As part of the proposed transaction, we also expect to enter into certain manufacturing services agreements with FUJIFILM pursuant to which FUJIFILM would use the Hillerød facility to produce commercial products for us, such as TYSABRI, as well as other third-party products. We determined that the operations to be disposed of in the proposed transaction did not meet the criteria to be classified as discontinued operations under the applicable guidance. In February 2019 the assets and liabilities related to our Hillerød, Denmark manufacturing operations met the criteria to be classified as held for sale. The following table presents information related to the carrying value of the major classes of assets and liabilities that were reclassified as held for sale in our condensed consolidated balance sheets: (In millions) As of June 30, 2019 Assets: Inventory $ 20.6 Property, plant and equipment, net 641.9 Operating lease assets 2.2 Goodwill 69.5 Other assets 62.4 Valuation allowance on disposal group on assets held for sale (113.2 ) Assets held for sale $ 683.4 Liabilities: Accrued expenses and other liabilities $ 33.4 Long-term operating lease liabilities 1.4 Deferred tax liability 53.8 Liabilities held for sale $ 88.6 For the six months ended June 30, 2019 , we recorded a loss of approximately $174.5 million in our condensed consolidated statements of income. This estimated loss includes a pre-tax loss of $113.2 million , which reflects a $2.3 million decrease to our original estimate as of March 31, 2019, reflecting our current estimated fair value of the assets and liabilities held for sale, adjusting for our expected costs to sell our Hillerød, Denmark manufacturing operations of approximately $10.0 million and our estimate of the fair value of an adverse commitment of approximately $120.0 million associated with the guarantee of future minimum batch production at the Hillerød facility. The value of this adverse commitment was determined using a probability-weighted estimate of future manufacturing activity. In addition, we recorded a tax expense of $61.3 million related to the proposed transaction. Our total estimated loss is based on current exchange rates and business conditions, and any changes to these factors through the closing date of the transaction will result in adjustments to the carrying values of the related assets and liabilities as well as a corresponding adjustment to the loss amount recognized on the sale. Following the closing of the proposed transaction, the final purchase price will be adjusted by an amount equal to the difference between our current estimates of working capital and inventory balances that will be transferred to FUJIFILM and the amounts that are ultimately transferred. In addition, upon closing of the proposed transaction, we expect to separately sell certain raw materials remaining at the Hillerød facility to FUJIFILM at carrying value. Our estimate of the fair value of assets and liabilities expected to be sold to FUJIFILM is a Level 3 measurement and is based on the expected consideration from the sale, including the valuation of the adverse commitment, as discussed above. The proposed transaction remains subject to certain closing conditions. We expect to complete the proposed transaction in the third quarter of 2019.

Revenues Revenues

Revenues Revenues	6 Months Ended
	Jun. 30, 2019
Revenue from Contract with Customer [Abstract]
Revenues	Revenues Product Revenues Revenues by product are summarized as follows: For the Three Months (In millions) 2019 2018 United States Rest of World Total United States Rest of World Total Multiple Sclerosis (MS): TECFIDERA $ 869.8 $ 280.4 $ 1,150.2 $ 825.8 $ 261.0 $ 1,086.8 Interferon* 379.7 174.7 554.4 444.7 180.8 625.5 TYSABRI 264.3 211.0 475.3 265.5 201.7 467.2 FAMPYRA — 24.1 24.1 — 23.0 23.0 Subtotal: MS product revenues 1,513.8 690.2 2,204.0 1,536.0 666.5 2,202.5 Spinal Muscular Atrophy: SPINRAZA 230.6 257.6 488.2 205.9 216.8 422.7 Biosimilars: BENEPALI — 120.3 120.3 — 115.6 115.6 IMRALDI — 47.3 47.3 — — — FLIXABI — 16.8 16.8 — 11.2 11.2 Subtotal: Biosimilar product revenues — 184.4 184.4 — 126.8 126.8 Other: FUMADERM — 3.7 3.7 — 5.5 5.5 Total product revenues $ 1,744.4 $ 1,135.9 $ 2,880.3 $ 1,741.9 $ 1,015.6 $ 2,757.5 *Interferon includes AVONEX and PLEGRIDY. For the Six Months (In millions) 2019 2018 United States Rest of World Total United States Rest of World Total Multiple Sclerosis (MS): TECFIDERA $ 1,587.5 $ 561.5 $ 2,149.0 $ 1,554.7 $ 519.0 $ 2,073.7 Interferon* 707.0 348.3 1,055.3 816.0 359.8 1,175.8 TYSABRI 509.3 426.4 935.7 515.2 414.1 929.3 FAMPYRA — 47.0 47.0 — 47.4 47.4 ZINBRYTA — — — — 1.4 1.4 Subtotal: MS product revenues 2,803.8 1,383.2 4,187.0 2,885.9 1,341.7 4,227.6 Spinal Muscular Atrophy: SPINRAZA 453.9 552.8 1,006.7 393.9 392.7 786.6 Biosimilars: BENEPALI — 244.3 244.3 — 236.5 236.5 IMRALDI — 83.0 83.0 — — — FLIXABI — 31.5 31.5 — 17.8 17.8 Subtotal: Biosimilar product revenues — 358.8 358.8 — 254.3 254.3 Other: FUMADERM — 7.8 7.8 — 12.5 12.5 Total product revenues $ 3,257.7 $ 2,302.6 $ 5,560.3 $ 3,279.8 $ 2,001.2 $ 5,281.0 *Interferon includes AVONEX and PLEGRIDY. We recognized revenues from two wholesalers accounting for 30.5% and 18.2% of gross product revenues for the three months ended June 30, 2019 , and 30.9% and 16.3% of gross product revenues for the six months ended June 30, 2019 . We recognized revenues from two wholesalers accounting for 32.5% and 18.6% of gross product revenues for the three months ended June 30, 2018 , and 33.2% and 17.3% of gross product revenues for the six months ended June 30, 2018 . 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, 2018 $ 127.8 $ 888.8 $ 34.7 $ 1,051.3 Current provisions relating to sales in current year 308.8 1,391.0 10.6 1,710.4 Adjustments relating to prior years (0.4 ) (41.9 ) (0.1 ) (42.4 ) Payments/credits relating to sales in current year (189.6 ) (827.6 ) (0.6 ) (1,017.8 ) Payments/credits relating to sales in prior years (121.9 ) (505.2 ) (11.9 ) (639.0 ) Balance, as of June 30, 2019 $ 124.7 $ 905.1 $ 32.7 $ 1,062.5 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, net $ 199.0 $ 176.6 Component of accrued expenses and other 863.5 874.7 Total revenue-related reserves $ 1,062.5 $ 1,051.3 Revenues from Anti-CD20 Therapeutic Programs Revenues from anti-CD20 therapeutic programs are summarized as follows: For the Three Months For the Six Months (In millions) 2019 2018 2019 2018 Biogen’s share of pre-tax profits in the U.S. for RITUXAN, RITUXAN HYCELA and GAZYVA $ 377.2 $ 359.0 $ 768.0 $ 708.6 Other revenues from anti-CD20 therapeutic programs 199.2 131.4 325.8 225.0 Total revenues from anti-CD20 therapeutic programs $ 576.4 $ 490.4 $ 1,093.8 $ 933.6 For additional information on our collaboration arrangements with Genentech, please read Note 19, Collaborative and Other Relationships, to our consolidated financial statements included in our 2018 Form 10-K. Other Revenues Other revenues are summarized as follows: For the Three Months For the Six Months (In millions) 2019 2018 2019 2018 Revenues from collaborative and other relationships: (Loss) profit earned under our 50% share of the co-promotion losses on ZINBRYTA in the U.S. with AbbVie $ (0.1 ) $ (2.5 ) $ (0.5 ) $ (7.2 ) Revenues earned under our technical development agreement, manufacturing services agreements and royalty revenues on biosimilar products with Samsung Bioepis 52.2 14.7 77.0 32.6 Other royalty and corporate revenues: Royalty 2.7 17.3 6.6 27.9 Other corporate 105.2 79.1 369.3 219.7 Total other revenues $ 160.0 $ 108.6 $ 452.4 $ 273.0 Other corporate revenues primarily reflect amounts earned under contract manufacturing agreements with our strategic partners, including Bioverativ Inc. (Bioverativ). During the three and six months ended June 30, 2019 , we recognized $34.5 million and $241.3 million , respectively, in revenues under the manufacturing and supply agreement with Bioverativ entered into in connection with the spin-off of our hemophilia business, compared to $47.6 million and $94.6 million , respectively, in the prior year comparative periods. For additional information on our collaboration arrangement with Samsung Bioepis, please read Note 17, Collaborative and Other Relationships, to these condensed consolidated financial statements. For additional information on our collaboration arrangement with AbbVie Inc., please read Note 19, Collaborative and Other Relationships, to our consolidated financial statements included in our 2018 Form 10-K. For additional information on our manufacturing and supply agreement with Bioverativ, please read Note 3, Hemophilia Spin-Off , to our consolidated financial statements included in our 2018 Form 10-K.

Inventory

Inventory	6 Months Ended
	Jun. 30, 2019
Inventory Disclosure [Abstract]
Inventory	Inventory The components of inventory are summarized as follows: (In millions) As of As of Raw materials $ 205.0 $ 196.3 Work in process 427.6 606.7 Finished goods 144.1 133.5 Total inventory $ 776.7 $ 936.5 Balance Sheet Classification: Inventory $ 776.7 $ 929.9 Investments and other assets — 6.6 Total inventory $ 776.7 $ 936.5 In the first quarter of 2019 we sold to Bioverativ most of the remaining hemophilia-related inventory on hand with a cost basis totaling $173.5 million pursuant to the terms of the manufacturing and supply agreement with Bioverativ entered into in connection with the spin-off of our hemophilia business. Long-term inventory, which primarily consists of work in process, is included in investments and other assets in our condensed consolidated balance sheets. Proposed Divestiture of Hillerød, Denmark Manufacturing Operations In March 2019 we entered into a share purchase agreement with FUJIFILM under which FUJIFILM will acquire all of the outstanding shares of our subsidiary that owns our biologics manufacturing operations in Hillerød, Denmark. Upon closing of the proposed transaction, we expect to receive up to $890.0 million in cash, subject to certain working capital adjustments and other contractual terms. As a result, $20.6 million of work in process inventory was reclassified to assets held for sale in our condensed consolidated balance sheets as of June 30, 2019 . Following the closing of the proposed transaction, the final purchase price will be adjusted by an amount equal to the difference between our current estimates of working capital and inventory balances that will be transferred to FUJIFILM and the amounts that are ultimately transferred. In addition, upon closing of the proposed transaction, we expect to separately sell certain raw materials remaining at the Hillerød facility to FUJIFILM at carrying value. For additional information on the proposed divestiture of our Hillerød, Denmark manufacturing operations, please read Note 3, Divestitures , to these condensed consolidated financial statements.

Intangible Assets and Goodwill

Intangible Assets and Goodwill	6 Months Ended
	Jun. 30, 2019
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 June 30, 2019 As of December 31, 2018 (In millions) Estimated Life Cost Accumulated Amortization Net Cost Accumulated Amortization Net Out-licensed patents 13-23 years $ 543.3 $ (542.4 ) $ 0.9 $ 543.3 $ (542.3 ) $ 1.0 Developed technology 15-23 years 3,005.3 (2,754.1 ) 251.2 3,005.3 (2,734.8 ) 270.5 In-process research and development Indefinite until commercialization 1,175.6 — 1,175.6 476.0 — 476.0 Trademarks and tradenames Indefinite 64.0 — 64.0 64.0 — 64.0 Acquired and in-licensed rights and patents 4-18 years 3,638.7 (1,449.1 ) 2,189.6 3,638.7 (1,330.2 ) 2,308.5 Total intangible assets $ 8,426.9 $ (4,745.6 ) $ 3,681.3 $ 7,727.3 $ (4,607.3 ) $ 3,120.0 For the three and six months ended June 30, 2019 , amortization and impairment of acquired intangible assets totaled $70.1 million and $138.3 million , respectively, compared to $107.4 million and $211.3 million , respectively, in the prior year comparative periods. The decrease in amortization and impairment of acquired intangible assets was primarily due to a net overall decrease in our expected rate of amortization for acquired intangible assets. This decrease was primarily due to lower amortization subsequent to the impairment in the fourth quarter of 2018 of the U.S. license to Forward Pharma A/S' (Forward Pharma) intellectual property, including Forward Pharma's intellectual property related to TECFIDERA, and higher expected lifetime revenues of TYSABRI. For the three and six months ended June 30, 2019 and 2018 , we had no impairment charges. 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 June 30, 2019 , was $246.6 million . IPR&D In connection with our acquisition of NST on June 7, 2019, we acquired IPR&D programs with an estimated fair value of $700.0 million . For additional information on our acquisition of NST, please read Note 2, Acquisitions , to these condensed consolidated financial statements. 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 Pharma International Ltd., an affiliate of Elan Corporation plc. Acquired and in-licensed rights and patents also includes our rest of world license to Forward Pharma's intellectual property, including Forward Pharma's intellectual property related to TECFIDERA, and other amounts related to our other marketed products and other programs acquired through business combinations. The net book value of the TYSABRI asset as of June 30, 2019 , was $1,933.3 million and the net book value of the TECFIDERA asset as of June 30, 2019 , was $55.2 million . For additional information on our TECFIDERA license rights, please read Note 7, Intangible Assets and Goodwill , to our consolidated financial statements included in our 2018 Form 10-K. Estimated Future Amortization of Intangible Assets Our amortization expense is based on the economic consumption and impairment of intangible assets. Our most significant intangible assets are related to our TYSABRI, AVONEX, SPINRAZA and TECFIDERA products and other programs acquired through business combinations. Annually, during our long-range planning cycle, we perform an analysis of anticipated lifetime revenues of our TYSABRI, AVONEX, SPINRAZA and TECFIDERA products. This analysis is also updated whenever events or changes in circumstances would significantly affect the anticipated lifetime revenues of any of these products. Impairments are recorded in the period in which they are incurred. Our most recent long-range planning cycle was completed in the second quarter of 2019. Based upon this most recent analysis, the estimated future amortization of acquired intangible assets for the next five years is expected to be as follows: (In millions) As of 2019 (remaining six months) $ 130.0 2020 255.0 2021 215.0 2022 215.0 2023 220.0 2024 210.0 Goodwill The following table provides a roll forward of the changes in our goodwill balance: (In millions) As of Goodwill, beginning of period $ 5,706.4 Increase to goodwill 112.6 Reclassification of goodwill to assets held for sale (69.5 ) Other (0.3 ) Goodwill, end of period $ 5,749.2 The increase in goodwill during the six months ended June 30, 2019 , was related to our acquisition of NST. For additional information on our acquisition of NST, please read Note 2, Acquisitions , to these condensed consolidated financial statements. The reclassification of goodwill to assets held for sale relates to an allocation based upon the relative fair value of the proposed divestiture of our Hillerød, Denmark manufacturing operations. In connection with our Hillerød, Denmark manufacturing operations meeting the criteria to be classified as held for sale due to the proposed divestiture, goodwill was reviewed for impairment, and based upon this review, no impairments were recognized. As of June 30, 2019 , we had no accumulated impairment losses related to goodwill. For additional information on the proposed divestiture of our Hillerød, Denmark manufacturing operations, please read Note 3, Divestitures , to these condensed consolidated financial statements.

Fair Value Measurements

Fair Value Measurements	6 Months Ended
	Jun. 30, 2019
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 June 30, 2019 (In millions) Total Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets: Cash equivalents $ 1,120.4 $ — $ 1,120.4 $ — Marketable debt securities: Corporate debt securities 1,687.7 — 1,687.7 — Government securities 604.4 — 604.4 — Mortgage and other asset backed securities 246.0 — 246.0 — Marketable equity securities 510.0 176.6 333.4 — Derivative contracts 88.2 — 88.2 — Plan assets for deferred compensation 29.5 — 29.5 — Total $ 4,286.2 $ 176.6 $ 4,109.6 $ — Liabilities: Derivative contracts $ 17.5 $ — $ 17.5 $ — Contingent consideration obligations 401.3 — — 401.3 Total $ 418.8 $ — $ 17.5 $ 401.3 As of December 31, 2018 (In millions) Total Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets: Cash equivalents $ 705.5 $ — $ 705.5 $ — Marketable debt securities: Corporate debt securities 2,459.2 — 2,459.2 — Government securities 969.6 — 969.6 — Mortgage and other asset backed securities 260.5 — 260.5 — Marketable equity securities 615.4 51.7 563.7 — Derivative contracts 66.9 — 66.9 — Plan assets for deferred compensation 25.4 — 25.4 — Total $ 5,102.5 $ 51.7 $ 5,050.8 $ — Liabilities: Derivative contracts $ 24.6 $ — $ 24.6 $ — Contingent consideration obligations 409.8 — — 409.8 Total $ 434.4 $ — $ 24.6 $ 409.8 There have been no impairments of our assets measured and carried at fair value during the three and six months ended June 30, 2019 . In addition, there were no changes in valuation techniques or inputs utilized or transfers between fair value measurement levels during the three and six months ended June 30, 2019 . The fair value of Level 2 instruments classified as cash equivalents, marketable debt securities and our marketable equity security investment in Ionis Pharmaceuticals, Inc. (Ionis) were determined through third-party pricing services or an option pricing valuation model. For additional information on our agreement with Ionis, please read Note 19, Collaborative and Other Relationships, to our consolidated financial statements included in our 2018 Form 10-K. For a description of our validation procedures related to prices provided by third-party pricing services and our option pricing valuation model, please read Note 1, Summary of Significant Accounting Policies - Fair Value Measurements, to our consolidated financial statements included in our 2018 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 June 30, 2019 As of December 31, 2018 (In millions) Fair Value Carrying Value Fair Value Carrying Value 2.900% Senior Notes due September 15, 2020 $ 1,507.5 $ 1,491.1 $ 1,489.5 $ 1,480.8 3.625% Senior Notes due September 15, 2022 1,032.3 996.0 1,000.4 995.5 4.050% Senior Notes due September 15, 2025 1,867.4 1,738.7 1,745.1 1,737.8 5.200% Senior Notes due September 15, 2045 1,953.2 1,722.7 1,802.6 1,722.4 Total $ 6,360.4 $ 5,948.5 $ 6,037.6 $ 5,936.5 The fair values of each of our series of Senior Notes were determined through market, observable and corroborated sources. For additional information on our debt instruments, please read Note 12, Indebtedness, to our consolidated financial statements included in our 2018 Form 10-K. Contingent Consideration Obligations In connection with our acquisitions of Convergence Pharmaceuticals Ltd., Stromedix Inc. and Biogen International Neuroscience GmbH in 2015, 2012 and 2010, respectively, we agreed to make additional payments based upon the achievement of certain milestone events. The following table provides a roll forward of the fair values of our contingent consideration obligations, which includes Level 3 measurements: For the Three Months For the Six Months (In millions) 2019 2018 2019 2018 Fair value, beginning of period $ 421.3 $ 498.0 $ 409.8 $ 523.6 Changes in fair value (20.0 ) 1.9 (8.5 ) (3.7 ) Payments — — — (20.0 ) Fair value, end of period $ 401.3 $ 499.9 $ 401.3 $ 499.9 As of June 30, 2019 and December 31, 2018 , $255.1 million and $265.0 million , respectively, of the fair value of our total contingent consideration obligations was reflected as a component of other long-term liabilities in our condensed consolidated balance sheets with the remaining balance reflected as a component of accrued expenses and other. For the three and six months ended June 30, 2019 , changes in the fair value of our contingent consideration obligations were primarily due to changes in the probability and expected timing of achieving certain development milestones, partially offset by a decrease in interest rates used to revalue our contingent consideration liabilities and the passage of time. For the three and six months ended June 30, 2018 , changes in the fair value of our contingent consideration obligations were primarily due to an increase in interest rates used to revalue our contingent consideration liabilities, the passage of time and a milestone payment.

Financial Instruments

Financial Instruments	6 Months Ended
	Jun. 30, 2019
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 in our condensed consolidated balance sheets: (In millions) As of As of Commercial paper $ 338.7 $ 231.2 Overnight reverse repurchase agreements 97.0 — Money market funds 405.7 279.5 Short-term debt securities 279.0 194.8 Total $ 1,120.4 $ 705.5 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. Our marketable equity securities gains (losses) are recorded in other income (expense), net in our condensed consolidated statements of income. The following tables summarize our marketable debt and equity securities: As of June 30, 2019 (In millions) Amortized Gross Unrealized Gains Gross Unrealized Losses Fair Value Corporate debt securities Current $ 933.4 $ 1.1 $ — $ 934.5 Non-current 748.9 4.4 (0.1 ) 753.2 Government securities Current 293.6 0.4 — 294.0 Non-current 309.7 0.9 (0.2 ) 310.4 Mortgage and other asset backed securities Current 0.3 — — 0.3 Non-current 244.5 1.5 (0.3 ) 245.7 Total marketable debt securities $ 2,530.4 $ 8.3 $ (0.6 ) $ 2,538.1 Marketable equity securities Current $ 117.9 $ 46.3 $ — $ 164.2 Non-current 111.4 247.9 (13.5 ) 345.8 Total marketable equity securities $ 229.3 $ 294.2 $ (13.5 ) $ 510.0 As of December 31, 2018 (In millions) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Corporate debt securities Current $ 1,608.4 $ — $ (0.9 ) $ 1,607.5 Non-current 854.9 0.7 (3.9 ) 851.7 Government securities Current 706.1 0.1 (0.4 ) 705.8 Non-current 264.0 0.1 (0.3 ) 263.8 Mortgage and other asset backed securities Current 0.1 — — 0.1 Non-current 260.5 0.4 (0.5 ) 260.4 Total marketable debt securities $ 3,694.0 $ 1.3 $ (6.0 ) $ 3,689.3 Marketable equity securities, non-current $ 496.2 $ 127.7 $ (8.5 ) $ 615.4 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 June 30, 2019 As of December 31, 2018 (In millions) Amortized Estimated Amortized Estimated Fair Value Due in one year or less $ 1,227.3 $ 1,228.8 $ 2,314.6 $ 2,313.4 Due after one year through five years 1,196.1 1,202.2 1,235.9 1,232.7 Due after five years 107.0 107.1 143.5 143.2 Total available-for-sale securities $ 2,530.4 $ 2,538.1 $ 3,694.0 $ 3,689.3 The average maturity of our marketable debt securities available-for-sale as of June 30, 2019 and December 31, 2018 , were approximately 14 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 Six Months (In millions) 2019 2018 2019 2018 Proceeds from maturities and sales $ 1,766.6 $ 2,733.7 $ 3,255.8 $ 6,802.7 Realized gains $ 1.0 $ 0.8 $ 1.6 $ 2.6 Realized losses $ (0.3 ) $ (0.8 ) $ (0.6 ) $ (10.2 ) Strategic Investments As of June 30, 2019 , our strategic investment portfolio was comprised of investments totaling $569.8 million , of which $164.2 million was reflected as a component of other current assets in our condensed consolidated balance sheet, with the remaining balance included in investments and other assets. As of December 31, 2018 , our strategic investment portfolio was comprised of investments totaling $676.3 million , which is included in investments and other assets in our condensed consolidated balance sheet. Our strategic investment portfolio includes investments in equity securities of certain biotechnology companies, which are reflected within our disclosures included in Note 7, Fair Value Measurements , to these condensed consolidated financial statements, venture capital funds where the underlying investments are in equity securities of certain biotechnology companies and non-marketable equity securities. Our investments in equity securities include shares of Ionis common stock acquired in June 2018. This investment is classified as a Level 2 marketable security due to certain holding period restrictions and is remeasured each reporting period and carried at fair value. The effect of the holding period restrictions on our Ionis stock valuation are estimated using an option pricing valuation model. The most significant assumptions within the model are the term of the restrictions and the stock price volatility, which is based upon historical volatility of similar companies. We also use a constant maturity risk-free interest rate to match the remaining term of the restrictions on our investment in Ionis common stock and a dividend yield of zero based upon the fact that Ionis and similar companies generally have not historically granted cash dividends. The remainder of our investments in equity securities of certain publicly-traded biotechnology companies are regularly measured and carried at fair value and classified as Level 1. The decrease in our strategic investment portfolio for the six months ended June 30, 2019, primarily reflects the sale of a portion of our investment in Ionis common stock for approximately $213.3 million as well as the sale of our investment in a non-marketable equity security, partially offset by an increase in the fair value of our remaining investment in Ionis common stock. For additional information on our June 2018 investment in Ionis common stock, please read Note 19, Collaborative and Other Relationships, to our consolidated financial statements included in our 2018 Form 10-K.

Derivative Instruments

Derivative Instruments	6 Months Ended
	Jun. 30, 2019
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 June 30, 2019 and December 31, 2018 , had durations of 1 to 18 months and 1 to 12 months , respectively. These contracts have been designated as cash flow hedges and unrealized gains or losses on the portion of these foreign currency forward contracts that are included in the effectiveness test are reported in accumulated other comprehensive income (loss) (referred to as AOCI in the tables below). Realized gains and losses of such contracts are recognized in revenues 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. We recognize all cash flow hedge reclassifications from accumulated other comprehensive income and fair value changes of excluded portions in the same line item in our condensed consolidated statements of income that has been impacted by the hedged item. 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 (In millions) As of As of Euro $ 2,111.3 $ 1,701.4 British pound 117.2 215.3 Swiss franc 69.7 131.4 Japanese yen 56.0 98.8 Canadian dollar 48.1 92.2 Total foreign currency forward contracts $ 2,402.3 $ 2,239.1 The pre-tax portion of the fair value of these foreign currency forward contracts that were included in accumulated other comprehensive income (loss) in total equity reflected net gains of $6.3 million and $27.3 million as of June 30, 2019 and December 31, 2018 , respectively. We expect the net gains of $6.3 million to be settled over the next 18 months , of which $9.8 million of these gains are expected to be settled over the next 12 months, with any amounts in accumulated other comprehensive income (loss) to be reported as an adjustment to revenues or operating expenses. 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 June 30, 2019 and December 31, 2018 , 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 in our condensed consolidated statements of income: For the Three Months Ended June 30, Net Gains/(Losses) Reclassified from AOCI into Operating Income (in millions) Net Gains/(Losses) Recognized in Operating Income (in millions) Location 2019 2018 Location 2019 2018 Revenues $ 29.8 $ (10.4 ) Revenues $ 3.7 $ 7.9 Operating expenses $ (0.7 ) $ (0.4 ) Operating expenses $ (0.3 ) $ (0.1 ) For the Six Months Ended June 30, Net Gains/(Losses) Reclassified from AOCI into Operating Income (in millions) Net Gains/(Losses) Recognized in Operating Income (in millions) Location 2019 2018 Location 2019 2018 Revenues $ 44.6 $ (43.3 ) Revenues $ 7.4 $ 7.0 Operating expenses $ (1.2 ) $ 0.9 Operating expenses $ (1.2 ) $ (0.4 ) 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, 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 our 2.90% Senior Notes attributable to changes in interest rates. The carrying value of our 2.90% Senior Notes as of June 30, 2019 and December 31, 2018, includes approximately $5.5 million and $14.5 million , respectively, related to changes in the fair value of these interest rate swap contracts. 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 with no net impact recorded in income. Any net interest payments made or received on the interest rate swap contracts are recorded as a component of interest expense in our condensed consolidated statements of income. Net Investment Hedges - Hedging Instruments In February 2012 we entered into a joint venture agreement with Samsung BioLogics, establishing an entity, Samsung Bioepis, to develop, manufacture and market biosimilar products. In June 2018 we exercised our option under our joint venture agreement to increase our ownership percentage in Samsung Bioepis from approximately 5% to approximately 49.9% . The share purchase transaction was completed in November 2018 and, upon closing, we paid 759.5 billion South Korean won ( $676.6 million ) to Samsung BioLogics. Our investment in the equity of Samsung Bioepis is exposed to the currency fluctuations in the South Korean won. In order to mitigate the currency fluctuations between the U.S. dollar and South Korean won, we have entered into foreign currency forward contracts. Foreign currency forward contracts in effect as of June 30, 2019 , had remaining durations of four months . These contracts have been designated as net investment hedges. We recognize changes in the spot exchange rate in accumulated other comprehensive income (loss). The pre-tax portion of the fair value of these foreign currency forward contracts that were included in accumulated other comprehensive income (loss) in total equity reflected net gains of $19.6 million and net losses of $3.8 million as of June 30, 2019 and December 31, 2018 , respectively. We exclude fair value changes related to the forward rate from our hedging relationship and will amortize the forward points in other income (expense), net in our condensed consolidated statements of income over the term of the contract. The pre-tax portion of the fair value of the forward points that were included in accumulated other comprehensive income (loss) in total equity reflected gains of $9.6 million and $7.3 million as of June 30, 2019 and December 31, 2018 , respectively. The following tables summarize the effect of our net investment hedge in our condensed consolidated financial statements: For the Three Months Ended June 30, Net Gains/(Losses) Recognized in Other Comprehensive Income (Effective Portion) (in millions) Net Gains/(Losses) Recognized in Other Comprehensive Income (Amounts Excluded from Effectiveness Testing) (in millions) Net Gains/(Losses) Recognized in Net Income (Amounts Excluded from Effectiveness Testing) (in millions) Location 2019 2018 Location 2019 2018 Location 2019 2018 Gains (losses) on net investment hedge $ 11.6 $ — Gains (losses) on net investment hedge $ 2.3 $ — Other income (expense) $ 2.2 $ — For the Six Months Ended June 30, Net Gains/(Losses) Recognized in Other Comprehensive Income (Effective Portion) (in millions) Net Gains/(Losses) Recognized in Other Comprehensive Income (Amounts Excluded from Effectiveness Testing) (in millions) Net Gains/(Losses) Recognized in Net Income (Amounts Excluded from Effectiveness Testing) (in millions) Location 2019 2018 Location 2019 2018 Location 2019 2018 Gains (losses) on net investment hedge $ 23.4 $ — Gains (losses) on net investment hedge $ 6.7 $ — Other income (expense) $ 4.4 $ — For additional information on our collaboration arrangement with Samsung Bioepis, please read Note 17, Collaborative and Other Relationships , to these condensed consolidated financial statements. 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 forward contracts was $934.4 million and $735.1 million as of June 30, 2019 and December 31, 2018 , respectively. Net gains of $0.9 million and net losses of $3.9 million , related to these contracts were recognized as a component of other income (expense), net for the three and six months ended June 30, 2019 , respectively, compared to net gains of $5.2 million and net losses of $0.4 million , respectively, in the prior year comparative periods. Summary of Derivatives While certain of our derivative instruments are subject to netting arrangements with our counterparties, we do not offset derivative assets and liabilities in our condensed consolidated balance sheets. The amounts in the table below would not be materially different if the derivative assets and liabilities were offset. The following table summarizes the fair value and presentation in our condensed consolidated balance sheets of our outstanding derivative instruments, including those designated as hedging instruments: (In millions) Balance Sheet Location As of As of Cash Flow Hedging Instruments: Asset derivative instruments Other current assets $ 81.8 $ 65.8 Liability derivative instruments Accrued expenses and other $ 5.5 $ 6.9 Other long-term liabilities $ 2.2 $ — Fair Value Hedging Instruments: Liability derivative instruments Other long-term liabilities $ 5.5 $ 14.5 Other Derivatives: Asset derivative instruments Other current assets $ 6.4 $ 1.1 Liability derivative instruments Accrued expenses and other $ 4.3 $ 3.2

Property, Plant and Equipment P

Property, Plant and Equipment Property, Plant and Equipment	6 Months Ended
	Jun. 30, 2019
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 $1,886.2 million and $1,797.4 million as of June 30, 2019 and December 31, 2018 , respectively. For the three and six months ended June 30, 2019 , depreciation expense totaled $47.0 million and $99.9 million , respectively, compared to $64.2 million and $129.2 million , respectively, in the prior year comparative periods. Solothurn, Switzerland Facility We are building a large-scale biologics manufacturing facility in Solothurn, Switzerland. We expect this facility to be partially operational by the end of 2020. Upon completion, the facility will include 393,000 square feet related to a large-scale biologics manufacturing facility, 290,000 square feet of warehouse, utilities and support space and 51,000 square feet of administrative space. As of June 30, 2019 and December 31, 2018 , we had approximately $1.8 billion and $1.6 billion , respectively, capitalized as construction in progress related to this facility. As of June 30, 2019 , we had contractual commitments of approximately $57.0 million outstanding related to the construction of this facility. Proposed Divestiture of Hillerød, Denmark Manufacturing Operations In March 2019 we entered into a share purchase agreement with FUJIFILM under which FUJIFILM will acquire all of the outstanding shares of our subsidiary that owns our biologics manufacturing operations in Hillerød, Denmark. As a result, $641.9 million of property, plant and equipment, which is primarily comprised of $318.4 million for buildings and $290.8 million for machinery and equipment, was reclassified to assets held for sale in our condensed consolidated balance sheets as of June 30, 2019 . Additionally, we ceased recording depreciation on these assets as depreciation is not recorded during the period in which a long-lived asset or disposal group is classified as held for sale, even if the asset or disposal group continues to generate revenue during the period. For additional information on the proposed divestiture of our Hillerød, Denmark manufacturing operations, please read Note 3, Divestitures , to these condensed consolidated financial statements.

Leases

Leases	6 Months Ended
	Jun. 30, 2019
Leases [Abstract]
Leases	Leases We lease real estate, including laboratory and office space, and certain equipment. Our leases have remaining lease terms ranging from less than one year to nine years . Certain leases include one or more options to renew, exercised at our sole discretion, with renewal terms that can extend the lease term from one year to six years . In addition, we sublease certain real estate to third parties. Our sublease portfolio consists of operating leases, with remaining lease terms ranging from less than one year to nine years . Our subleases do not include an option to renew as they are coterminous with our operating leases. All of our leases qualify as operating leases. The following table summarizes the presentation in our condensed consolidated balance sheets of our operating leases: (In millions) Balance sheet location As of June 30, 2019 Assets: Operating lease assets Operating lease assets $ 434.4 Liabilities Current operating lease liabilities Accrued expenses and other $ 72.3 Non-current operating lease liabilities Long-term operating lease liabilities 423.0 Total operating lease liabilities $ 495.3 The following table summarizes the effect of lease costs in our condensed consolidated statements of income: For the Three Months For the Six Months (In millions) Income Statement Location 2019 2019 Operating lease cost Research and development $ 3.4 $ 3.7 Selling, general and administrative 17.9 41.4 Sublease income Selling, general and administrative (6.0 ) (13.1 ) Other (income) expense, net (0.9 ) (1.9 ) Net lease cost $ 14.4 $ 30.1 The minimum lease payments for the next five years and thereafter is expected to be as follows: (In millions) As of 2019 (remaining six months) $ 50.2 2020 80.5 2021 74.9 2022 70.9 2023 69.4 2024 66.7 Thereafter 147.6 Total lease payments $ 560.2 Less: interest 64.9 Present value of operating lease liabilities $ 495.3 Under the prior lease guidance minimum rental commitments under non-cancelable leases, net of income from subleases, for each of the next five years and total thereafter as of December 31, 2018, were as follows: (In millions) 2019 2020 2021 2022 2023 Thereafter Total Minimum lease payments $ 87.0 $ 80.7 $ 75.9 $ 71.7 $ 71.0 $ 215.3 $ 601.6 Less: income from subleases (1) (26.8 ) (25.6 ) (23.7 ) (24.0 ) (24.3 ) (58.4 ) (182.8 ) Net minimum lease payments $ 60.2 $ 55.1 $ 52.2 $ 47.7 $ 46.7 $ 156.9 $ 418.8 (1) Represents sublease income expected to be received for the vacated manufacturing facility in Cambridge, MA, the vacated portion of our Weston, MA facility and other facilities throughout the world. The weighted average remaining lease term and weighted average discount rate of our operating leases are as follows: As of June 30, 2019 Weighted average remaining lease term in years 7.4 Weighted average discount rate 3.3 % Supplemental disclosure of cash flow information related to our operating leases included in cash flows provided by operating activities in our condensed consolidated statements of cash flows is as follows: For the Three Months For the Six Months (In millions) 2019 2019 Cash paid for amounts included in the measurement of lease liabilities $ 27.4 $ 46.8 Operating lease assets obtained in exchange for lease obligations $ 7.3 $ 12.7 Proposed Divestiture of Hillerød, Denmark Manufacturing Operations In March 2019 we entered into a share purchase agreement with FUJIFILM under which FUJIFILM will acquire all of the outstanding shares of our subsidiary that owns our biologics manufacturing operations in Hillerød, Denmark. As a result, $2.2 million of operating lease assets and $2.2 million of operating lease liabilities were reclassified to assets and liabilities held for sale in our condensed consolidated balance sheets as of June 30, 2019 . For additional information on the proposed divestiture of our Hillerød, Denmark manufacturing operations, please read Note 3, Divestitures , to these condensed consolidated financial statements.

Equity

Equity	6 Months Ended
	Jun. 30, 2019
Equity [Abstract]
Equity	Equity Share Repurchases In March 2019 our Board of Directors authorized a program to repurchase up to $5.0 billion of our common stock (2019 Share Repurchase Program). Our 2019 Share Repurchase Program does not have an expiration date. All share repurchases under our 2019 Share Repurchase Program will be retired. Under our 2019 Share Repurchase Program, we repurchased and retired approximately 3.9 million shares of our common stock at a cost of approximately $909.9 million during the three and six months ended June 30, 2019 . Approximately $4.1 billion remained available under our 2019 Share Repurchase program as of June 30, 2019 . In August 2018 our Board of Directors authorized a program to repurchase up to $3.5 billion of our common stock (2018 Share Repurchase Program), which was completed as of June 30, 2019. All share repurchases under our 2018 Share Repurchase Program were retired. Under our 2018 Share Repurchase Program, we repurchased and retired approximately 6.5 million and 8.9 million shares of our common stock at a cost of approximately $1.5 billion and $2.1 billion during the three and six months ended June 30, 2019 , respectively. In July 2016 our Board of Directors authorized a program to repurchase up to $5.0 billion of our common stock (2016 Share Repurchase Program), which was completed as of June 30, 2018. All share repurchases under our 2016 Share Repurchase Program were retired. Under our 2016 Share Repurchase Program, we repurchased and retired approximately 9.6 million and 10.5 million shares of our common stock at a cost of approximately $2.75 billion and $3.0 billion during the three and six months ended June 30, 2018 , respectively. Accumulated Other Comprehensive Income (Loss) The following tables summarize 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 Gains (Losses) on Net Investment Hedge Unfunded Status of Postretirement Benefit Plans, Net of Tax Currency Translation Adjustments Total Balance, December 31, 2018 $ (4.0 ) $ 34.7 $ 3.5 $ (31.3 ) $ (243.3 ) $ (240.4 ) Other comprehensive income (loss) before reclassifications 11.0 22.3 30.1 0.7 (28.1 ) 36.0 Amounts reclassified from accumulated other comprehensive income (loss) (0.8 ) (43.3 ) (4.4 ) — — (48.5 ) Net current period other comprehensive income (loss) 10.2 (21.0 ) 25.7 0.7 (28.1 ) (12.5 ) Balance, June 30, 2019 $ 6.2 $ 13.7 $ 29.2 $ (30.6 ) $ (271.4 ) $ (252.9 ) (In millions) Unrealized Gains (Losses) on Securities Available for Sale, Net of Tax Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax Gains (Losses) on Net Investment Hedge Unfunded Status of Postretirement Benefit Plans, Net of Tax Currency Translation Adjustments Total Balance, December 31, 2017 $ (1.6 ) $ (104.5 ) $ — $ (36.8 ) $ (175.5 ) $ (318.4 ) Amounts reclassified, net of tax, upon adoption of ASU No. 2016-01 1.5 — — — — 1.5 Balance, January 1, 2018 (0.1 ) (104.5 ) — (36.8 ) (175.5 ) (316.9 ) Other comprehensive income (loss) before reclassifications (7.6 ) 61.7 — 0.4 (47.3 ) 7.2 Amounts reclassified from accumulated other comprehensive income (loss) 6.0 42.1 — — — 48.1 Net current period other comprehensive income (loss) (1.6 ) 103.8 — 0.4 (47.3 ) 55.3 Balance, June 30, 2018 $ (1.7 ) $ (0.7 ) $ — $ (36.4 ) $ (222.8 ) $ (261.6 ) 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 Six Months 2019 2018 2019 2018 Gains (losses) on securities available for sale Other income (expense) $ 0.7 $ — $ 1.0 $ (7.6 ) Income tax benefit (expense) (0.1 ) — (0.2 ) 1.6 Gains (losses) on cash flow hedges Revenues 29.8 (10.4 ) 44.6 (43.3 ) Operating expenses (0.7 ) (0.4 ) (1.2 ) 0.9 Other income (expense) — — 0.1 0.1 Income tax benefit (expense) (0.1 ) 0.1 (0.2 ) 0.2 Gains (losses) on net investment hedge Other income (expense) 2.2 — 4.4 — Income tax benefit (expense) — — — — Total reclassifications, net of tax $ 31.8 $ (10.7 ) $ 48.5 $ (48.1 )

Earnings per Share

Earnings per Share	6 Months Ended
	Jun. 30, 2019
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 Six Months (In millions) 2019 2018 2019 2018 Numerator: Net income attributable to Biogen Inc. $ 1,494.1 $ 866.6 $ 2,902.9 $ 2,039.5 Denominator: Weighted average number of common shares outstanding 190.3 207.1 193.4 209.2 Effect of dilutive securities: Stock options and employee stock purchase plan — — — — Time-vested restricted stock units 0.1 0.1 0.2 0.2 Market stock units — 0.1 0.1 0.1 Performance stock units settled in stock — — — — Dilutive potential common shares 0.1 0.2 0.3 0.3 Shares used in calculating diluted earnings per share 190.4 207.3 193.7 209.5 Amounts excluded from the calculation of net income per diluted share because their effects were anti-dilutive were insignificant.

Share-based Payments

Share-based Payments	6 Months Ended
	Jun. 30, 2019
Share-based Payment Arrangement [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 Six Months (In millions) 2019 2018 2019 2018 Research and development $ 28.1 $ 17.6 $ 49.8 $ 39.5 Selling, general and administrative 54.8 25.2 82.6 53.7 Subtotal 82.9 42.8 132.4 93.2 Capitalized share-based compensation costs (2.8 ) (2.8 ) (6.1 ) (6.2 ) Share-based compensation expense included in total cost and expenses 80.1 40.0 126.3 87.0 Income tax effect (13.6 ) (6.5 ) (21.0 ) (14.1 ) Share-based compensation expense included in net income attributable to Biogen Inc. $ 66.5 $ 33.5 $ 105.3 $ 72.9 The following table summarizes share-based compensation expense associated with each of our share-based compensation programs: For the Three Months For the Six Months (In millions) 2019 2018 2019 2018 Market stock units $ 7.8 $ 7.1 $ 15.5 $ 13.2 Time-vested restricted stock units 31.8 30.6 66.5 66.7 Cash settled performance units 0.4 (0.7 ) (0.6 ) 3.6 Performance units 0.3 2.0 0.8 1.2 Performance stock units settled in stock 12.1 1.4 14.1 2.1 Performance stock units settled in cash 0.9 0.3 1.9 0.4 Employee stock purchase plan 3.4 2.1 8.0 6.0 NST stock options 26.2 — 26.2 — Subtotal 82.9 42.8 132.4 93.2 Capitalized share-based compensation costs (2.8 ) (2.8 ) (6.1 ) (6.2 ) Share-based compensation expense included in total cost and expenses $ 80.1 $ 40.0 $ 126.3 $ 87.0 We estimate the fair value of our obligations associated with our performance units, cash settled performance units and performance stock units settled in cash at the end of each reporting period through expected settlement. Cumulative adjustments to these obligations are recognized each quarter to reflect changes in the stock price and estimated outcome of the performance-related conditions. Stock option expense reflects the accelerated vesting of stock options previously granted to NST employees as a result of our acquisition of NST. For additional information on our acquisition of NST, please read Note 2, Acquisitions , to these condensed consolidated financial statements.

Income Taxes

Income Taxes	6 Months Ended
	Jun. 30, 2019
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes 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 Six Months 2019 2018 2019 2018 Statutory rate 21.0 % 21.0 % 21.0 % 21.0 % State taxes 0.9 0.6 0.6 0.8 Taxes on foreign earnings (4.6 ) (1.8 ) (4.6 ) (2.0 ) Credits and net operating loss utilization (0.9 ) (0.7 ) (0.8 ) (0.8 ) Purchased intangible assets 0.4 0.6 0.4 0.6 Denmark assets held for sale — — 2.2 — Internal reorganization of certain intellectual property rights (5.0 ) — (2.4 ) — Global Intangible Low-Taxed Income (GILTI) 1.4 1.8 1.7 1.6 Other permanent items 0.4 0.4 0.3 0.4 Other 0.5 0.5 0.1 0.3 Effective tax rate 14.1 % 22.4 % 18.5 % 21.9 % Changes in Tax Rate For the three months ended June 30, 2019 , compared to the same period in 2018 , the decrease in our effective tax rate was primarily due to the combination of an internal reorganization of certain intellectual property rights related to the intercompany sale of the intellectual property (the effective tax rate decrease from this internal reorganization is not expected to recur post 2019) and a higher effective tax rate in 2018 resulting from the sale of inventory, the tax effect of which had been included within prepaid taxes at January 1, 2018, at a higher effective tax rate. For the six months ended June 30, 2019 , compared to the same period in 2018 , the decrease in our effective tax rate was primarily due to the combination of the internal reorganization of certain intellectual property rights, offset by a $61.3 million tax expense related to the proposed divestiture of our subsidiary that owns our Hillerød, Denmark manufacturing operations and a higher effective tax rate in 2018 resulting from the sale of inventory, the tax effect of which had been included within prepaid taxes at January 1, 2018, at a higher effective tax rate. Specifically in regard to the Hillerød, Denmark manufacturing operations, although we are recognizing a loss on the proposed divestiture of such subsidiary, the proposed divestiture requires us to write off certain deferred tax assets upon the classification of the operations as held for sale and results in a taxable gain in certain jurisdictions. As a result of the internal reorganization of certain intellectual property rights, we have recorded a deferred tax asset of $856.7 million and a deferred tax liability of $685.3 million in the second quarter of 2019. 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. The U.S. Internal Revenue Service and other national tax authorities routinely examine our intercompany transfer pricing with respect to intellectual property related transactions and it is possible that they may disagree with one or more positions we have taken with respect to such valuations. Federal and State Uncertain Tax Positions It is reasonably possible that we will adjust the value of our uncertain tax positions related to certain transfer pricing, collaboration and other issues as we receive additional information from various taxing authorities, including reaching settlements with such authorities. International Uncertain Tax Positions We have made payments totaling approximately $60.0 million to the Danish Tax Authority (SKAT) for assessments received for 2009, 2011 and 2013 regarding withholding taxes on certain payments made by our subsidiary that owns our biologics manufacturing operations in Hillerød, Denmark. We continue to dispute the assessments for all of these periods and believe that the tax positions taken related to these payments are valid. Any amount refunded by SKAT associated with this withholding tax receivable will be paid to our subsidiary that owns our biologics manufacturing operations in Hillerød, Denmark. Proposed Divestiture of Hillerød, Denmark Manufacturing Operations In March 2019 we entered into a share purchase agreement with FUJIFILM under which FUJIFILM will acquire all of the outstanding shares of our subsidiary that owns our biologics manufacturing operations in Hillerød, Denmark. This withholding tax receivable from SKAT will be included within the assets that will be transferred to FUJIFILM as part of the proposed transaction. Under the share purchase agreement, FUJIFILM is required to remit any future proceeds refunded by SKAT to us. We have assessed the collectability of the receivable from FUJIFILM and regard it as a contingent gain, which does not meet the probable threshold for recognition under ASC 450, Contingencies , and therefore we have recorded a pre-tax charge of $60.0 million to write the asset down to zero as a component of the loss on assets and liabilities held for sale in the first quarter of 2019. We have also reclassified $53.8 million of our deferred tax liability to liabilities held for sale in our condensed consolidated balance sheets as of June 30, 2019 . For additional information on the proposed divestiture of our Hillerød, Denmark manufacturing operations, please read Note 3, Divestitures , to these condensed consolidated financial statements.

Other Consolidated Financial St

Other Consolidated Financial Statement Detail	6 Months Ended
	Jun. 30, 2019
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 Six Months (In millions) 2019 2018 2019 2018 Interest income $ 29.1 $ 28.7 $ 60.3 $ 55.4 Interest expense (47.7 ) (51.7 ) (95.6 ) (102.2 ) Gain (loss) on investments, net (173.4 ) 5.3 203.0 (9.1 ) Foreign exchange gains (losses), net 1.7 (13.0 ) (0.5 ) (14.0 ) Other, net (7.1 ) (3.8 ) (7.3 ) (5.6 ) Total other income (expense), net $ (197.4 ) $ (34.5 ) $ 159.9 $ (75.5 ) Gain (loss) on investments, net, as reflected in the table above, relate to debt securities, equity securities of certain biotechnology companies, venture capital funds where the underlying investments are in equity securities of certain biotechnology companies and non-marketable equity securities. The following table summarizes our gain (loss) on investments, net that relates to our equity securities held as of June 30, 2019 : For the Three Months For the Six Months (In millions) 2019 2018 2019 2018 Net gains (losses) recognized during the period on equity securities (174.2 ) 5.4 201.9 (1.5 ) Less: Net gains (losses) recognized during the period on equity securities sold during the period (42.9 ) — 42.4 (0.5 ) Unrealized gains (losses) recognized during the period on equity securities held as of June 30, 2019 (131.3 ) 5.4 159.5 (1.0 ) Accrued Expenses and Other Accrued expenses and other consists of the following: (In millions) As of As of Revenue-related reserves for discounts and allowances $ 863.5 $ 874.7 Royalties and licensing fees 212.8 224.7 Employee compensation and benefits 199.4 320.9 Current portion of contingent consideration obligations 146.2 444.8 Collaboration expenses 130.4 261.6 Construction in progress 38.2 125.2 Other 727.5 609.3 Total accrued expenses and other $ 2,318.0 $ 2,861.2 Other Long-term Liabilities Other long-term liabilities were $1,355.8 million and $1,389.4 million as of June 30, 2019 and December 31, 2018 , respectively, and included accrued income taxes totaling $801.5 million and $791.4 million , respectively.

Collaborative and Other Relatio

Collaborative and Other Relationships	6 Months Ended
	Jun. 30, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaborative and Other Relationships	Collaborative and Other Relationships Eisai Co., Ltd. BAN2401 and Elenbecestat Collaboration We have a collaboration agreement with Eisai Co., Ltd. (Eisai) to jointly develop and commercialize BAN2401, a monoclonal antibody that targets amyloid beta aggregates, and elenbecestat, the oral BACE (base amyloid cleaving enzyme) inhibitor, two Eisai product candidates for the treatment of AD (the BAN2401 and Elenbecestat Collaboration). The BAN2401 and Elenbecestat Collaboration also provided Eisai with an option to jointly develop and commercialize aducanumab (Aducanumab Option), and an option to jointly develop and commercialize one of our anti-tau monoclonal antibodies (Anti-Tau Option). In October 2017 Eisai exercised its Aducanumab Option and we entered into a new collaboration agreement for the joint development and commercialization of aducanumab (Aducanumab Collaboration Agreement). Eisai has not yet exercised its Anti-Tau Option. For additional information on our BAN2401 and Elenbecestat Collaboration, please read Note 19, Collaborative and Other Relationships , to our consolidated financial statements included in our 2018 Form 10-K. For the three and six months ended June 30, 2019 and 2018 , sales and marketing expense related to the BAN2401 and Elenbecestat Collaboration was immaterial. A summary of development expense related to the BAN2401 and Elenbecestat Collaboration is as follows: For the Three Months For the Six Months (In millions) 2019 2018 2019 2018 Total development expense incurred by the collaboration related to the advancement of BAN2401 and Elenbecestat $ 68.6 $ 54.6 $ 136.6 $ 111.1 Biogen's share of BAN2401 and Elenbecestat development expense reflected in research and development expense in our condensed consolidated statements of income $ 34.3 $ 27.3 $ 68.3 $ 55.5 Aducanumab Collaboration Agreement For the period through March 31, 2018, we were responsible for 100% of development costs incurred by the collaboration for the advancement of aducanumab (aducanumab development expense). For the period April 1, 2018 through December 31, 2018, Eisai reimbursed us for 15% of aducanumab development expense incurred and, beginning January 1, 2019, is reimbursing us for 45% of aducanumab development expense incurred. In March 2019 we and Eisai announced the decision to discontinue the global Phase 3 trials, ENGAGE and EMERGE, designed to evaluate the efficacy and safety of aducanumab in patients with mild cognitive impairment due to AD and mild AD dementia. As a result of this decision, in the first quarter of 2019, we accrued approximately $45.0 million related to the termination of various clinical trials and research and development contracts net of the expected 45% Eisai reimbursement of development costs incurred by the collaboration for the advancement of aducanumab. Sales and marketing expense incurred was shared in proportion to the same region-based profit split that would have been utilized to co-promote aducanumab had it been commercialized. For additional information on the Aducanumab Collaboration Agreement, please read Note 19, Collaborative and Other Relationships , to our consolidated financial statements included in our 2018 Form 10-K. A summary of development and sales and marketing expense related to the Aducanumab Collaboration Agreement is as follows: For the Three Months For the Six Months (In millions) 2019 2018 2019 2018 Total aducanumab development expense $ 3.2 $ 76.5 $ 165.8 $ 140.0 Biogen's share of aducanumab development expense reflected in research and development expense in our condensed consolidated statements of income $ 1.7 $ 65.0 $ 91.2 $ 128.5 Total aducanumab sales and marketing expense incurred by the collaboration $ 0.2 $ 14.0 $ 21.2 $ 21.2 Biogen's share of aducanumab sales and marketing expense reflected in selling, general and administrative expense in our condensed consolidated statements of income $ 0.1 $ 9.2 $ 11.7 $ 13.9 In addition, we and Eisai co-promote AVONEX, TYSABRI and TECFIDERA in Japan in certain settings and Eisai distributes AVONEX, TYSABRI, TECFIDERA and PLEGRIDY in India and other Asia-Pacific markets, excluding China. Other Research and Discovery Arrangements These arrangements may include the potential for future milestone payments based on the achievement of certain clinical and commercial development payable over a period of several years. Skyhawk Therapeutics, Inc. In January 2019 we entered into a collaboration and research and development services agreement with Skyhawk Therapeutics, Inc. (Skyhawk) pursuant to which the companies will leverage Skyhawk's SkySTAR technology platform with the goal of discovering innovative small molecule treatments for patients with neurological diseases, including MS and SMA. We will be responsible for the development and potential commercialization of any therapies resulting from this collaboration and we may also pay Skyhawk up to a total of approximately $2.0 billion in additional milestone payments as well as potential royalties on net commercial sales. In connection with this agreement, we made an upfront payment of $74.0 million to Skyhawk, of which $38.5 million was recorded as research and development expense in our condensed consolidated statements of income and $35.5 million was recorded as prepaid research and development expenditures within investments and other assets in our condensed consolidated balance sheets and will be expensed as the services are provided. 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 products. In June 2018 we exercised our option under our joint venture agreement to increase our ownership percentage in Samsung Bioepis from approximately 5% to approximately 49.9% . The share purchase transaction was completed in November 2018 and, upon closing, we paid 759.5 billion South Korean won ( $676.6 million ) to Samsung BioLogics. As of June 30, 2019 , our ownership percentage remained at approximately 49.9% . We recognize our share of the results of operations related to our investment in Samsung Bioepis under the equity method of accounting one quarter in arrears when the results of the entity become available, which is reflected as equity in income (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 initial investment, we suspended recognizing additional losses. In the first quarter of 2019 we restarted recognizing our share of Samsung Bioepis' income (losses), and we began recognizing amortization on certain basis differences resulting from our November 2018 investment. Upon investment, the equity method of accounting requires us to identify and allocate differences between the fair value of our investment and the carrying value of our interest in the underlying net assets of the investee. These basis differences are amortized over their economic life. The total basis difference was approximately $675 million , consisting of approximately $115 million attributed to inventory, approximately $615 million attributed to developed technology and approximately $170 million attributed to IPR&D. A deferred tax liability of $225 million was established for the acquired assets that had no tax basis. The basis differences related to inventory and developed technology will be amortized, net of tax, over their estimated useful lives of 1.5 years and 15 years, respectively, one quarter in arrears. Our joint venture partner, Samsung BioLogics, is currently subject to an ongoing criminal investigation that we continue to monitor. While this investigation could impact the operations of Samsung Bioepis and its business, we have assessed the value of our investment in Samsung Bioepis and continue to believe that the fair value of the investment is in excess of its net book value. For the three and six months ended June 30, 2019 , we recognized losses on our investment of $16.3 million and $45.0 million , respectively. These losses reflect our share of income totaling $5.5 million and losses totaling $8.5 million , respectively, and amortization of basis differences totaling $21.8 million and $36.5 million , respectively. As of June 30, 2019 and December 31, 2018 , the carrying value of our investment in Samsung Bioepis totaled 712.8 billion South Korean won ( $615.5 million ) and 759.5 billion South Korean won ( $680.6 million ), respectively, which is classified as a component of investments and other assets in our condensed consolidated balance sheets. Commercial Agreement We reflect revenues on sales of BENEPALI, IMRALDI 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. We share 50% of the profit or loss related to our commercial agreement with Samsung Bioepis, which is recognized in collaboration profit (loss) sharing in our condensed consolidated statements of income. For the three and six months ended June 30, 2019 , we recognized net profit-sharing expense of $63.4 million and $121.5 million , respectively, to reflect Samsung Bioepis' 50% sharing of the net collaboration profits, compared to $39.7 million and $83.5 million , respectively, 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 six months ended June 30, 2019 , we recognized $52.2 million and $77.0 million , respectively, in revenues related to these services, which is reflected in collaborative and other relationships revenues as a component of other revenues in our condensed consolidated statements of income, compared to $14.7 million and $32.6 million , respectively, in the prior year comparative periods. For additional information on our collaboration arrangement with Samsung Bioepis and our other significant collaboration arrangements, please read Note 19, Collaborative and Other Relationships, to our consolidated financial statements included in our 2018 Form 10-K.

Investments in Variable Interes

Investments in Variable Interest Entities	6 Months Ended
	Jun. 30, 2019
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. The following are our significant variable interest entities. Neurimmune SubOne AG We have a collaboration and license agreement with Neurimmune SubOne AG (Neurimmune) for the development and commercialization of antibodies for the treatment of AD, including aducanumab. We are responsible for the development, manufacturing and commercialization of all collaboration products. This agreement is effective for the longer of the duration of certain patents relating to a licensed product or 12 years from the first commercial sale of any product using such a licensed compound. 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 we are required to fund 100% of the research and development costs incurred in support of the collaboration. In March 2019 we and Eisai announced the decision to discontinue the global Phase 3 trials, ENGAGE and EMERGE, of aducanumab. Research and development costs for which we reimbursed Neurimmune are reflected in research and development expense in our condensed consolidated statements of income. During the three and six months ended June 30, 2019 and 2018 , amounts reimbursed were immaterial. The assets and liabilities of Neurimmune are not significant to our condensed consolidated financial position or results of operations as it is a research and development organization. We have provided no financing to Neurimmune other than contractually required amounts. Unconsolidated Variable Interest Entities We have relationships with 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 June 30, 2019 and December 31, 2018 , the carrying value of our investments in certain biotechnology companies representing potential unconsolidated variable interest entities totaled $24.0 million and $28.7 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 on our investments in Neurimmune and other variable interest entities, please read Note 20, Investments in Variable Interest Entities, to our consolidated financial statements included in our 2018 Form 10-K.

Litigation

Litigation	6 Months Ended
	Jun. 30, 2019
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 2018 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 IMRALDI Patent Litigation In September 2018 Fresenius Kabi Deutschland GmbH (Fresenius Kabi) commenced proceedings for damages and injunctive relief against Biogen France SAS in the Tribunal de Grande Instance de Paris, alleging that IMRALDI, the adalimumab biosimilar product of Samsung Bioepis UK Limited that Biogen commercializes in Europe, infringes the French counterpart of European Patent No. 3 148 510 (the ‘510 Patent), which was issued in June 2018 and expires in May 2035. A hearing has been scheduled for late 2019. In October 2018 Fresenius Kabi commenced preliminary injunction proceedings against Biogen (Denmark) Manufacturing ApS and Biogen Denmark A/S in Denmark's Maritime and Commercial High Court alleging infringement of the Danish Utility Models. In June 2019 the Danish court denied Fresenius Kabi's request for a preliminary injunction and Fresenius Kabi has appealed that decision. In November 2018 Fresenius Kabi commenced infringement proceedings for damages and injunctive relief against Biogen Italia S.R.L. in the District Court of Milan relating to the Italian counterpart of the ‘510 Patent, and against Biogen GmbH in the Düsseldorf Regional Court relating to the German counterpart of the ‘510 Patent. A hearing in the proceeding in Germany has been set for March 2020. No hearing has been set in the proceeding in Italy. An estimate of the possible loss or range of loss in the above matters cannot be made at this time. In August 2018 Biogen Idec Ltd. (Biogen UK) and Samsung Bioepis UK Limited filed an action in the United Kingdom Patents Court to revoke the United Kingdom counterpart of the ‘510 Patent. Fresenius Kabi counterclaimed for infringement, damages and injunctive relief. In July 2019 the United Kingdom Patents Court entered a consent order in which it declared that the United Kingdom counterpart of the '510 Patent is invalid, ordered the patent revoked and dismissed Fresenius Kabi's counterclaims. In December 2018 Biogen B.V. and Samsung Bioepis UK Limited filed an action in the District Court of the Hague, Netherlands to revoke the Dutch counterpart of the ‘510 Patent. A trial is scheduled in the Dutch matter for October 2019. In July 2019 Gedeon Richter PLC commenced proceedings against Biogen GmbH in the Düsseldorf Regional Court alleging infringement of the German counterpart of European Patent No. 3 212 667, which was issued in September 2018 and expires in October 2035, and seeking damages and injunctive relief. A hearing has been set for November 2020. An estimate of the possible loss or range of loss cannot be made at this time. Qui Tam Litigation In July 2015 a qui tam action filed by Michael Bawduniak on behalf of the U.S. 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 was denied in part. No trial date has been set. The U.S. has not made an intervention decision. An estimate of the possible loss or range of loss cannot be made at this time. In July 2018 we and certain other drug manufacturers and pharmacy benefit managers were served with a qui tam action filed by John Borzilleri on behalf of the U.S. and certain states in the U.S. District Court for the District of Rhode Island. The case was filed under seal in January 2014 and unsealed in April 2018 after the U.S. declined to intervene. The case alleges agreements with pharmacy benefit managers 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. We, the other defendants and the U.S. have moved to dismiss the case and the motions are pending. No trial date has been set. 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 in October 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. In June 2019 the U.S. Court of Appeals for the First Circuit affirmed the judgment below dismissing the complaint with prejudice. Other Matters Hatch-Waxman Act Litigation relating to TECFIDERA Orange-Book Listed Patents In 2017, 2018 and 2019 we initiated patent infringement proceedings against multiple parties pursuant to the Drug Price Competition and Patent Term Restoration Act of 1984, commonly known as the Hatch-Waxman Act, in the U.S. District Courts. Patent infringement proceedings pursuant to the Hatch-Waxman Act are pending against Accord Healthcare Inc., Alkem Laboratories Ltd., Amneal Pharmaceuticals LLC, Aurobindo Pharma U.S.A., Inc., Banner Life Sciences LLC, Cipla Limited, Glenmark Pharmaceuticals Ltd., Graviti Pharmaceuticals Pvt. Ltd., Hetero USA Inc., Lupin Atlantis Holdings SA, Macleods Pharmaceuticals, Ltd., MSN Laboratories Pvt. Ltd., Pharmathen S.A., Prinston Pharmaceutical Inc., Sandoz Inc., Sawai USA, Inc., Shilpa Medicare Limited, Slayback Pharma LLC, Torrent Pharmaceuticals Ltd., TWi Pharmaceuticals, Inc., Windlas Healthcare Pvt. Ltd. and Zydus Pharmaceuticals (USA) Inc. in the U.S. District Court for the District of Delaware and against Mylan Pharmaceuticals Inc. in the U.S. District Court for the Northern District of West Virginia. A trial date has not been set in the Delaware action against Banner Life Sciences LLC. A trial date has been set for December 2019 in the other Delaware actions, and a trial date has been set for February 2020 in the West Virginia action against Mylan Pharmaceuticals Inc. Petition for Inter Partes Review In July 2018 Mylan Pharmaceuticals Inc. filed a petition with the U.S. Patent Trial and Appeal Board seeking inter partes review of 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 per day as provided for in our TECFIDERA label. On February 6, 2019, the U.S. Patent Trial and Appeal Board instituted inter partes review of the '514 Patent. European Patent Office Oppositions In 2016 the European Patent Office (EPO) revoked our European patent number 2 137 537 (the '537 Patent), which includes claims covering the treatment of MS with 480 mg of dimethyl fumarate as provided for in our TECFIDERA label. We have appealed to the Technical Boards of Appeal of the EPO and the appeal is pending. A hearing has been set for March 2020. In March 2018 the EPO revoked Forward Pharma’s European Patent No. 2 801 355, which expires in October 2025. Forward Pharma has filed an appeal to the Technical Boards of Appeal of the EPO and the appeal is pending. A hearing has been set for June 2020. 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 Technical Boards of Appeal and the Enlarged Board of Appeal, if applicable, to make a final determination. TYSABRI Patent Revocation Matters In November 2017 Bioeq GMBH, affiliated with the Polpharma Group, brought an action in the Polish Patent Office seeking to revoke Polish Patent Number 215263 (the Polish ‘263 Patent), the Polish patent corresponding to our European Patent Number 1 485 127 (the EU ‘127 Patent) (“Administration of agents to treat inflammation”). The Polish ‘263 Patent concerns administration of natalizumab (TYSABRI) to treat MS. The Polish ‘263 Patent expires in February 2023. Swiss Pharma International AG, also affiliated with the Polpharma Group, filed actions in the District Court of The Hague (January 2016), the German Patents Court (March 2016) and the Commercial Court of Rome (November 2017) seeking to invalidate the Dutch, German and Italian counterparts of the EU '127 Patent, which also concerns administration of natalizumab (TYSABRI) to treat MS and expires in February 2023. The Dutch and German counterparts were ruled invalid and we have appealed. No date for a hearing on the merits has been set in the Italian action. '755 Patent Litigation In May 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 (the '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, Pfizer, Novartis and EMD Serono filed counterclaims seeking declaratory judgments of patent invalidity and non-infringement and seeking monetary relief in the form of costs and attorneys' fees. 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 '755 Patent is invalid, and seeking monetary relief in the form of attorneys' fees, costs and expenses. In September 2018 the trial court entered judgment against EMD Serono and Pfizer that the '755 Patent is infringed and valid and ordered a new trial on damages. In October 2018 EMD Serono and Pfizer filed an appeal from the judgment in the U.S. Court of Appeals for the Federal Circuit, which is pending. The trial court has not yet scheduled the new damages trial or a trial against Bayer and Novartis. 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 assist patients taking drugs sold by Biogen and Biogen's co-pay assistance programs. We are cooperating with the government. Tax Matter In the second quarter of 2018 the State Treasury of Goias, Brazil issued tax assessments for the period 2013 through February 2018 relating to tax on the circulation of goods and totaling approximately $70.0 million including interest and penalties. We dispute the assessments and have filed defenses with the Administrative Court of Appeals for the State of Goias, which are pending. We have not formed an opinion that an unfavorable outcome of the dispute is either probable or remote. 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.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Overview	Business Overview Biogen is a global biopharmaceutical company focused on discovering, developing and delivering worldwide innovative therapies for people living with serious neurological and neurodegenerative diseases as well as related therapeutic adjacencies. Our core growth areas include multiple sclerosis (MS) and neuroimmunology, neuromuscular disorders, including spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS), movement disorders, including Parkinson's disease and progressive supranuclear palsy, Alzheimer's disease (AD) and dementia and ophthalmology. We are also focused on discovering, developing and delivering worldwide innovative therapies in our emerging growth areas of immunology, neurocognitive disorders, acute neurology and pain. In addition, we commercialize biosimilars of advanced biologics. We support our drug discovery and development efforts through the commitment of significant resources to discovery, research and development programs and business development opportunities. Our marketed products include TECFIDERA, AVONEX, PLEGRIDY, TYSABRI and FAMPYRA for the treatment of MS, SPINRAZA for the treatment of 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, RITUXAN HYCELA for the treatment of non-Hodgkin's lymphoma and CLL, GAZYVA for the treatment of CLL and follicular lymphoma, OCREVUS for the treatment of primary progressive MS (PPMS) and relapsing MS (RMS) and other potential anti-CD20 therapies pursuant to our collaboration arrangements with Genentech, Inc. (Genentech), a wholly-owned member of the Roche Group. For additional information on our collaboration arrangements with Genentech, please read Note 19, Collaborative and Other Relationships , to our consolidated financial statements included in our Annual Report on Form 10-K for the year ended December 31, 2018 ( 2018 Form 10-K). Our innovative drug development and commercialization activities are complemented by our biosimilar products that expand access to medicines and reduce the cost burden for healthcare systems. Through Samsung Bioepis Co., Ltd. (Samsung Bioepis), our joint venture with Samsung BioLogics Co., Ltd. (Samsung BioLogics), we market and sell BENEPALI, an etanercept biosimilar referencing ENBREL, IMRALDI, an adalimumab biosimilar referencing HUMIRA, and FLIXABI, an infliximab biosimilar referencing REMICADE, in the European Union (E.U.). For additional information on our collaboration arrangement with Samsung Bioepis, please read Note 17, Collaborative and Other Relationships , to these unaudited condensed consolidated financial statements (condensed consolidated financial statements).
Basis of presentation	In the opinion of management, our 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 audited consolidated financial statements and the accompanying notes included in our 2018 Form 10-K. Our accounting policies are described in the “ Notes to Consolidated Financial Statements ” in our 2018 Form 10-K and updated, as necessary, in this report. The year-end condensed consolidated balance sheet data presented for comparative purposes was derived from our audited financial statements but does not include all disclosures required by U.S. GAAP. The results of operations for the three and six months ended June 30, 2019 , 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 and delivering worldwide innovative therapies for people living with serious neurological and neurodegenerative diseases as well as related therapeutic adjacencies.
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 are 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.
Assets and liabilities held for sale	Upon determining that a long-lived asset or disposal group meets the criteria to be classified as held for sale, we cease depreciation and separately present such assets and liabilities of the disposal group in our condensed consolidated balance sheet. We initially measure a long-lived asset or disposal group that is classified as held for sale at the lower of its carrying value or fair value less any costs to sell. Any loss resulting from this measurement is recognized in the period in which the held for sale criteria are met. Conversely, gains are not recognized on the sale of a long-lived asset or disposal group until the date of sale. We assess the fair value of a long-lived asset or disposal group less any costs to sell each reporting period it remains classified as held for sale and recognize any subsequent changes as an adjustment to the carrying value of the asset or disposal group, as long as the remeasured carrying value does not exceed the carrying value less costs to sell of the asset or disposal group at the time it was initially classified as held for sale.
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 below, we do not believe that the adoption of recently issued standards have or may have a material impact on our condensed consolidated financial statements or disclosures. Leases In February 2016 the FASB issued Accounting Standards Update (ASU) No. 2016-02, Leases (Topic 842) , a new standard issued to increase transparency and comparability among organizations related to their leasing activities. This standard established a right-of-use model that requires all lessees to recognize right-of-use assets and lease liabilities on their balance sheet that arise from leases as well as provide disclosures with respect to certain qualitative and quantitative information related to a company's leasing arrangements to meet the objective of allowing users of financial statements to assess the amount, timing and uncertainty of cash flows arising from leases. The FASB subsequently issued the following amendments to ASU 2016-02 that have the same effective date and transition date: ASU No. 2018-01, Leases (Topic 842): Land Easement Practical Expedient for Transition to Topic 842 , ASU No. 2018-10, Codification Improvements to Topic 842, Leases , ASU No. 2018-11, Leases (Topic 842): Targeted Improvements , ASU No. 2018-20, Narrow-Scope Improvement for Lessors , and ASU No. 2019-01, Leases (Topic 842): Codification Improvements . We adopted these amendments with ASU 2016-02 (collectively, the new leasing standards) effective January 1, 2019. We adopted the new leasing standards using the modified retrospective transition approach, as of January 1, 2019, with no restatement of prior periods or cumulative adjustment to retained earnings. Upon adoption, we elected the package of transition practical expedients, which allowed us to carry forward prior conclusions related to whether any expired or existing contracts are or contain leases, the lease classification for any expired or existing leases and initial direct costs for existing leases. We also elected the practical expedient to not reassess certain land easements and made an accounting policy election to not recognize leases with an initial term of 12 months or less within our condensed consolidated balance sheets and to recognize those lease payments on a straight-line basis in our condensed consolidated statements of income over the lease term. Upon adoption of the new leasing standards we recognized an operating lease asset of approximately $463.0 million and a corresponding operating lease liability of approximately $526.0 million , which are included in our condensed consolidated balance sheet. The adoption of the new leasing standards did not have an impact on our condensed consolidated statements of income. We determine if an arrangement is a lease at contract inception. Operating lease assets represent our right to use an underlying asset for the lease term and operating lease liabilities represent our obligation to make lease payments arising from the lease. Operating lease assets and liabilities are recognized at the commencement date of the lease based upon the present value of lease payments over the lease term. When determining the lease term, we include options to extend or terminate the lease when it is reasonably certain that we will exercise that option. We use the implicit rate when readily determinable and use our incremental borrowing rate when the implicit rate is not readily determinable based upon the information available at the commencement date in determining the present value of the lease payments. Our incremental borrowing rate is determined using a secured borrowing rate for the same currency and term as the associated lease. The lease payments used to determine our operating lease assets may include lease incentives, stated rent increases and escalation clauses linked to rates of inflation when determinable and are recognized in our operating lease assets in our condensed consolidated balance sheets. In addition, our contracts contain lease and non-lease components. We separate lease payments for the identified assets from any non-lease payments included in the contract. For certain equipment leases, such as vehicles, we apply a portfolio approach to effectively account for the operating lease assets and liabilities. Our operating leases are reflected in operating lease assets, accrued expenses and other and in long-term operating lease liabilities in our condensed consolidated balance sheets. Lease expense for minimum lease payments is recognized on a straight-line basis over the lease term. We also have real estate lease agreements which are subleased to third parties. Operating leases for which we are the sublessor are included in accrued expenses and other and other long-term liabilities in our condensed consolidated balance sheets. We recognize sublease income on a straight-line basis over the lease term in our condensed consolidated statements of income. For additional information on the adoption of the new leasing standards, please read Note 11, Leases , to these condensed consolidated financial statements. Credit Losses In June 2016 the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments . The FASB has subsequently issued amendments to ASU 2016-13, which have the same effective date and transition date of January 1, 2020. These standards require that credit losses be reported using an expected losses model rather than the incurred losses model that is currently used, and establishes additional disclosures related to credit risks. For available-for-sale debt securities with unrealized losses, these standards now require allowances to be recorded instead of reducing the amortized cost of the investment. These standards limit the amount of credit losses to be recognized for available-for-sale debt securities to the amount by which carrying value exceeds fair value and requires the reversal of previously recognized credit losses if fair value increases. Based on the composition of our investment portfolio and other financial assets, current market conditions and historical credit loss activity, the adoption of these standards is not expected to have a material impact on our consolidated financial position and results of operations and related disclosures. Debt Securities 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 . This standard amends the amortization period for certain purchased callable debt securities held at a premium by shortening the amortization period to the earliest call date. This standard became effective for us on January 1, 2019, and was adopted using a modified retrospective transition approach. The adoption of this standard did not result in a significant adjustment to our marketable debt securities. Fair Value Measurements In August 2018 the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement . This standard modifies certain disclosure requirements on fair value measurements. This standard will become effective for us on January 1, 2020. We do not expect that the adoption of this standard will have a material impact on our disclosures. Derivative Instruments and Hedging Activities In October 2018 the FASB issued ASU No. 2018-16, Derivatives and Hedging (Topic 815): Inclusion of the Secured Overnight Financing Rate (SOFR) Overnight Index Swap (OIS) Rate as a Benchmark Interest Rate for Hedge Accounting Purposes . This standard permits the use of the OIS rate based on the SOFR as a United States (U.S.) benchmark interest rate for hedge accounting purposes under Accounting Standards Codification (ASC) 815, Derivative and Hedging . This standard became effective for us on January 1, 2019, and did not have an impact on our condensed consolidated results of operations or financial position. Collaborative Arrangements In November 2018 the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 . This standard makes targeted improvements for collaborative arrangements as follows: • Clarifies that certain transactions between collaborative arrangement participants should be accounted for as revenue under ASC 606, Revenue from Contracts with Customers , when the collaborative arrangement participant is a customer in the context of a unit of account. In those situations, all the guidance in ASC 606 should be applied, including recognition, measurement, presentation and disclosure requirements; • Adds unit-of-account guidance to ASC 808, Collaborative Arrangements , to align with the guidance in ASC 606 (that is, a distinct good or service) when an entity is assessing whether the collaborative arrangement or a part of the arrangement is within the scope of ASC 606; and • Precludes a company from presenting transactions with collaborative arrangement participants that are not directly related to sales to third parties with revenue recognized under ASC 606 if the collaborative arrangement participant is not a customer. This standard will become effective for us on January 1, 2020; however, early adoption is permitted. A retrospective transition approach is required for either all contracts or only for contracts that are not completed at the date of initial application of ASC 606, with a cumulative adjustment to opening retained earnings, as of January 1, 2018. We are currently evaluating the potential impact that this standard may have on our consolidated financial position, results of operations and related disclosures.

Summary of separately identifia

Summary of separately identifiable assets acquired and liabilities assumed (Tables)	6 Months Ended
	Jun. 30, 2019
Acquisitions [Abstract]
Schedule of Business Acquisitions, by Acquisition [Table Text Block]	The following table summarizes the estimated fair values of the separately identifiable assets acquired and liabilities assumed as of June 7, 2019: (In millions) Cash and cash equivalents $ 107.8 Marketable securities 7.5 In-process research and development intangible assets 700.0 Goodwill 112.6 Deferred tax liability (77.0 ) Other, net 1.3 Total purchase price $ 852.2

Divestitures (Tables)

Divestitures (Tables)	6 Months Ended
	Jun. 30, 2019
Divestitures [Abstract]
Assets and liabilities held for sale	The following table presents information related to the carrying value of the major classes of assets and liabilities that were reclassified as held for sale in our condensed consolidated balance sheets: (In millions) As of June 30, 2019 Assets: Inventory $ 20.6 Property, plant and equipment, net 641.9 Operating lease assets 2.2 Goodwill 69.5 Other assets 62.4 Valuation allowance on disposal group on assets held for sale (113.2 ) Assets held for sale $ 683.4 Liabilities: Accrued expenses and other liabilities $ 33.4 Long-term operating lease liabilities 1.4 Deferred tax liability 53.8 Liabilities held for sale $ 88.6

Revenues (Tables)

Revenues (Tables)	6 Months Ended
	Jun. 30, 2019
Revenue from Contract with Customer [Abstract]
Revenues by product	Revenues by product are summarized as follows: For the Three Months (In millions) 2019 2018 United States Rest of World Total United States Rest of World Total Multiple Sclerosis (MS): TECFIDERA $ 869.8 $ 280.4 $ 1,150.2 $ 825.8 $ 261.0 $ 1,086.8 Interferon* 379.7 174.7 554.4 444.7 180.8 625.5 TYSABRI 264.3 211.0 475.3 265.5 201.7 467.2 FAMPYRA — 24.1 24.1 — 23.0 23.0 Subtotal: MS product revenues 1,513.8 690.2 2,204.0 1,536.0 666.5 2,202.5 Spinal Muscular Atrophy: SPINRAZA 230.6 257.6 488.2 205.9 216.8 422.7 Biosimilars: BENEPALI — 120.3 120.3 — 115.6 115.6 IMRALDI — 47.3 47.3 — — — FLIXABI — 16.8 16.8 — 11.2 11.2 Subtotal: Biosimilar product revenues — 184.4 184.4 — 126.8 126.8 Other: FUMADERM — 3.7 3.7 — 5.5 5.5 Total product revenues $ 1,744.4 $ 1,135.9 $ 2,880.3 $ 1,741.9 $ 1,015.6 $ 2,757.5 *Interferon includes AVONEX and PLEGRIDY. For the Six Months (In millions) 2019 2018 United States Rest of World Total United States Rest of World Total Multiple Sclerosis (MS): TECFIDERA $ 1,587.5 $ 561.5 $ 2,149.0 $ 1,554.7 $ 519.0 $ 2,073.7 Interferon* 707.0 348.3 1,055.3 816.0 359.8 1,175.8 TYSABRI 509.3 426.4 935.7 515.2 414.1 929.3 FAMPYRA — 47.0 47.0 — 47.4 47.4 ZINBRYTA — — — — 1.4 1.4 Subtotal: MS product revenues 2,803.8 1,383.2 4,187.0 2,885.9 1,341.7 4,227.6 Spinal Muscular Atrophy: SPINRAZA 453.9 552.8 1,006.7 393.9 392.7 786.6 Biosimilars: BENEPALI — 244.3 244.3 — 236.5 236.5 IMRALDI — 83.0 83.0 — — — FLIXABI — 31.5 31.5 — 17.8 17.8 Subtotal: Biosimilar product revenues — 358.8 358.8 — 254.3 254.3 Other: FUMADERM — 7.8 7.8 — 12.5 12.5 Total product revenues $ 3,257.7 $ 2,302.6 $ 5,560.3 $ 3,279.8 $ 2,001.2 $ 5,281.0 *Interferon includes AVONEX and PLEGRIDY.
Analysis of change 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, 2018 $ 127.8 $ 888.8 $ 34.7 $ 1,051.3 Current provisions relating to sales in current year 308.8 1,391.0 10.6 1,710.4 Adjustments relating to prior years (0.4 ) (41.9 ) (0.1 ) (42.4 ) Payments/credits relating to sales in current year (189.6 ) (827.6 ) (0.6 ) (1,017.8 ) Payments/credits relating to sales in prior years (121.9 ) (505.2 ) (11.9 ) (639.0 ) Balance, as of June 30, 2019 $ 124.7 $ 905.1 $ 32.7 $ 1,062.5
Total reserves included in 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, net $ 199.0 $ 176.6 Component of accrued expenses and other 863.5 874.7 Total revenue-related reserves $ 1,062.5 $ 1,051.3
Revenues from anti-CD20 therapeutic programs	Revenues from anti-CD20 therapeutic programs are summarized as follows: For the Three Months For the Six Months (In millions) 2019 2018 2019 2018 Biogen’s share of pre-tax profits in the U.S. for RITUXAN, RITUXAN HYCELA and GAZYVA $ 377.2 $ 359.0 $ 768.0 $ 708.6 Other revenues from anti-CD20 therapeutic programs 199.2 131.4 325.8 225.0 Total revenues from anti-CD20 therapeutic programs $ 576.4 $ 490.4 $ 1,093.8 $ 933.6
Other revenues	Other revenues are summarized as follows: For the Three Months For the Six Months (In millions) 2019 2018 2019 2018 Revenues from collaborative and other relationships: (Loss) profit earned under our 50% share of the co-promotion losses on ZINBRYTA in the U.S. with AbbVie $ (0.1 ) $ (2.5 ) $ (0.5 ) $ (7.2 ) Revenues earned under our technical development agreement, manufacturing services agreements and royalty revenues on biosimilar products with Samsung Bioepis 52.2 14.7 77.0 32.6 Other royalty and corporate revenues: Royalty 2.7 17.3 6.6 27.9 Other corporate 105.2 79.1 369.3 219.7 Total other revenues $ 160.0 $ 108.6 $ 452.4 $ 273.0

Inventory (Tables)

Inventory (Tables)	6 Months Ended
	Jun. 30, 2019
Inventory Disclosure [Abstract]
Components of inventory	The components of inventory are summarized as follows: (In millions) As of As of Raw materials $ 205.0 $ 196.3 Work in process 427.6 606.7 Finished goods 144.1 133.5 Total inventory $ 776.7 $ 936.5 Balance Sheet Classification: Inventory $ 776.7 $ 929.9 Investments and other assets — 6.6 Total inventory $ 776.7 $ 936.5

Intangible Assets and Goodwill

Intangible Assets and Goodwill (Tables)	6 Months Ended
	Jun. 30, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible assets	Intangible assets, net of accumulated amortization, impairment charges and adjustments, are summarized as follows: As of June 30, 2019 As of December 31, 2018 (In millions) Estimated Life Cost Accumulated Amortization Net Cost Accumulated Amortization Net Out-licensed patents 13-23 years $ 543.3 $ (542.4 ) $ 0.9 $ 543.3 $ (542.3 ) $ 1.0 Developed technology 15-23 years 3,005.3 (2,754.1 ) 251.2 3,005.3 (2,734.8 ) 270.5 In-process research and development Indefinite until commercialization 1,175.6 — 1,175.6 476.0 — 476.0 Trademarks and tradenames Indefinite 64.0 — 64.0 64.0 — 64.0 Acquired and in-licensed rights and patents 4-18 years 3,638.7 (1,449.1 ) 2,189.6 3,638.7 (1,330.2 ) 2,308.5 Total intangible assets $ 8,426.9 $ (4,745.6 ) $ 3,681.3 $ 7,727.3 $ (4,607.3 ) $ 3,120.0
Estimated future amortization for acquired intangible assets	Based upon this most recent analysis, the estimated future amortization of acquired intangible assets for the next five years is expected to be as follows: (In millions) As of 2019 (remaining six months) $ 130.0 2020 255.0 2021 215.0 2022 215.0 2023 220.0 2024 210.0
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 $ 5,706.4 Increase to goodwill 112.6 Reclassification of goodwill to assets held for sale (69.5 ) Other (0.3 ) Goodwill, end of period $ 5,749.2

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 2019
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 June 30, 2019 (In millions) Total Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets: Cash equivalents $ 1,120.4 $ — $ 1,120.4 $ — Marketable debt securities: Corporate debt securities 1,687.7 — 1,687.7 — Government securities 604.4 — 604.4 — Mortgage and other asset backed securities 246.0 — 246.0 — Marketable equity securities 510.0 176.6 333.4 — Derivative contracts 88.2 — 88.2 — Plan assets for deferred compensation 29.5 — 29.5 — Total $ 4,286.2 $ 176.6 $ 4,109.6 $ — Liabilities: Derivative contracts $ 17.5 $ — $ 17.5 $ — Contingent consideration obligations 401.3 — — 401.3 Total $ 418.8 $ — $ 17.5 $ 401.3 As of December 31, 2018 (In millions) Total Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets: Cash equivalents $ 705.5 $ — $ 705.5 $ — Marketable debt securities: Corporate debt securities 2,459.2 — 2,459.2 — Government securities 969.6 — 969.6 — Mortgage and other asset backed securities 260.5 — 260.5 — Marketable equity securities 615.4 51.7 563.7 — Derivative contracts 66.9 — 66.9 — Plan assets for deferred compensation 25.4 — 25.4 — Total $ 5,102.5 $ 51.7 $ 5,050.8 $ — Liabilities: Derivative contracts $ 24.6 $ — $ 24.6 $ — Contingent consideration obligations 409.8 — — 409.8 Total $ 434.4 $ — $ 24.6 $ 409.8
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 June 30, 2019 As of December 31, 2018 (In millions) Fair Value Carrying Value Fair Value Carrying Value 2.900% Senior Notes due September 15, 2020 $ 1,507.5 $ 1,491.1 $ 1,489.5 $ 1,480.8 3.625% Senior Notes due September 15, 2022 1,032.3 996.0 1,000.4 995.5 4.050% Senior Notes due September 15, 2025 1,867.4 1,738.7 1,745.1 1,737.8 5.200% Senior Notes due September 15, 2045 1,953.2 1,722.7 1,802.6 1,722.4 Total $ 6,360.4 $ 5,948.5 $ 6,037.6 $ 5,936.5
Fair value of contingent consideration obligations	The following table provides a roll forward of the fair values of our contingent consideration obligations, which includes Level 3 measurements: For the Three Months For the Six Months (In millions) 2019 2018 2019 2018 Fair value, beginning of period $ 421.3 $ 498.0 $ 409.8 $ 523.6 Changes in fair value (20.0 ) 1.9 (8.5 ) (3.7 ) Payments — — — (20.0 ) Fair value, end of period $ 401.3 $ 499.9 $ 401.3 $ 499.9

Financial Instruments (Tables)

Financial Instruments (Tables)	6 Months Ended
	Jun. 30, 2019
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 in our condensed consolidated balance sheets: (In millions) As of As of Commercial paper $ 338.7 $ 231.2 Overnight reverse repurchase agreements 97.0 — Money market funds 405.7 279.5 Short-term debt securities 279.0 194.8 Total $ 1,120.4 $ 705.5
Marketable debt and equity securities	The following tables summarize our marketable debt and equity securities: As of June 30, 2019 (In millions) Amortized Gross Unrealized Gains Gross Unrealized Losses Fair Value Corporate debt securities Current $ 933.4 $ 1.1 $ — $ 934.5 Non-current 748.9 4.4 (0.1 ) 753.2 Government securities Current 293.6 0.4 — 294.0 Non-current 309.7 0.9 (0.2 ) 310.4 Mortgage and other asset backed securities Current 0.3 — — 0.3 Non-current 244.5 1.5 (0.3 ) 245.7 Total marketable debt securities $ 2,530.4 $ 8.3 $ (0.6 ) $ 2,538.1 Marketable equity securities Current $ 117.9 $ 46.3 $ — $ 164.2 Non-current 111.4 247.9 (13.5 ) 345.8 Total marketable equity securities $ 229.3 $ 294.2 $ (13.5 ) $ 510.0 As of December 31, 2018 (In millions) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Corporate debt securities Current $ 1,608.4 $ — $ (0.9 ) $ 1,607.5 Non-current 854.9 0.7 (3.9 ) 851.7 Government securities Current 706.1 0.1 (0.4 ) 705.8 Non-current 264.0 0.1 (0.3 ) 263.8 Mortgage and other asset backed securities Current 0.1 — — 0.1 Non-current 260.5 0.4 (0.5 ) 260.4 Total marketable debt securities $ 3,694.0 $ 1.3 $ (6.0 ) $ 3,689.3 Marketable equity securities, non-current $ 496.2 $ 127.7 $ (8.5 ) $ 615.4
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 June 30, 2019 As of December 31, 2018 (In millions) Amortized Estimated Amortized Estimated Fair Value Due in one year or less $ 1,227.3 $ 1,228.8 $ 2,314.6 $ 2,313.4 Due after one year through five years 1,196.1 1,202.2 1,235.9 1,232.7 Due after five years 107.0 107.1 143.5 143.2 Total available-for-sale securities $ 2,530.4 $ 2,538.1 $ 3,694.0 $ 3,689.3
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 Six Months (In millions) 2019 2018 2019 2018 Proceeds from maturities and sales $ 1,766.6 $ 2,733.7 $ 3,255.8 $ 6,802.7 Realized gains $ 1.0 $ 0.8 $ 1.6 $ 2.6 Realized losses $ (0.3 ) $ (0.8 ) $ (0.6 ) $ (10.2 )

Derivative Instruments (Tables)

Derivative Instruments (Tables)	6 Months Ended
	Jun. 30, 2019
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 (In millions) As of As of Euro $ 2,111.3 $ 1,701.4 British pound 117.2 215.3 Swiss franc 69.7 131.4 Japanese yen 56.0 98.8 Canadian dollar 48.1 92.2 Total foreign currency forward contracts $ 2,402.3 $ 2,239.1
Summary of the effect of cash flow 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 in our condensed consolidated statements of income: For the Three Months Ended June 30, Net Gains/(Losses) Reclassified from AOCI into Operating Income (in millions) Net Gains/(Losses) Recognized in Operating Income (in millions) Location 2019 2018 Location 2019 2018 Revenues $ 29.8 $ (10.4 ) Revenues $ 3.7 $ 7.9 Operating expenses $ (0.7 ) $ (0.4 ) Operating expenses $ (0.3 ) $ (0.1 ) For the Six Months Ended June 30, Net Gains/(Losses) Reclassified from AOCI into Operating Income (in millions) Net Gains/(Losses) Recognized in Operating Income (in millions) Location 2019 2018 Location 2019 2018 Revenues $ 44.6 $ (43.3 ) Revenues $ 7.4 $ 7.0 Operating expenses $ (1.2 ) $ 0.9 Operating expenses $ (1.2 ) $ (0.4 )
Summary of the effect of derivatives designated as net investment hedging instruments on our consolidated statement of income	The following tables summarize the effect of our net investment hedge in our condensed consolidated financial statements: For the Three Months Ended June 30, Net Gains/(Losses) Recognized in Other Comprehensive Income (Effective Portion) (in millions) Net Gains/(Losses) Recognized in Other Comprehensive Income (Amounts Excluded from Effectiveness Testing) (in millions) Net Gains/(Losses) Recognized in Net Income (Amounts Excluded from Effectiveness Testing) (in millions) Location 2019 2018 Location 2019 2018 Location 2019 2018 Gains (losses) on net investment hedge $ 11.6 $ — Gains (losses) on net investment hedge $ 2.3 $ — Other income (expense) $ 2.2 $ — For the Six Months Ended June 30, Net Gains/(Losses) Recognized in Other Comprehensive Income (Effective Portion) (in millions) Net Gains/(Losses) Recognized in Other Comprehensive Income (Amounts Excluded from Effectiveness Testing) (in millions) Net Gains/(Losses) Recognized in Net Income (Amounts Excluded from Effectiveness Testing) (in millions) Location 2019 2018 Location 2019 2018 Location 2019 2018 Gains (losses) on net investment hedge $ 23.4 $ — Gains (losses) on net investment hedge $ 6.7 $ — Other income (expense) $ 4.4 $ —
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 derivative instruments, including those designated as hedging instruments: (In millions) Balance Sheet Location As of As of Cash Flow Hedging Instruments: Asset derivative instruments Other current assets $ 81.8 $ 65.8 Liability derivative instruments Accrued expenses and other $ 5.5 $ 6.9 Other long-term liabilities $ 2.2 $ — Fair Value Hedging Instruments: Liability derivative instruments Other long-term liabilities $ 5.5 $ 14.5 Other Derivatives: Asset derivative instruments Other current assets $ 6.4 $ 1.1 Liability derivative instruments Accrued expenses and other $ 4.3 $ 3.2

Leases (Tables)

Leases (Tables)	6 Months Ended
	Jun. 30, 2019
Leases [Abstract]
Schedule of operating leases	The following table summarizes the presentation in our condensed consolidated balance sheets of our operating leases: (In millions) Balance sheet location As of June 30, 2019 Assets: Operating lease assets Operating lease assets $ 434.4 Liabilities Current operating lease liabilities Accrued expenses and other $ 72.3 Non-current operating lease liabilities Long-term operating lease liabilities 423.0 Total operating lease liabilities $ 495.3
Operating lease cost	The following table summarizes the effect of lease costs in our condensed consolidated statements of income: For the Three Months For the Six Months (In millions) Income Statement Location 2019 2019 Operating lease cost Research and development $ 3.4 $ 3.7 Selling, general and administrative 17.9 41.4 Sublease income Selling, general and administrative (6.0 ) (13.1 ) Other (income) expense, net (0.9 ) (1.9 ) Net lease cost $ 14.4 $ 30.1
Operating lease liability maturity	The minimum lease payments for the next five years and thereafter is expected to be as follows: (In millions) As of 2019 (remaining six months) $ 50.2 2020 80.5 2021 74.9 2022 70.9 2023 69.4 2024 66.7 Thereafter 147.6 Total lease payments $ 560.2 Less: interest 64.9 Present value of operating lease liabilities $ 495.3
Minimum lease payments	Under the prior lease guidance minimum rental commitments under non-cancelable leases, net of income from subleases, for each of the next five years and total thereafter as of December 31, 2018, were as follows: (In millions) 2019 2020 2021 2022 2023 Thereafter Total Minimum lease payments $ 87.0 $ 80.7 $ 75.9 $ 71.7 $ 71.0 $ 215.3 $ 601.6 Less: income from subleases (1) (26.8 ) (25.6 ) (23.7 ) (24.0 ) (24.3 ) (58.4 ) (182.8 ) Net minimum lease payments $ 60.2 $ 55.1 $ 52.2 $ 47.7 $ 46.7 $ 156.9 $ 418.8 (1)
Operating lease weighted average remaining term and discount rate	The weighted average remaining lease term and weighted average discount rate of our operating leases are as follows: As of June 30, 2019 Weighted average remaining lease term in years 7.4 Weighted average discount rate 3.3 %
Operating lease supplemental cash flow disclosure	Supplemental disclosure of cash flow information related to our operating leases included in cash flows provided by operating activities in our condensed consolidated statements of cash flows is as follows: For the Three Months For the Six Months (In millions) 2019 2019 Cash paid for amounts included in the measurement of lease liabilities $ 27.4 $ 46.8 Operating lease assets obtained in exchange for lease obligations $ 7.3 $ 12.7

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income (Loss) (Tables)	6 Months Ended
	Jun. 30, 2019
Equity [Abstract]
Schedule of Accumulated Other Comprehensive Income (Loss)	The following tables summarize 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 Gains (Losses) on Net Investment Hedge Unfunded Status of Postretirement Benefit Plans, Net of Tax Currency Translation Adjustments Total Balance, December 31, 2018 $ (4.0 ) $ 34.7 $ 3.5 $ (31.3 ) $ (243.3 ) $ (240.4 ) Other comprehensive income (loss) before reclassifications 11.0 22.3 30.1 0.7 (28.1 ) 36.0 Amounts reclassified from accumulated other comprehensive income (loss) (0.8 ) (43.3 ) (4.4 ) — — (48.5 ) Net current period other comprehensive income (loss) 10.2 (21.0 ) 25.7 0.7 (28.1 ) (12.5 ) Balance, June 30, 2019 $ 6.2 $ 13.7 $ 29.2 $ (30.6 ) $ (271.4 ) $ (252.9 ) (In millions) Unrealized Gains (Losses) on Securities Available for Sale, Net of Tax Unrealized Gains (Losses) on Cash Flow Hedges, Net of Tax Gains (Losses) on Net Investment Hedge Unfunded Status of Postretirement Benefit Plans, Net of Tax Currency Translation Adjustments Total Balance, December 31, 2017 $ (1.6 ) $ (104.5 ) $ — $ (36.8 ) $ (175.5 ) $ (318.4 ) Amounts reclassified, net of tax, upon adoption of ASU No. 2016-01 1.5 — — — — 1.5 Balance, January 1, 2018 (0.1 ) (104.5 ) — (36.8 ) (175.5 ) (316.9 ) Other comprehensive income (loss) before reclassifications (7.6 ) 61.7 — 0.4 (47.3 ) 7.2 Amounts reclassified from accumulated other comprehensive income (loss) 6.0 42.1 — — — 48.1 Net current period other comprehensive income (loss) (1.6 ) 103.8 — 0.4 (47.3 ) 55.3 Balance, June 30, 2018 $ (1.7 ) $ (0.7 ) $ — $ (36.4 ) $ (222.8 ) $ (261.6 )
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 Six Months 2019 2018 2019 2018 Gains (losses) on securities available for sale Other income (expense) $ 0.7 $ — $ 1.0 $ (7.6 ) Income tax benefit (expense) (0.1 ) — (0.2 ) 1.6 Gains (losses) on cash flow hedges Revenues 29.8 (10.4 ) 44.6 (43.3 ) Operating expenses (0.7 ) (0.4 ) (1.2 ) 0.9 Other income (expense) — — 0.1 0.1 Income tax benefit (expense) (0.1 ) 0.1 (0.2 ) 0.2 Gains (losses) on net investment hedge Other income (expense) 2.2 — 4.4 — Income tax benefit (expense) — — — — Total reclassifications, net of tax $ 31.8 $ (10.7 ) $ 48.5 $ (48.1 )

Earnings per Share (Tables)

Earnings per Share (Tables)	6 Months Ended
	Jun. 30, 2019
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 Six Months (In millions) 2019 2018 2019 2018 Numerator: Net income attributable to Biogen Inc. $ 1,494.1 $ 866.6 $ 2,902.9 $ 2,039.5 Denominator: Weighted average number of common shares outstanding 190.3 207.1 193.4 209.2 Effect of dilutive securities: Stock options and employee stock purchase plan — — — — Time-vested restricted stock units 0.1 0.1 0.2 0.2 Market stock units — 0.1 0.1 0.1 Performance stock units settled in stock — — — — Dilutive potential common shares 0.1 0.2 0.3 0.3 Shares used in calculating diluted earnings per share 190.4 207.3 193.7 209.5

Share-Based Payments (Tables)

Share-Based Payments (Tables)	6 Months Ended
	Jun. 30, 2019
Share-based Payment Arrangement [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 Six Months (In millions) 2019 2018 2019 2018 Research and development $ 28.1 $ 17.6 $ 49.8 $ 39.5 Selling, general and administrative 54.8 25.2 82.6 53.7 Subtotal 82.9 42.8 132.4 93.2 Capitalized share-based compensation costs (2.8 ) (2.8 ) (6.1 ) (6.2 ) Share-based compensation expense included in total cost and expenses 80.1 40.0 126.3 87.0 Income tax effect (13.6 ) (6.5 ) (21.0 ) (14.1 ) Share-based compensation expense included in net income attributable to Biogen Inc. $ 66.5 $ 33.5 $ 105.3 $ 72.9
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 Six Months (In millions) 2019 2018 2019 2018 Market stock units $ 7.8 $ 7.1 $ 15.5 $ 13.2 Time-vested restricted stock units 31.8 30.6 66.5 66.7 Cash settled performance units 0.4 (0.7 ) (0.6 ) 3.6 Performance units 0.3 2.0 0.8 1.2 Performance stock units settled in stock 12.1 1.4 14.1 2.1 Performance stock units settled in cash 0.9 0.3 1.9 0.4 Employee stock purchase plan 3.4 2.1 8.0 6.0 NST stock options 26.2 — 26.2 — Subtotal 82.9 42.8 132.4 93.2 Capitalized share-based compensation costs (2.8 ) (2.8 ) (6.1 ) (6.2 ) Share-based compensation expense included in total cost and expenses $ 80.1 $ 40.0 $ 126.3 $ 87.0

Income Taxes (Tables)

Income Taxes (Tables)	6 Months Ended
	Jun. 30, 2019
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 Six Months 2019 2018 2019 2018 Statutory rate 21.0 % 21.0 % 21.0 % 21.0 % State taxes 0.9 0.6 0.6 0.8 Taxes on foreign earnings (4.6 ) (1.8 ) (4.6 ) (2.0 ) Credits and net operating loss utilization (0.9 ) (0.7 ) (0.8 ) (0.8 ) Purchased intangible assets 0.4 0.6 0.4 0.6 Denmark assets held for sale — — 2.2 — Internal reorganization of certain intellectual property rights (5.0 ) — (2.4 ) — Global Intangible Low-Taxed Income (GILTI) 1.4 1.8 1.7 1.6 Other permanent items 0.4 0.4 0.3 0.4 Other 0.5 0.5 0.1 0.3 Effective tax rate 14.1 % 22.4 % 18.5 % 21.9 %

Other Consolidated Financial _2

Other Consolidated Financial Statement Detail (Tables)	6 Months Ended
	Jun. 30, 2019
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 Six Months (In millions) 2019 2018 2019 2018 Interest income $ 29.1 $ 28.7 $ 60.3 $ 55.4 Interest expense (47.7 ) (51.7 ) (95.6 ) (102.2 ) Gain (loss) on investments, net (173.4 ) 5.3 203.0 (9.1 ) Foreign exchange gains (losses), net 1.7 (13.0 ) (0.5 ) (14.0 ) Other, net (7.1 ) (3.8 ) (7.3 ) (5.6 ) Total other income (expense), net $ (197.4 ) $ (34.5 ) $ 159.9 $ (75.5 )
Gain (loss) on investments in equity securities	The following table summarizes our gain (loss) on investments, net that relates to our equity securities held as of June 30, 2019 : For the Three Months For the Six Months (In millions) 2019 2018 2019 2018 Net gains (losses) recognized during the period on equity securities (174.2 ) 5.4 201.9 (1.5 ) Less: Net gains (losses) recognized during the period on equity securities sold during the period (42.9 ) — 42.4 (0.5 ) Unrealized gains (losses) recognized during the period on equity securities held as of June 30, 2019 (131.3 ) 5.4 159.5 (1.0 )
Accrued expenses and other	Accrued expenses and other consists of the following: (In millions) As of As of Revenue-related reserves for discounts and allowances $ 863.5 $ 874.7 Royalties and licensing fees 212.8 224.7 Employee compensation and benefits 199.4 320.9 Current portion of contingent consideration obligations 146.2 444.8 Collaboration expenses 130.4 261.6 Construction in progress 38.2 125.2 Other 727.5 609.3 Total accrued expenses and other $ 2,318.0 $ 2,861.2

Collaborative and Other Relat_2

Collaborative and Other Relationships (Tables)	6 Months Ended
	Jun. 30, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Summary of Activity Related to BAN2401 and Elenbecestat Collaboration	A summary of development expense related to the BAN2401 and Elenbecestat Collaboration is as follows: For the Three Months For the Six Months (In millions) 2019 2018 2019 2018 Total development expense incurred by the collaboration related to the advancement of BAN2401 and Elenbecestat $ 68.6 $ 54.6 $ 136.6 $ 111.1 Biogen's share of BAN2401 and Elenbecestat development expense reflected in research and development expense in our condensed consolidated statements of income $ 34.3 $ 27.3 $ 68.3 $ 55.5
Summary of Activity Related to Aducanumab Collaboration	A summary of development and sales and marketing expense related to the Aducanumab Collaboration Agreement is as follows: For the Three Months For the Six Months (In millions) 2019 2018 2019 2018 Total aducanumab development expense $ 3.2 $ 76.5 $ 165.8 $ 140.0 Biogen's share of aducanumab development expense reflected in research and development expense in our condensed consolidated statements of income $ 1.7 $ 65.0 $ 91.2 $ 128.5 Total aducanumab sales and marketing expense incurred by the collaboration $ 0.2 $ 14.0 $ 21.2 $ 21.2 Biogen's share of aducanumab sales and marketing expense reflected in selling, general and administrative expense in our condensed consolidated statements of income $ 0.1 $ 9.2 $ 11.7 $ 13.9

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Details) $ in Millions	6 Months Ended
	Jun. 30, 2019USD ($)segment	Jan. 01, 2019USD ($)	Dec. 31, 2018USD ($)
Operating lease assets	$ 434.4		$ 0
Operating lease liabilities	$ 495.3
Number of reportable segments | segment	1
Interest in subsidiary (less than given percentage)	100.00%
Accounting Standards Update 2016-02
Operating lease assets		$ 463
Operating lease liabilities		$ 526

Business Acquisition (Details)

Business Acquisition (Details) - USD ($) $ in Millions	Jun. 30, 2019	Jun. 07, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Goodwill	$ 5,749.2		$ 5,706.4
Nightstar
Business Acquisition [Line Items]
Cash and Equivalents		$ 107.8
Marketable Securities		7.5
In-process research and development		700
Goodwill		112.6
Deferred Tax Liability		(77)
Other, net		1.3
Total purchase price		$ 852.2

Business Acquisition (Details T

Business Acquisition (Details Textual) - Nightstar - USD ($) $ / shares in Units, $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 07, 2019
Business Acquisition [Line Items]
Price per share		$ 25.50
Total transaction value	$ 847.6
Payments for pre-combination equity compensation	4.6
Fair value, in-process research and development, discount rate		12.50%
In-process research and development		$ 700
Deferred tax liabilities, goodwill	35
Selling, general and administrative
Business Acquisition [Line Items]
Fair value of post-combination equity compensation	18.4
Research and development
Business Acquisition [Line Items]
Fair value of post-combination equity compensation	7.8
BIIB111
Business Acquisition [Line Items]
In-process research and development		480
BIIB112
Business Acquisition [Line Items]
In-process research and development		$ 220
Post-acquisition equity compensation
Business Acquisition [Line Items]
Fair value of post-combination equity compensation	$ 26.2

Divestitures (Details)

Divestitures (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Long Lived Assets and Liabilities Held-for-sale [Line Items]
Loss on assets and liabilities held for sale	$ 2.3	$ 0	$ (113.2)	$ 0
Denmark Manufacturing Operations
Long Lived Assets and Liabilities Held-for-sale [Line Items]
Property, plant and equipment	641.9		641.9
Operating lease assets	2.2		2.2
Goodwill	69.5		69.5
Other assets	62.4		62.4
Loss on assets and liabilities held for sale			(113.2)
Assets held for sale	683.4		683.4
Accrued expenses and other liabilities	33.4		33.4
Operating lease liabilities	2.2		2.2
Deferred tax liability	53.8		53.8
Liabilities held for sale	88.6		88.6
Long-term operating lease liabilities | Denmark Manufacturing Operations
Long Lived Assets and Liabilities Held-for-sale [Line Items]
Operating lease liabilities	$ 1.4		$ 1.4

Divestitures (Details Textual)

Divestitures (Details Textual) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loss on assets and liabilities held for sale	$ (2.3)	$ 0	$ 113.2	$ 0
Denmark Manufacturing Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Consideration expected to be received for sale of Denmark Manufacturing Operations	890		890
Loss on assets and liabilities held for sale, net of tax	2.3		174.5
Loss on assets and liabilities held for sale			113.2
Expected costs to sell disposal group			10
Future minimum batch production for Denmark Manufacturing Operations	$ 120		120
Tax expense on disposal group			$ 61.3

Revenues by Product (Details)

Revenues by Product (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Product revenues	$ 3,616.7	$ 3,356.5	$ 7,106.5	$ 6,487.6
TECFIDERA
Disaggregation of Revenue [Line Items]
Product revenues	1,150.2	1,086.8	2,149	2,073.7
TECFIDERA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	869.8	825.8	1,587.5	1,554.7
TECFIDERA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	280.4	261	561.5	519
Interferon
Disaggregation of Revenue [Line Items]
Product revenues	554.4	625.5	1,055.3	1,175.8
Interferon | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	379.7	444.7	707	816
Interferon | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	174.7	180.8	348.3	359.8
TYSABRI
Disaggregation of Revenue [Line Items]
Product revenues	475.3	467.2	935.7	929.3
TYSABRI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	264.3	265.5	509.3	515.2
TYSABRI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	211	201.7	426.4	414.1
FAMPYRA
Disaggregation of Revenue [Line Items]
Product revenues	24.1	23	47	47.4
FAMPYRA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0	0	0	0
FAMPYRA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	24.1	23	47	47.4
ZINBRYTA
Disaggregation of Revenue [Line Items]
Product revenues			0	1.4
ZINBRYTA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues			0	0
ZINBRYTA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues			0	1.4
MS Product Revenues
Disaggregation of Revenue [Line Items]
Product revenues	2,204	2,202.5	4,187	4,227.6
MS Product Revenues | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	1,513.8	1,536	2,803.8	2,885.9
MS Product Revenues | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	690.2	666.5	1,383.2	1,341.7
SPINRAZA
Disaggregation of Revenue [Line Items]
Product revenues	488.2	422.7	1,006.7	786.6
SPINRAZA | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	230.6	205.9	453.9	393.9
SPINRAZA | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	257.6	216.8	552.8	392.7
BENEPALI
Disaggregation of Revenue [Line Items]
Product revenues	120.3	115.6	244.3	236.5
BENEPALI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0	0	0	0
BENEPALI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	120.3	115.6	244.3	236.5
IMRALDI
Disaggregation of Revenue [Line Items]
Product revenues	47.3	0	83	0
IMRALDI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0	0	0	0
IMRALDI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	47.3	0	83	0
FLIXABI
Disaggregation of Revenue [Line Items]
Product revenues	16.8	11.2	31.5	17.8
FLIXABI | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0	0	0	0
FLIXABI | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	16.8	11.2	31.5	17.8
Biosimilars
Disaggregation of Revenue [Line Items]
Product revenues	184.4	126.8	358.8	254.3
Biosimilars | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0	0	0	0
Biosimilars | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	184.4	126.8	358.8	254.3
FUMADERM
Disaggregation of Revenue [Line Items]
Product revenues	3.7	5.5	7.8	12.5
FUMADERM | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	0	0	0	0
FUMADERM | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	3.7	5.5	7.8	12.5
Product, net
Disaggregation of Revenue [Line Items]
Product revenues	2,880.3	2,757.5	5,560.3	5,281
Product, net | U.S.
Disaggregation of Revenue [Line Items]
Product revenues	1,744.4	1,741.9	3,257.7	3,279.8
Product, net | Rest of World
Disaggregation of Revenue [Line Items]
Product revenues	$ 1,135.9	$ 1,015.6	$ 2,302.6	$ 2,001.2

Reserves for Discounts and Allo

Reserves for Discounts and Allowances (Details 1) $ in Millions	6 Months Ended
	Jun. 30, 2019USD ($)
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	$ 1,051.3
Current Provisions Relating To Sales In Current Year	1,710.4
Adjustments Relating To Prior Years	(42.4)
Payments/Returns Relating To Sales in Current Year	(1,017.8)
Payments/Returns Relating To Sales in Prior Year	(639)
Ending Balance	1,062.5
Discounts
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	127.8
Current Provisions Relating To Sales In Current Year	308.8
Adjustments Relating To Prior Years	(0.4)
Payments/Returns Relating To Sales in Current Year	(189.6)
Payments/Returns Relating To Sales in Prior Year	(121.9)
Ending Balance	124.7
Contractual adjustments
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	888.8
Current Provisions Relating To Sales In Current Year	1,391
Adjustments Relating To Prior Years	(41.9)
Payments/Returns Relating To Sales in Current Year	(827.6)
Payments/Returns Relating To Sales in Prior Year	(505.2)
Ending Balance	905.1
Returns
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	34.7
Current Provisions Relating To Sales In Current Year	10.6
Adjustments Relating To Prior Years	(0.1)
Payments/Returns Relating To Sales in Current Year	(0.6)
Payments/Returns Relating To Sales in Prior Year	(11.9)
Ending Balance	$ 32.7

Reserves for Discounts and Al_2

Reserves for Discounts and Allowances (Details 2) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Total Reserves	$ 1,062.5	$ 1,051.3
Reduction of accounts receivable
Total Reserves	199	176.6
Component of accrued expenses and other
Total Reserves	$ 863.5	$ 874.7

Revenues from Anti-CD20 Therape

Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 3,616.7	$ 3,356.5	$ 7,106.5	$ 6,487.6
Genentech
Disaggregation of Revenue [Line Items]
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA	377.2	359	768	708.6
Other revenues from anti-CD20 therapeutic programs	199.2	131.4	325.8	225
Revenues from anti-CD20 therapeutic programs
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 576.4	$ 490.4	$ 1,093.8	$ 933.6

Other Revenues (Details)

Other Revenues (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Total revenues	$ 3,616.7	$ 3,356.5	$ 7,106.5	$ 6,487.6
Royalty
Disaggregation of Revenue [Line Items]
Total revenues	2.7	17.3	6.6	27.9
Other corporate revenues
Disaggregation of Revenue [Line Items]
Total revenues	105.2	79.1	369.3	219.7
Other
Disaggregation of Revenue [Line Items]
Total revenues	160	108.6	452.4	273
Collaborative arrangement | AbbVie
Disaggregation of Revenue [Line Items]
Total revenues	(0.1)	(2.5)	(0.5)	(7.2)
Collaborative arrangement | Samsung Bioepis
Disaggregation of Revenue [Line Items]
Total revenues	$ 52.2	$ 14.7	$ 77	$ 32.6

Revenues (Details Textual)

Revenues (Details Textual) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Other Revenues	$ 3,616.7	$ 3,356.5	$ 7,106.5	$ 6,487.6
Distributor One
Disaggregation of Revenue [Line Items]
Percentage of revenues from major distributors	30.50%	32.50%	30.90%	33.20%
Distributor Two
Disaggregation of Revenue [Line Items]
Percentage of revenues from major distributors	18.20%	18.60%	16.30%	17.30%
Other
Disaggregation of Revenue [Line Items]
Other Revenues	$ 160	$ 108.6	$ 452.4	$ 273
Other | Bioverativ
Disaggregation of Revenue [Line Items]
Other Revenues	$ 34.5	$ 47.6	$ 241.3	$ 94.6

Inventory (Details)

Inventory (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Components of inventories
Raw materials	$ 205	$ 196.3
Work in process	427.6	606.7
Finished goods	144.1	133.5
Total inventory	776.7	936.5
Inventory, current	776.7	929.9
Inventory, Noncurrent	$ 0	$ 6.6

Inventory (Details Textual)

Inventory (Details Textual) $ in Millions	3 Months Ended
	Mar. 31, 2019USD ($)
Bioverativ
Inventory [Line Items]
Inventory sold to Bioverativ, cost	$ 173.5

Proposed Divestiture of Denmark

Proposed Divestiture of Denmark Manufacturing Operations (Details) $ in Millions	Jun. 30, 2019USD ($)
Denmark Manufacturing Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Consideration expected to be received for sale of Denmark Manufacturing Operations	$ 890
Work-in-process
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Inventory expected to be sold to FUJIFILM	$ 20.6

Intangible Assets and Goodwil_2

Intangible Assets and Goodwill (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 07, 2019	Dec. 31, 2018
Intangible assets
Total intangible assets, gross	$ 8,426.9		$ 8,426.9			$ 7,727.3
Accumulated Amortization	(4,745.6)		(4,745.6)			(4,607.3)
Intangible assets, net	3,681.3		3,681.3			3,120
Amortization and impairment of acquired intangible assets	70.1	$ 107.4	138.3	$ 211.3
Intangible Assets and Goodwill (Textual) [Abstract]
Expected future amortization expense, 2019 (remaining six months)	130		130
Expected future amortization expense, 2020	255		255
Expected future amortization expense, 2021	215		215
Expected future amortization expense, 2022	215		215
Expected future amortization expense, 2023	220		220
Expected future amortization expense, 2024	210		210
Out-licensed patents
Intangible assets
Cost	543.3		543.3			543.3
Net	0.9		0.9			1
Accumulated Amortization	(542.4)		$ (542.4)			(542.3)
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
Net	251.2		251.2			270.5
Accumulated Amortization	(2,754.1)		$ (2,754.1)			(2,734.8)
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	246.6		$ 246.6
Acquired and in-licensed rights and patents
Intangible assets
Cost	3,638.7		3,638.7			3,638.7
Net	2,189.6		2,189.6			2,308.5
Accumulated Amortization	(1,449.1)		$ (1,449.1)			(1,330.2)
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 | TYSABRI
Intangible assets
Net	1,933.3		$ 1,933.3
Acquired and in-licensed rights and patents | TECFIDERA
Intangible assets
Net	55.2		$ 55.2
In Process Research and Development
Intangible assets
Indefinite lived intangible assets useful life			Indefinite until commercialization
Cost and Net	1,175.6		$ 1,175.6			476
Accumulated Amortization	0		$ 0			0
Trademarks and Trade Names
Intangible assets
Indefinite lived intangible assets useful life			Indefinite
Cost and Net	64		$ 64			64
Accumulated Amortization	$ 0		$ 0			$ 0
Nightstar
Intangible assets
In-process research and development					$ 700

Intangible Assets and Goodwil_3

Intangible Assets and Goodwill (Details 1) $ in Millions	6 Months Ended
	Jun. 30, 2019USD ($)
Goodwill [Line Items]
Accumulated impairment losses related to goodwill	$ 0
Summary of roll forward of the changes in goodwill
Goodwill, beginning of period	5,706.4
Other	(0.3)
Goodwill, end of period	5,749.2
Denmark Manufacturing Operations
Summary of roll forward of the changes in goodwill
Goodwill held for sale	$ (69.5)

Fair Value Measurements (Detail

Fair Value Measurements (Details) - USD ($) $ in Millions	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Assets:
Marketable debt securities	$ 2,538.1		$ 3,689.3
Fair Value, Measurements, Recurring
Assets:
Cash equivalents	1,120.4		705.5
Marketable equity securities	510		615.4
Derivative contracts	88.2		66.9
Plan assets for deferred compensation	29.5		25.4
Total	4,286.2		5,102.5
Liabilities:
Derivative contracts	17.5		24.6
Contingent consideration obligations	401.3	$ 421.3	409.8	$ 499.9	$ 498	$ 523.6
Total	418.8		434.4
Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	1,687.7		2,459.2
Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	604.4		969.6
Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	246		260.5
Quoted Prices in Active Markets (Level 1) | Fair Value, Measurements, Recurring
Assets:
Cash equivalents	0		0
Marketable equity securities	176.6		51.7
Derivative contracts	0		0
Plan assets for deferred compensation	0		0
Total	176.6		51.7
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	1,120.4		705.5
Marketable equity securities	333.4		563.7
Derivative contracts	88.2		66.9
Plan assets for deferred compensation	29.5		25.4
Total	4,109.6		5,050.8
Liabilities:
Derivative contracts	17.5		24.6
Contingent consideration obligations	0		0
Total	17.5		24.6
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	1,687.7		2,459.2
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	604.4		969.6
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	246		260.5
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	401.3		409.8
Total	401.3		409.8
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 (Deta_2

Fair Value Measurements (Details 1) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Notes payable, carrying value	$ 5,948.5	$ 5,936.5
Debt instruments, fair value	6,360.4	6,037.6
2.900% Senior Notes due 2020
Debt Instrument [Line Items]
Notes payable, fair value	1,507.5	1,489.5
Notes payable, carrying value	1,491.1	1,480.8
3.625% Senior Notes due 2022
Debt Instrument [Line Items]
Notes payable, fair value	1,032.3	1,000.4
Notes payable, carrying value	996	995.5
4.050% Senior Notes due 2025
Debt Instrument [Line Items]
Notes payable, fair value	1,867.4	1,745.1
Notes payable, carrying value	1,738.7	1,737.8
5.200% Senior Notes due 2045
Debt Instrument [Line Items]
Notes payable, fair value	1,953.2	1,802.6
Notes payable, carrying value	$ 1,722.7	$ 1,722.4

Fair Value Measurements (Deta_3

Fair Value Measurements (Details 2) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration	$ (20)	$ 1.9	$ (8.5)	$ (3.7)
Payments	0	0	0	(20)
Fair Value, Measurements, Recurring
Business Acquisition, Contingent Consideration [Line Items]
Fair value, beginning of period	421.3	498	409.8	523.6
Fair value, end of period	$ 401.3	$ 499.9	$ 401.3	$ 499.9

Fair Value Measurements (Deta_4

Fair Value Measurements (Details Textual) - USD ($)	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Business Acquisition, Contingent Consideration [Line Items]
Asset impairment charges	$ 0
Fair value measurements, changes in valuation inputs	0
Contingent consideration obligations	$ 255,100,000	$ 265,000,000
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	Jun. 30, 2019	Dec. 31, 2018
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$ 1,120.4	$ 705.5
Commercial paper
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	338.7	231.2
Overnight reverse repurchase agreements
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	97	0
Money market funds
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	405.7	279.5
Short-term debt securities
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$ 279	$ 194.8

Financial Instruments (Details

Financial Instruments (Details 1) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 2,530.4	$ 3,694
Gross unrealized gains	8.3	1.3
Gross unrealized losses	(0.6)	(6)
Fair value	2,538.1	3,689.3
Marketable equity securities, fair value	2,538.1	3,689.3
Corporate debt securities Current
Debt Securities, Available-for-sale [Line Items]
Amortized cost	933.4	1,608.4
Gross unrealized gains	1.1	0
Gross unrealized losses	0	(0.9)
Fair value	934.5	1,607.5
Corporate debt securities Non-current
Debt Securities, Available-for-sale [Line Items]
Amortized cost	748.9	854.9
Gross unrealized gains	4.4	0.7
Gross unrealized losses	(0.1)	(3.9)
Fair value	753.2	851.7
Government securities Current
Debt Securities, Available-for-sale [Line Items]
Amortized cost	293.6	706.1
Gross unrealized gains	0.4	0.1
Gross unrealized losses	0	(0.4)
Fair value	294	705.8
Government securities Non-current
Debt Securities, Available-for-sale [Line Items]
Amortized cost	309.7	264
Gross unrealized gains	0.9	0.1
Gross unrealized losses	(0.2)	(0.3)
Fair value	310.4	263.8
Mortgage and other asset backed securities Current
Debt Securities, Available-for-sale [Line Items]
Amortized cost	0.3	0.1
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair value	0.3	0.1
Mortgage and other asset backed securities Non-current
Debt Securities, Available-for-sale [Line Items]
Amortized cost	244.5	260.5
Gross unrealized gains	1.5	0.4
Gross unrealized losses	(0.3)	(0.5)
Fair value	245.7	260.4
Marketable equity securities
Debt Securities, Available-for-sale [Line Items]
Marketable equity securities, amortized cost	229.3	496.2
Marketable equity securities, gross unrealized gains	294.2	127.7
Marketable equity securities, gross unrealized losses	(13.5)	(8.5)
Marketable equity securities, fair value	510	$ 615.4
Other current assets | Marketable equity securities
Debt Securities, Available-for-sale [Line Items]
Marketable equity securities, amortized cost	117.9
Marketable equity securities, gross unrealized gains	46.3
Marketable equity securities, gross unrealized losses	0
Marketable equity securities, fair value	164.2
Other noncurrent assets | Marketable equity securities
Debt Securities, Available-for-sale [Line Items]
Marketable equity securities, amortized cost	111.4
Marketable equity securities, gross unrealized gains	247.9
Marketable equity securities, gross unrealized losses	(13.5)
Marketable equity securities, fair value	$ 345.8

Financial Instruments (Detail_2

Financial Instruments (Details 2) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Summary of Contractual Maturities: Available-for-Sale Securities
Due in one year or less, amortized cost	$ 1,227.3	$ 2,314.6
Due in one year or less, estimated fair value	1,228.8	2,313.4
Due after one year through five years, amortized cost	1,196.1	1,235.9
Due after one year through five years, estimated fair value	1,202.2	1,232.7
Due after five years, amortized cost	107	143.5
Due after five years, estimated fair value	107.1	143.2
Amortized cost	2,530.4	3,694
Available-for-sale Securities	$ 2,538.1	$ 3,689.3

Financial Instruments (Detail_3

Financial Instruments (Details 3) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Proceeds from maturities and sales	$ 1,766.6	$ 2,733.7	$ 3,255.8	$ 6,802.7
Realized gains	1	0.8	1.6	2.6
Realized losses	$ (0.3)	$ (0.8)	$ (0.6)	$ (10.2)

Financial Instruments (Detail_4

Financial Instruments (Details Textual)	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Average maturity of marketable securities, months	14 months	12 months

Financial Instruments Financial

Financial Instruments Financial Instruments (Details Textual 2) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Business Acquisition [Line Items]
Proceeds from sales of strategic investments	$ 309.7	$ 0
Strategic Investments
Business Acquisition [Line Items]
Strategic investment portfolio	569.8
Ionis Pharmaceuticals
Business Acquisition [Line Items]
Proceeds from sales of strategic investments	213.3
Other current assets | Strategic Investments
Business Acquisition [Line Items]
Strategic investment portfolio	$ 164.2
Other noncurrent assets | Strategic Investments
Business Acquisition [Line Items]
Strategic investment portfolio			$ 676.3

Derivative Instruments (Details

Derivative Instruments (Details) $ in Millions, ₩ in Billions	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2018KRW (₩)	Jun. 30, 2018USD ($)	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)	Dec. 31, 2018USD ($)	Sep. 30, 2018
Derivatives [Line Items]
Gain/Loss on fair value of foreign currency forward contracts	$ 6.3	$ (27.3)			$ 6.3		$ (27.3)
Gains (losses) on net investment hedges	11.7			$ 0	25.7	$ 0
Net gains (losses) of other income (expense) related to foreign currency forward contracts	(0.9)			5.2	$ (3.9)	0.4
Minimum
Derivatives [Line Items]
Range of durations of foreign currency forward contracts					1 month		1 month
Maximum
Derivatives [Line Items]
Range of durations of foreign currency forward contracts					18 months		12 months
Not designated as hedging instrument
Derivatives [Line Items]
Aggregate notional amount	934.4	735.1			$ 934.4		$ 735.1
Other current assets | Designated as hedging instrument
Derivatives [Line Items]
Derivative asset, fair value	81.8	65.8			81.8		65.8
Other current assets | Not designated as hedging instrument
Derivatives [Line Items]
Derivative asset, fair value	6.4	1.1			6.4		1.1
Accrued expenses and other | Designated as hedging instrument
Derivatives [Line Items]
Derivative liability, fair value	5.5	6.9			5.5		6.9
Accrued expenses and other | Not designated as hedging instrument
Derivatives [Line Items]
Derivative liability, fair value	4.3	3.2			4.3		3.2
Other long-term liabilities | Designated as hedging instrument
Derivatives [Line Items]
Derivative liability, fair value	2.2	0			2.2		0
Foreign exchange contract | Designated as hedging instrument
Derivatives [Line Items]
Aggregate notional amount	2,402.3	2,239.1			2,402.3		2,239.1
Interest rate swap | Designated as hedging instrument
Derivatives [Line Items]
Aggregate notional amount	675				$ 675
Net Investment Hedging
Derivatives [Line Items]
Remaining duration of Net Investment Hedges					4 months
Unrealized gain (loss) on net investment hedges in AOCI					$ 19.6		3.8
Derivative qualifying as net investment hedge, excluded component	9.6	7.3			9.6		7.3
Gains (losses) on net investment hedges	11.6			0	23.4	0
Gains (losses) on net investment hedge, excluded component	2.3			0	6.7	0
Revenue | Cash flows, revenue | Foreign exchange contract
Derivatives [Line Items]
Gain (loss) on reclassification of cash flow hedges, effective portion	29.8			(10.4)	44.6	(43.3)
Gain (loss) recognized in net income, excluded component	3.7			7.9	7.4	7
Operating expense | Cash flows, operating expenses | Foreign exchange contract
Derivatives [Line Items]
Gain (loss) on reclassification of cash flow hedges, effective portion	(0.7)			(0.4)	(1.2)	0.9
Gain (loss) recognized in net income, excluded component	(0.3)			(0.1)	(1.2)	(0.4)
Other income (expense) | Net Investment Hedging
Derivatives [Line Items]
Gain (loss) recognized in net income, excluded component	2.2			$ 0	4.4	$ 0
Euro | Foreign exchange contract | Designated as hedging instrument
Derivatives [Line Items]
Aggregate notional amount	2,111.3	1,701.4			2,111.3		1,701.4
British pound | Foreign exchange contract | Designated as hedging instrument
Derivatives [Line Items]
Aggregate notional amount	117.2	215.3			117.2		215.3
Swiss franc | Foreign exchange contract | Designated as hedging instrument
Derivatives [Line Items]
Aggregate notional amount	69.7	131.4			69.7		131.4
Japan, Yen | Foreign exchange contract | Designated as hedging instrument
Derivatives [Line Items]
Aggregate notional amount	56	98.8			56		98.8
Canadian dollar | Foreign exchange contract | Designated as hedging instrument
Derivatives [Line Items]
Aggregate notional amount	$ 48.1	92.2			$ 48.1		92.2
2.900% Senior Notes due 2020
Derivatives [Line Items]
Senior Notes interest rate	2.90%				2.90%
2.900% Senior Notes due 2020 | Interest rate swap | Other long-term liabilities | Designated as hedging instrument
Derivatives [Line Items]
Derivative liability, fair value	$ 5.5	14.5			$ 5.5		$ 14.5
Samsung Bioepis
Derivatives [Line Items]
Percentage of stake in entity								5.00%
Equity Method Investment, Ownership Percentage	49.90%				49.90%
Payments to acquire additional investment in equity method investment		$ 676.6	₩ 759.5
Short-term derivative
Derivatives [Line Items]
Range of durations of foreign currency forward contracts					12 months
Gain/Loss on fair value of foreign currency forward contracts	$ 9.8				$ 9.8

Property, Plant and Equipment (

Property, Plant and Equipment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Accumulated depreciation	$ 1,886.2		$ 1,886.2		$ 1,797.4
Depreciation	47	$ 64.2	99.9	$ 129.2
Solothurn, Switzerland
Property, Plant and Equipment [Line Items]
Construction in progress	1,800		1,800		$ 1,600
Contractual commitments for the construction of the facility	57		57
Building
Property, Plant and Equipment [Line Items]
Property, plant and equipment held for sale	318.4		318.4
Machinery and Equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment held for sale	$ 290.8		$ 290.8

Leases (Details)

Leases (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Operating lease assets	$ 434.4	$ 434.4	$ 0
Long-term operating lease liabilities	423	423	0
Operating lease liabilities	495.3	495.3
Operating lease cost	14.4	30.1
Net lease cost	14.4	30.1
ASC 842 - Minimum lease payments 2019 (remaining six months)	50.2	50.2
ASC 842 - Minimum lease payments, 2020	80.5	80.5
ASC 842 - Minimum lease payments, 2021	74.9	74.9
ASC 842 - Minimum lease payments, 2022	70.9	70.9
ASC 842 - Minimum lease payments, 2023	69.4	69.4
ASC 842 - Minimum lease payments, 2024	66.7	66.7
ASC 842 - Minimum lease payments, thereafter	147.6	147.6
ASC 842 - Minimum lease payments	560.2	560.2
Less: Interest	$ 64.9	$ 64.9
ASC 840 - Minimum lease payments, 2019			87
ASC 840 - Minimum lease payments, 2020			80.7
ASC 840 - Minimum lease payments, 2021			75.9
ASC 840 - Minimum lease payments, 2022			71.7
ASC 940 - Minimum lease payments, 2023			71
ASC 840 - Minimum lease payments, thereafter			215.3
ASC 840 - Minimum lease payments total			601.6
ASC 840 - Sublease income, 2019			26.8
ASC 840 - Sublease income, 2020			25.6
ASC 840 - Sublease income, 2021			23.7
ASC 840 - Sublease income, 2022			24
ASC 840 - Sublease income, 2023			24.3
ASC 840 - Sublease income, thereafter			58.4
ASC 840 - Sublease income total			182.8
ASC 840 - Net minimum lease payments 2019			60.2
ASC 840 - Net minimum lease payments, 2020			55.1
ASC 840 - Net minimum lease payments, 2021			52.2
ASC 840 - Net minimum lease payments, 2022			47.7
ASC 840 - Net minimum lease payments, 2023			46.7
ASC 840 - Net minimum lease payments, thereafter			156.9
ASC 840 - Net minimum lease payments total			$ 418.8
Weighted average remaining lease term in years	7 years 4 months 24 days	7 years 4 months 24 days
Weighted average discount rate	3.30%	3.30%
Cash paid for amounts included in measurement of lease liabilities	$ 27.4	$ 46.8
Operating lease assets obtained in exchange for lease obligations	7.3	12.7
Research and development
Operating lease cost	3.4	3.7
Net lease cost	3.4	3.7
Selling, general and administrative
Operating lease cost	17.9	41.4
Sublease income	(6)	(13.1)
Net lease cost	17.9	41.4
Other income (expense)
Sublease income	(0.9)	(1.9)
Operating lease assets
Operating lease assets	434.4	434.4
Accrued expenses and other
Operating lease liability, current	72.3	72.3
Long-term operating lease liabilities
Long-term operating lease liabilities	$ 423	$ 423

Leases (Details Textual)

Leases (Details Textual) $ in Millions	Jun. 30, 2019USD ($)
Minimum
Operating Leased Assets [Line Items]
Lessee, Operating Lease, Term of Contract	1 year
Lessee, Operating Lease, Renewal Term	1 year
Maximum
Operating Leased Assets [Line Items]
Lessee, Operating Lease, Term of Contract	9 years
Lessee, Operating Lease, Renewal Term	6 years
Denmark Manufacturing Operations
Operating Leased Assets [Line Items]
Operating lease assets	$ 2.2
Operating lease liabilities	$ 2.2

Share Repurchases (Details)

Share Repurchases (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2016
Class of Stock [Line Items]
Payments for repurchase of common stock			$ 3,057.3	$ 3,000
2019 Share Repurchase Program
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount					$ 5,000
Repurchase of common stock, at cost, shares			3.9
Payments for repurchase of common stock			$ 909.9
Amount remaining under 2019 Share Repurchase Program	$ 4,100		$ 4,100
2018 Share Repurchase Program
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount						$ 3,500
Repurchase of common stock, at cost, shares	6.5		8.9
Payments for repurchase of common stock	$ 1,500		$ 2,100
2016 Share Repurchase Program
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount							$ 5,000
Repurchase of common stock, at cost, shares		9.6		10.5
Payments for repurchase of common stock		$ 2,750		$ 3,000

Accumulated Other Comprehensi_2

Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jan. 01, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance			$ (240,400,000)	$ (318,400,000)
Balance, January 1, 2018	$ (252,900,000)	$ (261,600,000)	(252,900,000)	(318,400,000)	$ (316,900,000)
Other comprehensive income (loss), before reclassifications, net of tax			36,000,000	7,200,000
Amounts reclassified from accumulated other comprehensive income, net of tax			(48,500,000)	48,100,000
Other comprehensive income (loss), net of tax	(33,500,000)	41,800,000	(12,900,000)	55,000,000
Accumulated other comprehensive income (loss), net of tax ending balance	(252,900,000)	(261,600,000)	(252,900,000)	(261,600,000)
Other comprehensive income (loss), net of tax	(33,100,000)	42,300,000	(12,500,000)	55,300,000
Unrealized gains (losses) on securities available for sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance			(4,000,000)	(1,600,000)
Balance, January 1, 2018	6,200,000	(1,700,000)	(4,000,000)	(1,600,000)	(100,000)
Other comprehensive income (loss), before reclassifications, net of tax			11,000,000	(7,600,000)
Amounts reclassified from accumulated other comprehensive income, net of tax			(800,000)	6,000,000
Other comprehensive income (loss), net of tax			10,200,000	(1,600,000)
Accumulated other comprehensive income (loss), net of tax ending balance	6,200,000	(1,700,000)	6,200,000	(1,700,000)
Unrealized gains (losses) on cash flow hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance			34,700,000	(104,500,000)
Balance, January 1, 2018	13,700,000	(700,000)	34,700,000	(104,500,000)	(104,500,000)
Other comprehensive income (loss), before reclassifications, net of tax			22,300,000	61,700,000
Amounts reclassified from accumulated other comprehensive income, net of tax			(43,300,000)	42,100,000
Other comprehensive income (loss), net of tax			(21,000,000)	103,800,000
Accumulated other comprehensive income (loss), net of tax ending balance	13,700,000	(700,000)	13,700,000	(700,000)
Gains (losses) on net investment hedge
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance			3,500,000	0
Balance, January 1, 2018	29,200,000	0	3,500,000	0	0
Other comprehensive income (loss), before reclassifications, net of tax			30,100,000	0
Amounts reclassified from accumulated other comprehensive income, net of tax			(4,400,000)	0
Other comprehensive income (loss), net of tax			25,700,000	0
Accumulated other comprehensive income (loss), net of tax ending balance	29,200,000	0	29,200,000	0
Unfunded status of postretirement benefit plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance			(31,300,000)	(36,800,000)
Balance, January 1, 2018	(30,600,000)	(36,400,000)	(31,300,000)	(36,800,000)	(36,800,000)
Other comprehensive income (loss), before reclassifications, net of tax			700,000	400,000
Amounts reclassified from accumulated other comprehensive income, net of tax			0	0
Other comprehensive income (loss), net of tax			700,000	400,000
Accumulated other comprehensive income (loss), net of tax ending balance	(30,600,000)	(36,400,000)	(30,600,000)	(36,400,000)
Currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax beginning balance			(243,300,000)	(175,500,000)
Balance, January 1, 2018	(271,400,000)	(222,800,000)	(243,300,000)	(175,500,000)	(175,500,000)
Other comprehensive income (loss), before reclassifications, net of tax			(28,100,000)	(47,300,000)
Amounts reclassified from accumulated other comprehensive income, net of tax			0	0
Other comprehensive income (loss), net of tax			(28,100,000)	(47,300,000)
Accumulated other comprehensive income (loss), net of tax ending balance	$ (271,400,000)	$ (222,800,000)	$ (271,400,000)	$ (222,800,000)
Accounting Standards Update 2016-01
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of new accounting principle in period of adoption					1,500,000
Accounting Standards Update 2016-01 | Unrealized gains (losses) on securities available for sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of new accounting principle in period of adoption					1,500,000
Accounting Standards Update 2016-01 | Unrealized gains (losses) on cash flow hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of new accounting principle in period of adoption					0
Accounting Standards Update 2016-01 | Gains (losses) on net investment hedge
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of new accounting principle in period of adoption					0
Accounting Standards Update 2016-01 | Unfunded status of postretirement benefit plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of new accounting principle in period of adoption					0
Accounting Standards Update 2016-01 | Currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of new accounting principle in period of adoption					$ 0

Reclassification out of Accumul

Reclassification out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Nonoperating Income (Expense)	$ (197.4)	$ (34.5)	$ 159.9	$ (75.5)
Income Tax Expense (Benefit)	248.1	263.7	670.6	586.2
Product revenues	(3,616.7)	(3,356.5)	(7,106.5)	(6,487.6)
Net income attributable to Biogen Inc.	1,494.1	866.6	2,902.9	2,039.5
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net income attributable to Biogen Inc.	31.8	(10.7)	48.5	(48.1)
Unrealized gains (losses) on securities available for sale | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Nonoperating Income (Expense)	0.7	0	1	(7.6)
Income Tax Expense (Benefit)	(0.1)	0	(0.2)	1.6
Unrealized gains (losses) on cash flow hedges | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Nonoperating Income (Expense)	0	0	0.1	0.1
Income Tax Expense (Benefit)	(0.1)	0.1	(0.2)	0.2
Product revenues	(29.8)	(10.4)	(44.6)	(43.3)
Operating Expenses	(0.7)	(0.4)	(1.2)	(0.9)
Gains (losses) on net investment hedge | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Nonoperating Income (Expense)	2.2	0	4.4	0
Income Tax Expense (Benefit)	$ 0	$ 0	$ 0	$ 0

Earnings per Share (Details)

Earnings per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net income attributable to Biogen Inc.	$ 1,494.1	$ 866.6	$ 2,902.9	$ 2,039.5
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	190.3	207.1	193.4	209.2
Effect of dilutive securities:
Dilutive potential common shares	0.1	0.2	0.3	0.3
Shares used in calculating diluted earnings per share	190.4	207.3	193.7	209.5
Stock options and employee stock purchase plan
Effect of dilutive securities:
Stock units	0	0	0	0
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	0.1	0.1	0.2	0.2
Market stock units
Effect of dilutive securities:
Stock units	0	0.1	0.1	0.1
Performance stock units settled in stock
Effect of dilutive securities:
Stock units	0	0	0	0

Share-Based Payments (Details)

Share-Based Payments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	$ 55	$ 41.4	$ 104	$ 87.6
Subtotal	82.9	42.8	132.4	93.2
Capitalized share-based compensation costs	(2.8)	(2.8)	(6.1)	(6.2)
Share-based compensation expense included in total costs and expenses	80.1	40	126.3	87
Income tax effect	(13.6)	(6.5)	(21)	(14.1)
Research and development
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	28.1	17.6	49.8	39.5
Selling, general and administrative
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	54.8	25.2	82.6	53.7
Total share-based compensation expense, net of tax
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	$ 66.5	$ 33.5	$ 105.3	$ 72.9

Share-Based Payments (Details 1

Share-Based Payments (Details 1) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	$ 55	$ 41.4	$ 104	$ 87.6
Subtotal	82.9	42.8	132.4	93.2
Capitalized share-based compensation costs	(2.8)	(2.8)	(6.1)	(6.2)
Share-based compensation expense included in total costs and expenses	80.1	40	126.3	87
Market stock units
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	7.8	7.1	15.5	13.2
Time-vested restricted stock units
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	31.8	30.6	66.5	66.7
Cash settled performance shares
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	0.4	(0.7)	(0.6)	3.6
Performance units
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	0.3	2	0.8	1.2
Performance stock units settled in stock
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	12.1	1.4	14.1	2.1
Performance stock units settled in cash
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	0.9	0.3	1.9	0.4
Employee stock purchase plan
Summary of share based compensation expense associated with different programs [Abstract]
Share-based compensation expense	3.4	2.1	8	6
Nightstar | NST stock option
Summary of share based compensation expense associated with different programs [Abstract]
Fair value of post-combination equity compensation	$ 26.2	$ 0	$ 26.2	$ 0

Income Taxes (Details)

Income Taxes (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Reconciliation between the U.S. federal statutory tax rate and effective tax rate
Statutory rate	21.00%	21.00%	21.00%	21.00%
State taxes	0.90%	0.60%	0.60%	0.80%
Taxes on foreign earnings	(4.60%)	(1.80%)	(4.60%)	(2.00%)
Credits and net operating loss utilization	(0.90%)	(0.70%)	(0.80%)	(0.80%)
Purchased intangible assets	0.40%	0.60%	0.40%	0.60%
Denmark assets held for sale	0.00%	0.00%	2.20%	0.00%
Internal reorganization of certain intellectual property rights	(5.00%)	0.00%	(2.40%)	0.00%
Effective Tax Rate Reconciliation, GILTI tax	1.40%	1.80%	1.70%	1.60%
Permanent items	0.40%	0.40%	0.30%	0.40%
Other	0.50%	0.50%	0.10%	0.30%
Effective tax rate	14.10%	22.40%	18.50%	21.90%

Income Taxes (Details Textual)

Income Taxes (Details Textual) $ in Millions	6 Months Ended
	Jun. 30, 2019USD ($)
Foreign tax authority
Income Tax Contingency [Line Items]
Income tax payment, including penalties and interest	$ 60
Denmark Manufacturing Operations
Income Tax Contingency [Line Items]
Tax expense on disposal group	61.3
Deferred tax liability held for sale	53.8
Intellectual Property
Income Tax Contingency [Line Items]
Deferred tax asset recorded on transfer of intellectual property	856.7
Deferred tax liability recorded on transfer of intellectual property	$ 685.3

Other Consolidated Financial _3

Other Consolidated Financial Statement Detail (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Other Income (Expense), Net
Interest income	$ 29.1	$ 28.7	$ 60.3	$ 55.4
Interest expense	(47.7)	(51.7)	(95.6)	(102.2)
Gain (loss) on investments, net	(173.4)	5.3	203	(9.1)
Foreign exchange gains (losses), net	1.7	(13)	(0.5)	(14)
Other, net	(7.1)	(3.8)	(7.3)	(5.6)
Total other income (expense), net	(197.4)	(34.5)	159.9	(75.5)
Equity Securities, FV-NI, Gain (Loss), Alternative [Abstract]
Net gains (losses) recognized during the period on equity securities	(174.2)	5.4	201.9	(1.5)
Less: Net gains (losses) recognized during the period on equity securities sold during the period	(42.9)	0	42.4	(0.5)
Unrealized gains (losses) recognized during the period on equity securities held as of June 30, 2019	(131.3)	$ 5.4	159.5	$ (1)
Accrued Expenses and Other
Revenue-related reserves for discounts and allowances	1,062.5		1,062.5		$ 1,051.3
Royalties and licensing fees	212.8		212.8		224.7
Employee compensation and benefits	199.4		199.4		320.9
Current portion of contingent consideration obligations	146.2		146.2		444.8
Collaboration expenses	130.4		130.4		261.6
Construction in progress	38.2		38.2		125.2
Other	727.5		727.5		609.3
Total accrued expenses and other	2,318		2,318		2,861.2
Component of accrued expenses and other
Accrued Expenses and Other
Revenue-related reserves for discounts and allowances	$ 863.5		$ 863.5		$ 874.7

Other Consolidated Financial _4

Other Consolidated Financial Statement (Details Textual) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Other long-term liabilities	$ 1,355.8	$ 1,389.4
Accrued income taxes	$ 801.5	$ 791.4

Collaborative and Other Relat_3

Collaborative and Other Relationships - Collaborations (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended		9 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Eisai
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Percentage of future development costs related to Eisai				45.00%		15.00%
Loss on research and development contracts terminated with Eisai		$ 45
Eisai | BAN2401 and Elenbecestat
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration	$ 68.6		$ 54.6	$ 136.6	$ 111.1
Expense reflected within statements of income	34.3		27.3	68.3	55.5
Skyhawk Therapeutics
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Additional milestone payment to Skyhawk	2,000			2,000
Upfront and milestone payments made to collaborative partner		74
Research and development expense		38.5
Prepaid research and development expenses		$ 35.5
Research and development | Eisai | Aducanumab
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration	3.2		76.5	165.8	140
Expense reflected within statements of income	1.7		65	91.2	128.5
Selling, general and administrative | Eisai | Aducanumab
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expense incurred by the collaboration	0.2		14	21.2	21.2
Expense reflected within statements of income	$ 0.1		$ 9.2	$ 11.7	$ 13.9

Collaborative and Other Relat_4

Collaborative and Other Relationships - Equity Method Investments (Details) $ in Millions, ₩ in Billions	3 Months Ended				6 Months Ended
	Jun. 30, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2018KRW (₩)	Jun. 30, 2018USD ($)	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2019KRW (₩)	Dec. 31, 2018KRW (₩)	Nov. 07, 2018USD ($)	Sep. 30, 2018
Schedule of Equity Method Investments [Line Items]
Our share of Samsung Bioepis gains (losses)	$ 16.3			$ 0	$ 45	$ 0
Collaboration profit (loss) sharing	63.5			39.2	121.6	81.7
Total revenues	$ 3,616.7			3,356.5	$ 7,106.5	6,487.6
Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Percentage of stake in entity										5.00%
Percentage of stake in entity maximum	49.90%				49.90%		49.90%
Payments to acquire additional investment in equity method investment		$ 676.6	₩ 759.5
Amortization of basis differences									$ 675
Loss recorded on Samsung Bioepis joint venture	$ 16.3				$ 45
Our share of Samsung Bioepis gains (losses)	(5.5)				8.5
Amortization of basis differences	21.8				36.5
Investment in Samsung Bioepis	615.5	$ 680.6			$ 615.5		₩ 712.8	₩ 759.5
Biogen share of co-promotion profits or losses					50.00%
Collaboration profit (loss) sharing	63.4			39.7	$ 121.5	83.5
Collaborative arrangement | Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Total revenues	$ 52.2			$ 14.7	$ 77	$ 32.6
Inventory | Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Amortization of basis differences									115
Equity method investment basis difference amortization period					1 year 6 months
Developed technology | Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Amortization of basis differences									615
Equity method investment basis difference amortization period					15 years
In Process Research and Development | Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Amortization of basis differences									170
Deferred tax liability | Samsung Bioepis
Schedule of Equity Method Investments [Line Items]
Amortization of basis differences									$ 225

Investments in Variable Inter_2

Investments in Variable Interest Entities (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Investment in Variable Interest Entities (Textual)
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities	$ 24	$ 28.7

Litigation (Details)

Litigation (Details) $ in Millions	6 Months Ended
	Jun. 30, 2019USD ($)
Brazil
Income Tax Contingency [Line Items]
Brazil tax assessment, including interest and penalties	$ 70