Biogen (BIIB) 10-Q 24 Apr 15 2015 Q1 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
	Mar. 31, 2015	Apr. 17, 2015
Document and Entity Information [Abstract]
Entity Registrant Name	BIOGEN INC.
Entity Central Index Key	875045
Current Fiscal Year End Date	-19
Entity Filer Category	Large Accelerated Filer
Document Type	10-Q
Document Period End Date	31-Mar-15
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Amendment Flag	FALSE
Entity Common Stock, Shares Outstanding		235,229,512

Condensed_Consolidated_Stateme

Condensed Consolidated Statements of Income (Unaudited) (USD $)	3 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Revenues:
Product, net	$2,172,322	$1,742,765
Unconsolidated joint business	330,611	296,885
Other	52,030	90,101
Total revenues	2,554,963	2,129,751
Cost and expenses:
Cost of sales, excluding amortization of acquired intangible assets	312,431	279,245
Research and development	460,549	528,884
Selling, general and administrative	560,361	511,674
Amortization of acquired intangible assets	95,903	143,258
(Gain) loss on fair value remeasurement of contingent consideration	7,844	-799
Total cost and expenses	1,437,088	1,462,262
Gain on sale of rights	0	3,859
Income from operations	1,117,875	671,348
Other income (expense), net	-14,986	-5,601
Income before income tax expense and equity in loss of investee, net of tax	1,102,889	665,747
Income tax expense	281,881	178,414
Equity in loss of investee, net of tax	834	7,605
Net income	820,174	479,728
Net income (loss) attributable to noncontrolling interests, net of tax	-2,367	-228
Net income attributable to Biogen Inc.	$822,541	$479,956
Net income per share:
Basic earnings per share attributable to Biogen Inc.	$3.50	$2.03
Diluted earnings per share attributable to Biogen Inc.	$3.49	$2.02
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Inc.	234,995	236,786
Diluted earnings per share attributable to Biogen Inc.	235,630	237,849

Condensed_Consolidated_Stateme1

Condensed Consolidated Statements of Comprehensive Income (Unaudited) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Inc.	$822,541	$479,956
Other comprehensive income:
Unrealized gains (losses) on securities available for sale, net of tax of $766 and $1,014 for the three months ended March 31, 2015 and 2014, respectively	1,299	1,725
Unrealized gains (losses) on foreign currency forward contracts, net of tax of $412 and $265 for the three months ended March 31, 2015 and 2014, respectively	87,246	5,791
Unrealized gains (losses) on pension benefit obligation	1,258	817
Currency translation adjustment	-100,853	-2,944
Total other comprehensive income (loss), net of tax	-11,050	5,389
Comprehensive income attributable to Biogen Inc.	811,491	485,345
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	-2,267	-228
Comprehensive income	$809,224	$485,117

Condensed_Consolidated_Stateme2

Condensed Consolidated Statements of Comprehensive Income (Unaudited) (Parenthetical) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Statement of Comprehensive Income [Abstract]
Tax effect on net unrealized gains (losses) recognized on securities available for sale	$766	$1,014
Tax effect of net unrealized gains (losses) recognized on foreign currency forward contracts	$412	$265

Condensed_Consolidated_Balance

Condensed Consolidated Balance Sheets (Unaudited) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Current assets:
Cash and cash equivalents	$1,478,051	$1,204,924
Marketable securities	673,946	640,460
Accounts receivable, net	1,389,995	1,292,445
Due from unconsolidated joint business, net	311,956	283,360
Inventory	825,349	804,022
Other current assets	579,599	447,462
Total current assets	5,258,896	4,672,673
Marketable securities	1,377,325	1,470,652
Property, plant and equipment, net	1,739,628	1,765,683
Intangible assets, net	4,353,123	4,028,507
Goodwill	1,849,852	1,760,249
Investments and other assets	640,630	618,795
Total assets	15,219,454	14,316,559
Current liabilities:
Current portion of notes payable	3,254	3,136
Taxes payable	322,075	168,058
Accounts payable	236,128	229,178
Accrued expenses and other	1,384,774	1,819,334
Total current liabilities	1,946,231	2,219,706
Notes payable	580,672	582,061
Long-term deferred tax liability	130,564	50,656
Other long-term liabilities	892,529	650,096
Total liabilities	3,549,996	3,502,519
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	129	129
Additional paid-in capital	4,242,426	4,196,156
Accumulated other comprehensive loss	-70,538	-59,488
Retained earnings	10,106,460	9,283,919
Treasury stock, at cost	-2,611,706	-2,611,706
Total Biogen Inc. shareholders’ equity	11,666,771	10,809,010
Noncontrolling interests	2,687	5,030
Total equity	11,669,458	10,814,040
Total liabilities and equity	$15,219,454	$14,316,559

Condensed_Consolidated_Balance1

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Preferred stock, par value	$0.00	$0.00
Common stock, par value	$0.00	$0.00

Condensed_Consolidated_Stateme3

Condensed Consolidated Statements of Cash Flows (Unaudited) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Cash flows from operating activities:
Net income	$820,174	$479,728
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization	147,937	190,963
Share-based compensation	52,063	46,340
Deferred income taxes	16,739	-79,558
Other	-32,921	-71,476
Changes in operating assets and liabilities, net:
Accounts receivable	-128,287	-197,685
Inventory	-21,507	-16,980
Accrued expenses and other current liabilities	-205,467	-171,368
Current taxes payable	154,017	-18,640
Other changes in operating assets and liabilities, net	-69,711	-56,698
Net cash flows provided by operating activities	733,037	104,626
Cash flows from investing activities:
Proceeds from sales and maturities of marketable securities	373,563	757,512
Purchases of marketable securities	-322,958	-666,211
Acquisitions of business, net of cash acquired	-198,798	0
Purchases of property, plant and equipment	-97,830	-54,306
Contingent consideration related to Fumapharm AG acquisition	250,000	25,000
Other	-6,748	-6,002
Net cash flows provided by (used in) investing activities	-502,771	5,993
Cash flows from financing activities:
Proceeds from issuance of stock for share-based compensation arrangements	25,614	22,363
Excess tax benefit from stock options	66,429	79,456
Other	-11,287	13,056
Net cash flows provided by financing activities	80,756	114,875
Net increase in cash and cash equivalents	311,022	225,494
Effect of exchange rate changes on cash and cash equivalents	-37,895	545
Cash and cash equivalents, beginning of the period	1,204,924	602,562
Cash and cash equivalents, end of the period	$1,478,051	$828,601

Summary_of_Significant_Account

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Business	Summary of Significant Accounting Policies
	Business Overview
	Biogen is a global biopharmaceutical company focused on discovering, developing, manufacturing and delivering therapies for neurological, autoimmune and hematologic disorders. Our principal marketed products include AVONEX, PLEGRIDY, TECFIDERA, TYSABRI, and FAMPYRA for multiple sclerosis (MS), ALPROLIX for hemophilia B and ELOCTATE for hemophilia A. We also collaborate on the development and commercialization of RITUXAN for the treatment of non-Hodgkin's lymphoma, chronic lymphocytic leukemia and other conditions and share profits and losses for GAZYVA which is approved for the treatment of chronic lymphocytic leukemia.
	Basis of Presentation
	In the opinion of management, the accompanying unaudited condensed consolidated financial statements include all adjustments, consisting of normal recurring accruals, necessary for a fair presentation of our financial statements for interim periods in accordance with accounting principles generally accepted in the United States (U.S. GAAP). The information included in this quarterly report on Form 10-Q should be read in conjunction with our consolidated financial statements and the accompanying notes included in our Annual Report on Form 10-K for the year ended December 31, 2014 (2014 Form 10-K). Our accounting policies are described in the “Notes to Consolidated Financial Statements” in our 2014 Form 10-K and updated, as necessary, in this Form 10-Q. The year-end condensed consolidated balance sheet data presented for comparative purposes was derived from our audited financial statements, but does not include all disclosures required by U.S. GAAP. The results of operations for the three months ended March 31, 2015, are not necessarily indicative of the operating results for the full year or for any other subsequent interim period.
	Consolidation
	Our condensed consolidated financial statements reflect our financial statements, those of our wholly-owned subsidiaries and those of certain variable interest entities where we are the primary beneficiary. For consolidated entities where we own or are exposed to less than 100% of the economics, we record net income (loss) attributable to noncontrolling interests in our condensed consolidated statements of income equal to the percentage of the economic or ownership interest retained in such entities by the respective noncontrolling parties. Intercompany balances and transactions are eliminated in consolidation.
	In determining whether we are the primary beneficiary of an entity and therefore required to consolidate, we apply a qualitative approach that determines whether we have both (1) the power to direct the economically significant activities of the entity and (2) the obligation to absorb losses of, or the right to receive benefits from, the entity that could potentially be significant to that entity. These considerations impact the way we account for our existing collaborative relationships and other arrangements. We continuously assess whether we are the primary beneficiary of a variable interest entity as changes to existing relationships or future transactions may result in us consolidating or deconsolidating one or more of our collaborators or partners.
	Use of Estimates
	The preparation of our condensed consolidated financial statements requires us to make estimates, judgments, and assumptions that may affect the reported amounts of assets, liabilities, equity, revenues and expenses, and related disclosure of contingent assets and liabilities. On an on-going basis, we evaluate our estimates and judgments and methodologies. We base our estimates on historical experience and on various other assumptions that are believed to be reasonable, the results of which form the basis for making judgments about the carrying values of assets and liabilities. Actual results may differ from these estimates under different assumptions or conditions.
	Property, Plant and Equipment
	Property, plant and equipment are recorded at cost, net of accumulated depreciation. Accumulated depreciation on property, plant and equipment was $1,217.8 million and $1,186.4 million as of March 31, 2015 and December 31, 2014, respectively.

Acquisitions_Acquisitions

Acquisitions Acquisitions	3 Months Ended
	Mar. 31, 2015
Acquisitions [Abstract]
Acquisitions	Acquisitions
	Convergence Pharmaceuticals
	On February 12, 2015, we completed our acquisition of all of the outstanding stock of Convergence Pharmaceuticals (Convergence), a clinical-stage biopharmaceutical company with a focus on developing product candidates for neuropathic pain. Convergence’s lead candidate is its Phase 2 clinical candidate (CNV1014802), which has demonstrated clinical activity in proof-of-concept studies for trigeminal neuralgia (TGN), a chronic orphan disease. Additionally, CNV1014802 has potential applicability in several other neuropathic pain states.
	The purchase price consisted of a $200.1 million cash payment at closing, plus contingent consideration in the form of development and approval milestones up to a maximum of $450.0 million, of which $350.0 million is associated with the development and approval of CNV1014802 for the treatment of TGN. The acquisition was funded from our existing cash on hand and has been accounted for as the acquisition of a business. In addition to obtaining the rights to CNV1014802 and additional product candidates in preclinical development, we retained the services of key employees of Convergence.
	Upon acquisition, we recorded a liability of $238.5 million representing the fair value of the contingent consideration. This amount was estimated through a valuation model that incorporates industry-based probability adjusted assumptions relating to the achievement of these milestones and thus the likelihood of making the contingent payments. This fair value measurement is based upon significant inputs not observable in the market and therefore represents a Level 3 measurement. Subsequent changes in the fair value of this obligation will be recognized as adjustments to contingent consideration and reflected within our condensed consolidated statements of income. For additional information related to our fair value of this obligation, please read Note 7, Fair Value Measurements to these condensed consolidated financial statements.
	The purchase price consists of the following:
	(In millions)
	Cash portion of consideration	$	200.1
	Contingent consideration	238.5
	Total purchase price	$	438.6
	The following table summarizes the estimated fair values of the separately identifiable assets acquired and liabilities assumed as of February 12, 2015:
	(In millions)
	In-process research and development	$	424.6
	Other intangible assets	7.6
	Goodwill	92.3
	Deferred tax liability	(84.9		)
	Other, net	(1.0		)
	Total purchase price	$	438.6
	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 intangible assets acquired and filing Convergence's final tax return. 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.
	Our estimate of the fair value of the in-process research and development (IPR&D) programs acquired was determined through a probability adjusted cash flow analysis based on probability weighted net cash flow utilizing a discount rate of 11%. This valuation was primarily driven by the value associated with the lead candidate, CNV1014802, which is in development for the treatment of TGN and is expected to be completed no earlier than 2020, at a remaining cost of approximately $143.0 million. The fair value associated with CNV1014802 for the treatment of TGN was $170.0 million. We have recorded additional IPR&D assets related to the use of CNV1014802 in two additional neuropathic pain indications. The remaining costs of development for these two indications is approximately $446.0 million, with an expected completion of date of no earlier than 2022. These fair value measurements were based on significant inputs not observable in the market and thus represents Level 3 fair value measurements.
	The goodwill recognized is largely attributable to establishing a deferred tax liability for the acquired IPR&D intangible assets which have no tax basis. The goodwill is not tax deductible.
	Pro forma results of operations would not be materially different as a result of the acquisition of Convergence and therefore are not presented. Subsequent to the acquisition date, our results of operations include the results of operations of Convergence.

Accounts_Receivable

Accounts Receivable	3 Months Ended
	Mar. 31, 2015
Receivables [Abstract]
Accounts Receivable	Accounts Receivable
	Our accounts receivable primarily arise from product sales in the U.S. and Europe and mainly represent amounts due from our wholesale distributors, public hospitals and other government entities. Concentrations of credit risk with respect to our accounts receivable, which are typically unsecured, are limited due to the wide variety of customers and markets using our products, as well as their dispersion across many different geographic areas. The majority of our accounts receivable have standard payment terms which generally require payment within 30 to 90 days. We monitor the financial performance and credit worthiness of our customers so that we can properly assess and respond to changes in their credit profile. We provide reserves against trade receivables for estimated losses that may result from a customer’s inability to pay. Amounts determined to be uncollectible are charged or written-off against the reserve. To date, our historical reserves and write-offs of accounts receivable have not been significant.
	The credit and economic conditions within Spain and Portugal, among other members of the E.U. continue to remain uncertain. Uncertain credit and economic conditions have generally led to a lengthening of time to collect our accounts receivable in these countries. In Portugal and select regions in Spain, where our collections have slowed and a significant portion of these receivables are routinely being collected beyond our contractual payment terms and over periods in excess of one year, we have discounted our receivables and reduced related revenues based on the period of time that we estimate those amounts will be paid, to the extent such period exceeds one year, using the country’s market-based borrowing rate for such period. The related receivables are classified at the time of sale as non-current assets. We accrete interest income on these receivables, which is recognized as a component of other income (expense), net within our condensed consolidated statements of income.
	Our net accounts receivable balances from product sales in selected European countries are summarized as follows:
		As of March 31, 2015
	(In millions)	Current		Non-Current		Total
		Balance Included		Balance Included
		within Accounts		within Investments
		Receivable, net		and Other Assets
	Spain	$	71.1	$	2.2	$	73.3
	Portugal	$	12.9	$	7.7	$	20.6
		As of December 31, 2014
	(In millions)	Current		Non-Current		Total
		Balance Included		Balance Included
		within Accounts		within Investments
		Receivable, net		and Other Assets
	Spain	$	62.5	$	5	$	67.5
	Portugal	$	15.1	$	7.6	$	22.7
	Approximately $11.4 million and $11.1 million of the total net accounts receivable balances for these countries were overdue more than one year as of March 31, 2015 and December 31, 2014, respectively.
	Pricing of TYSABRI in Italy - AIFA
	In the fourth quarter of 2011, Biogen Italia SRL (formerly Biogen Idec Italia SRL), our Italian subsidiary, received notice from the Italian National Medicines Agency (Agenzia Italiana del Farmaco or AIFA) that sales of TYSABRI after mid-February 2009 exceeded a reimbursement limit established pursuant to a Price Determination Resolution granted by AIFA in December 2006. In December 2011, we filed an appeal against AIFA in administrative court in Rome, Italy seeking a ruling that the reimbursement limit is unenforceable. That appeal is pending.
	In June 2014, AIFA approved a resolution, effective for a 24 month term, setting the price for TYSABRI in Italy. This resolution also eliminated the reimbursement limit from February 2013 going forward. As a result, in June 2014, we recognized $53.5 million of TYSABRI revenues related to the periods beginning February 2013 that were previously deferred. AIFA and Biogen Italia SRL remain in discussions for a possible resolution for the period from mid-February 2009 through January 2013. In October 2014, we proposed a settlement to resolve all of AIFA's claims relating to sales of TYSABRI in excess of the reimbursement limit for this period for an aggregate lump sum repayment of EUR35.6 million. We have approximately EUR75 million recorded as accrued expenses and deferred revenue within our condensed consolidated balance sheets for this matter.
	For information relating to our agreement with AIFA relating to sales of TYSABRI in Italy, please read Note 4, Accounts Receivable to our consolidated financial statements included within our 2014 Form 10-K.

Reserves_for_Discounts_and_All

Reserves for Discounts and Allowances	3 Months Ended
	Mar. 31, 2015
Reserves for Discounts and Allowances [Abstract]
Reserves for discounts and allowances	Reserves for Discounts and Allowances
	An analysis of the change in reserves for discounts and allowances is summarized as follows:
	(In millions)	Discounts			Contractual			Returns			Total
					Adjustments
	Balance, as of December 31, 2014	$	47.6		$	387.1		$	49.1		$	483.8
	Current provisions relating to sales in current year	101.8			441.2			10.2			553.2
	Adjustments relating to prior years	(2.4		)	(10.2		)	(5.9		)	(18.5		)
	Payments/credits relating to sales in current year	(50.3		)	(166.7		)	(0.7		)	(217.7		)
	Payments/credits relating to sales in prior years	(41.3		)	(170.7		)	(4.6		)	(216.6		)
	Balance, as of March 31, 2015	$	55.4		$	480.7		$	48.1		$	584.2
	The total reserves above, included in our condensed consolidated balance sheets, are summarized as follows:
	(In millions)	As of			As of
		March 31,			December 31,
		2015			2014
	Reduction of accounts receivable	$	144.9		$	124.6
	Component of accrued expenses and other	439.3			359.2
	Total reserves	$	584.2		$	483.8

Inventory

Inventory	3 Months Ended
	Mar. 31, 2015
Inventory Disclosure [Abstract]
Inventory	Inventory
	The components of inventory are summarized as follows:
	(In millions)	As of		As of
		March 31,		December 31,
		2015		2014
	Raw materials	$	139.7	$	128.3
	Work in process	513.1		511.5
	Finished goods	172.6		164.2
	Total inventory	$	825.3	$	804

Intangible_Assets_and_Goodwill

Intangible Assets and Goodwill	3 Months Ended
	Mar. 31, 2015
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 March 31, 2015							As of December 31, 2014
	(In millions)	Estimated		Cost		Accumulated			Net		Cost		Accumulated			Net
		Life				Amortization							Amortization
	Out-licensed patents	13-23 years		$	543.3	$	(487.8	)	$	55.5	$	543.3	$	(481.7	)	$	61.6
	Developed	15-23 years		3,005.30		(2,447.5		)	557.8		3,005.30		(2,396.8		)	608.5
	technology
	In-process research and development	Indefinite until commercialization		726.1		—			726.1		314.1		—			314.1
	Trademarks and	Indefinite		64		—			64		64		—			64
	tradenames
	Acquired and in-licensed rights	6-17 years		3,288.90		(339.2		)	2,949.70		3,280.40		(300.1		)	2,980.30
	and patents
	Total intangible assets			$	7,627.60	$	(3,274.5	)	$	4,353.10	$	7,207.10	$	(3,178.6	)	$	4,028.50
	For the three months ended March 31, 2015, amortization of acquired intangible assets totaled $95.9 million, as compared to $143.3 million in the prior year comparative period. The decrease in amortization of acquired intangible assets was primarily driven by a $34.7 million impairment charge recorded in the first quarter of 2014 related to one of our out-licensed patents, higher expected lifetime revenues of TYSABRI and a decrease in AVONEX revenues during the quarter.
	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 March 31, 2015 was $548.7 million. We amortize this intangible asset using the economic consumption method based on actual and expected revenues generated from the sales of our AVONEX product.
	In-process Research and Development (IPR&D)
	IPR&D represents the fair value assigned to research and development assets that we acquire that have not reached technological feasibility at the date of acquisition. Upon commercialization, we determine the estimated useful life. In connection with our acquisition of Convergence in February 2015, we acquired IPR&D programs with an estimated fair value of $424.6 million. For a more detailed description of this transaction, 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 the TYSABRI rights from Elan Corporation plc (Elan). Elan was acquired by Perrigo Company plc (Perrigo) in December 2013. The net book value of this asset as of March 31, 2015 was $2,886.6 million. We amortize this intangible asset over the estimated useful life using an economic consumption method based on actual and expected revenues generated from the sales of our TYSABRI product.
	Estimated Future Amortization of Intangible Assets
	Our amortization expense is based on the economic consumption of the intangible assets. Our most significant intangible assets are related to our AVONEX and TYSABRI products. Annually, during our long-range planning cycle, we perform an analysis of anticipated lifetime revenues of AVONEX and TYSABRI. This analysis is updated whenever events or changes in circumstances would significantly affect the anticipated lifetime revenues of either product.
	Our most recent long range planning cycle was updated in the third quarter of 2014. Our analysis included an increase in the expected future product revenues of TYSABRI, resulting in a decrease in the rate of amortization expense as compared to prior quarters. Our analysis also included a decrease in the expected future product revenues of AVONEX, resulting in an increase in the rate of amortization expense as compared to prior quarters. The results of our TYSABRI and AVONEX analyses were impacted by changes in the estimated impact of TECFIDERA, as well as other existing and potential oral and alternative MS formulations, including PLEGRIDY, that may compete with TYSABRI and AVONEX. Based upon this recent analysis, the estimated future amortization for acquired intangible assets is expected to be as follows:
	(In millions)	As of
		March 31,
		2015
	2015 (remaining nine months)	$	269
	2016	313.3
	2017	286.1
	2018	281.6
	2019	270.4
	2020	263.4
	Total	$	1,683.80
	Goodwill
	The following table provides a roll forward of the changes in our goodwill balance:
	(In millions)	As of
		March 31,
		2015
	Goodwill, beginning of period	$	1,760.20
	Increase to goodwill	92.3
	Other	(2.7		)
	Goodwill, end of period	$	1,849.90
	The increase in goodwill during the three months ended March 31, 2015 was related to our acquisition of Convergence. Other includes changes related to foreign exchange. For additional information related to this transaction, please read Note 2, Acquisitions to these condensed consolidated financial statements.
	As of March 31, 2015, we had no accumulated impairment losses related to goodwill.

Fair_Value_Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2015
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 March 31, 2015 (In millions)	Total			Quoted Prices			Significant Other		Significant
					in Active			Observable Inputs		Unobservable
					Markets			(Level 2)		Inputs
					(Level 1)					(Level 3)
	Assets:
	Cash equivalents	$	752.5		$	—		$	752.5	$	—
	Marketable debt securities:
	Corporate debt securities	991			—			991		—
	Government securities	850.6			—			850.6		—
	Mortgage and other asset backed securities	209.6			—			209.6		—
	Marketable equity securities	6.2			6.2			—		—
	Venture capital investments	12.1			—			—		12.1
	Derivative contracts	168.2			—			168.2		—
	Plan assets for deferred compensation	44.1			—			44.1		—
	Total	$	3,034.30		$	6.2		$	3,016.00	$	12.1
	Liabilities:
	Derivative contracts	$	12.6		$	—		$	12.6	$	—
	Contingent consideration obligations	461.8			—			—		461.8
	Total	$	474.4		$	—		$	12.6	$	461.8
	As of December 31, 2014 (In millions)	Total			Quoted Prices			Significant		Significant
					in Active			Other		Unobservable
					Markets			Observable		Inputs
					(Level 1)			Inputs		(Level 3)
								(Level 2)
	Assets:
	Cash equivalents	$	716.3		$	—		$	716.3	$	—
	Marketable debt securities:
	Corporate debt securities	1,063.00			—			1,063.00		—
	Government securities	849.8			—			849.8		—
	Mortgage and other asset backed securities	198.3			—			198.3		—
	Marketable equity securities	6.9			6.9			—		—
	Venture capital investments	14.5			—			—		14.5
	Derivative contracts	72.7			—			72.7		—
	Plan assets for deferred compensation	36.9			—			36.9		—
	Total	$	2,958.40		$	6.9		$	2,937.00	$	14.5
	Liabilities:
	Derivative contracts	$	5.4		$	—		$	5.4	$	—
	Contingent consideration obligations	215.5			—			—		215.5
	Total	$	220.9		$	—		$	5.4	$	215.5
	There have been no impairments of our assets measured and carried at fair value during the three months ended March 31, 2015. In addition, there were no changes in valuation techniques or inputs utilized or transfers between fair value measurement levels during the three months ended March 31, 2015. The fair value of Level 2 instruments classified as cash equivalents and marketable debt securities were determined through third party pricing services. For a description of our validation procedures related to prices provided by third party pricing services, refer to Note 1, Summary of Significant Accounting Policies: Fair Value Measurements, to our consolidated financial statements included within our 2014 Form 10-K.
	Marketable Equity Securities and Venture Capital Investments
	Our marketable equity securities represent investments in publicly traded equity securities. Our venture capital investments, which are all Level 3 measurements, include investments in certain venture capital funds, accounted for at fair value, that primarily invest in small privately-owned, venture-backed biotechnology companies.
	The following table provides a roll forward of the fair value of our venture capital investments, which includes Level 3 measurements:
		For the Three Months
		Ended March 31,
	(In millions)	2015			2014
	Fair value, beginning of period	$	14.5		$	21.9
	Unrealized gains included in earnings	—			2.9
	Unrealized losses included in earnings	(2.4		)	(1.2		)
	Fair value, end of period	$	12.1		$	23.6
	Debt Instruments
	The fair and carrying values of our debt instruments, which are Level 2 liabilities, are summarized as follows:
		As of March 31, 2015						As of December 31, 2014
	(In millions)	Fair			Carrying			Fair		Carrying
		Value			Value			Value		Value
	Notes payable to Fumedica	$	12.9		$	12.1		$	12.6	$	11.7
	6.875% Senior Notes due March 1, 2018	633.9			571.8			634.6		573.5
	Total	$	646.8		$	583.9		$	647.2	$	585.2
	The fair value of our notes payable to Fumedica was estimated using market observable inputs, including current interest and foreign currency exchange rates. The fair value of our 6.875% Senior Notes was determined through market, observable, and corroborated sources. For additional information related to our debt instruments, please read Note 12, Indebtedness to our consolidated financial statements included within our 2014 Form 10-K.
	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
Ended March 31,
(In millions)	2015			2014
Fair value, beginning of period	$	215.5		$	280.9
Additions	238.5			—
Changes in fair value	7.8			(0.8		)
Payments	—			(5.0		)
Fair value, end of period	$	461.8		$	275.1
As of March 31, 2015 and December 31, 2014, approximately $397.5 million and $200.0 million, respectively, of the fair value of our total contingent consideration obligations were reflected as components of other long-term liabilities within our condensed consolidated balance sheets with the remaining balances reflected as a component of accrued expenses and other.
In connection with our acquisition of Convergence in February 2015, we recorded a liability of $238.5 million, representing the fair value of the contingent consideration. This valuation was based on probability weighted net cash outflow projections of $450.0 million, discounted using a rate of 2%, which is the estimated cost of debt financing for market participants.
Acquired IPR&D
In connection with our acquisition of Convergence, we also allocated $424.6 million of the total purchase price to acquired IPR&D, which was capitalized as an intangible asset. The amount allocated to acquired IPR&D was based on significant inputs not observable in the market and thus represented a Level 3 fair value measurement. These assets will be tested for impairment annually until commercialization, after which time the IPR&D will be amortized over its estimated useful life. For a more detailed description of this transaction, please read Note 2, Acquisitions to these condensed consolidated financial statements.

Financial_Instruments

Financial Instruments	3 Months Ended
	Mar. 31, 2015
Investments, All Other Investments [Abstract]
Financial Instruments	Financial Instruments
	Marketable Securities
	The following tables summarize our marketable debt and equity securities:
	As of March 31, 2015 (In millions)	Fair			Gross			Gross			Amortized
		Value			Unrealized			Unrealized			Cost
					Gains			Losses
	Available-for-sale:
	Corporate debt securities
	Current	$	385.3		$	0.1		$	(0.1	)	$	385.3
	Non-current	605.7			0.8			(0.4		)	605.3
	Government securities
	Current	288.5			—			(0.1		)	288.6
	Non-current	562.1			0.6			(0.1		)	561.6
	Mortgage and other asset backed securities
	Current	0.1			—			—			0.1
	Non-current	209.5			0.4			(0.1		)	209.2
	Total marketable debt securities	$	2,051.20		$	1.9		$	(0.8	)	$	2,050.10
	Marketable equity securities, non-current	$	6.2		$	0.3		$	—		$	5.9
	As of December 31, 2014 (In millions)	Fair			Gross			Gross			Amortized
		Value			Unrealized			Unrealized			Cost
					Gains			Losses
	Available-for-sale:
	Corporate debt securities
	Current	$	370.4		$	—		$	(0.2	)	$	370.6
	Non-current	692.6			0.2			(1.5		)	693.9
	Government securities
	Current	269.9			—			(0.1		)	270
	Non-current	579.9			0.3			(0.4		)	580
	Mortgage and other asset backed securities
	Current	0.2			—			—			0.2
	Non-current	198.1			0.2			(0.2		)	198.1
	Total marketable debt securities	$	2,111.10		$	0.7		$	(2.4	)	$	2,112.80
	Marketable equity securities, non-current	$	6.9		$	1.2		$	(0.2	)	$	5.9
	The following table summarizes our financial assets with maturities of less than 90 days from the date of purchase included within cash and cash equivalents on the accompanying condensed consolidated balance sheet:
	(In millions)	As of			As of
		March 31,			December 31,
		2015			2014
	Commercial paper	$	10.3		$	54.2
	Overnight reverse repurchase agreements	308.6			305
	Money market funds	427.2			321.2
	Short-term debt securities	6.4			35.9
	Total	$	752.5		$	716.3
	The carrying values of our commercial paper, including accrued interest, overnight reverse repurchase agreements, money market funds and our short-term debt securities approximate fair value due to their short term maturities.
	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 March 31, 2015						As of December 31, 2014
	(In millions)	Estimated			Amortized			Estimated			Amortized
		Fair Value			Cost			Fair Value			Cost
	Due in one year or less	$	673.9		$	674		$	640.5		$	640.8
	Due after one year through five years	1,252.00			1,251.00			1,343.70			1,345.20
	Due after five years	125.3			125.1			126.9			126.8
	Total available-for-sale securities	$	2,051.20		$	2,050.10		$	2,111.10		$	2,112.80
	The average maturity of our marketable debt securities available-for-sale as of March 31, 2015 and December 31, 2014 was approximately 15 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
		Ended March 31,
	(In millions)	2015			2014
	Proceeds from maturities and sales	$	373.6		$	757.5
	Realized gains	$	0.2		$	0.2
	Realized losses	$	(0.3	)	$	(0.1	)
	Strategic Investments
	As of March 31, 2015 and December 31, 2014, our strategic investment portfolio was comprised of investments totaling $45.3 million and $47.8 million, respectively, which are included in investments and other assets in our accompanying condensed consolidated balance sheets.
	Our strategic investment portfolio includes investments in marketable equity securities of certain biotechnology companies and our investments in venture capital funds accounted for at fair value which totaled $18.3 million and $21.4 million as of March 31, 2015 and December 31, 2014, respectively. Our strategic investment portfolio also includes other equity investments in privately-held companies and additional investments in venture capital funds accounted for under the cost method. The carrying value of these investments totaled $27.0 million and $26.4 million as of March 31, 2015 and December 31, 2014, respectively.

Derivative_Instruments

Derivative Instruments	3 Months Ended
	Mar. 31, 2015
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 are earned in currencies other than the U.S. dollar. The value of revenues 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.
	Foreign currency forward contracts in effect as of March 31, 2015 and December 31, 2014 had durations of 1 to 21 months and 1 to 15 months, respectively. These contracts have been designated as cash flow hedges and accordingly, to the extent effective, any unrealized gains or losses on these foreign currency forward contracts are reported in accumulated other comprehensive income (loss) (referred to as AOCI in the tables below). Realized gains and losses for the effective portion of such contracts are recognized in revenue when the sale of product in the currency being hedged is recognized. To the extent ineffective, hedge transaction gains and losses are reported in other income (expense), net.
	The notional value of foreign currency forward contracts that were entered into to hedge forecasted revenues is summarized as follows:
		Notional Amount
	Foreign Currency: (In millions)	As of			As of
		March 31,			December 31,
		2015			2014
	Euro	$	1,482.30		$	1,174.60
	Canadian dollar	45.3			56.7
	British pound sterling	29.2			34.5
	Japanese yen	16.6			16.6
	Australian dollar	15.1			19.9
	Total foreign currency forward contracts	$	1,588.50		$	1,302.30
	The portion of the fair value of these foreign currency forward contracts that was included in accumulated other comprehensive income (loss) within total equity reflected gains of $159.8 million and $72.1 million as of March 31, 2015 and December 31, 2014, respectively. We expect all contracts to be settled over the next 21 months and any amounts in accumulated other comprehensive income (loss) to be reported as an adjustment to revenue. 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 March 31, 2015 and December 31, 2014, credit risk did not change the fair value of our foreign currency forward contracts.
	The following table summarizes the effect of derivatives designated as hedging instruments on our condensed consolidated statements of income:
	For the Three Months Ended March 31,
	Net Gains/(Losses)								Net Gains/(Losses)
	Reclassified from AOCI into Operating Income								Recognized into Net Income
	(Effective Portion)								(Ineffective Portion)
	Location		2015			2014			Location	2015		2014
	Revenue		$	35		$	(4.7	)	Other income (expense)	$	2.2	$	(0.2	)
	Foreign Currency Forward Contracts - Other Derivatives
	We also enter into other foreign currency forward contracts, usually with one month durations, to mitigate the foreign currency risk related to certain balance sheet positions. We have not elected hedge accounting for these transactions.
	The aggregate notional amount of these outstanding foreign currency contracts was $494.2 million and $365.2 million as of March 31, 2015 and December 31, 2014, respectively. A net loss of $9.7 million related to these contracts was recognized as a component of other income (expense), net, for three months ended March 31, 2015, respectively, as compared to a net loss of $1.4 million, in the prior year comparative period.
	Summary of Derivatives
	While certain of our derivatives are subject to netting arrangements with our counterparties, we do not offset derivative assets and liabilities within our condensed consolidated balance sheets.
	The following table summarizes the fair value and presentation in our condensed consolidated balance sheets for our outstanding derivatives including those designated as hedging instruments:
	(In millions)	Balance Sheet Location	Fair Value As of March 31, 2015
	Hedging Instruments:
	Asset derivatives	Other current assets	$	160.1
		Investments and other assets	$	6.2
	Liability derivatives	Accrued expenses and other	$	1.3
		Other long-term liabilities	$	3.7
	Other Derivatives:
	Asset derivatives	Other current assets	$	1.9
	Liability derivatives	Accrued expenses and other	$	7.6
	(In millions)	Balance Sheet Location	Fair Value As of December 31, 2014
	Hedging Instruments:
	Asset derivatives	Other current assets	$	69.5
		Investments and other assets	$	1.9
	Other Derivatives:
	Asset derivatives	Other current assets	$	1.3
	Liability derivatives	Accrued expenses and other	$	5.4

Equity

Equity	3 Months Ended
	Mar. 31, 2015
Equity [Abstract]
Equity	Equity
	Total equity as of March 31, 2015 increased $855.4 million compared to December 31, 2014. This increase was primarily driven by net income attributable to Biogen Inc. of $822.5 million and an increase in additional paid in capital resulting from our share-based compensation arrangements totaling $46.3 million.
	Share Repurchases
	In February 2011, our Board of Directors authorized the repurchase of up to 20.0 million shares of common stock. This authorization does not have an expiration date. During the three months ended March 31, 2015 and 2014, we did not repurchase any shares of common stock. As of March 31, 2015, approximately 1.3 million shares of our common stock remain available for repurchase under the 2011 authorization.
	Noncontrolling Interests
	The following table reconciles equity attributable to noncontrolling interests (NCI):
		For the Three Months
		Ended March 31,
	(In millions)	2015			2014
	NCI, beginning of period	$	5		$	0.6
	Net income (loss) attributable to NCI, net of tax	(2.4		)	(0.2		)
	Fair value of net assets and liabilities acquired and assigned to NCI	—			4
	Translation adjustment and other	0.1			—
	NCI, end of period	$	2.7		$	4.4

Accumulated_Other_Comprehensiv

Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss)	3 Months Ended
	Mar. 31, 2015
Equity [Abstract]
Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss)
	The following table summarizes the changes in accumulated other comprehensive income (loss), net of tax by component:
	(In millions)	Unrealized Gains (Losses) on Securities Available for Sale				Unrealized Gains (Losses) on Foreign Currency Forward Contracts				Unfunded Status of Postretirement Benefit Plans			Translation Adjustments			Total
	Balance, as of December 31, 2014	$	(0.4	)		$	71.7			$	(31.6	)	$	(99.2	)	$	(59.5	)
	Other comprehensive income (loss) before reclassifications	1.2				122.1				1.3			(100.9		)	23.7
	Amounts reclassified from accumulated other comprehensive income (loss)	0.1				(34.8		)		—			—			(34.7		)
	Net current period other comprehensive income (loss)	1.3				87.3				1.3			(100.9		)	(11.1		)
	Balance, as of March 31, 2015	$	0.9			$	159			$	(30.3	)	$	(200.1	)	$	(70.5	)
	(In millions)	Unrealized Gains (Losses) on Securities Available for Sale				Unrealized Gains (Losses) on Foreign Currency Forward Contracts				Unfunded Status of Postretirement Benefit Plans			Translation Adjustments			Total
	Balance, as of December 31, 2013	$	5.6			$	(23.7	)		$	(19.6	)	$	10		$	(27.7	)
	Other comprehensive income (loss) before reclassifications	1.8				1.5				0.8			(2.9		)	1.2
	Amounts reclassified from accumulated other comprehensive income (loss)	(0.1		)		4.3				—			—			4.2
	Net current period other comprehensive income (loss)	1.7				5.8				0.8			(2.9		)	5.4
	Balance, as of March 31, 2014	$	7.3			$	(17.9	)		$	(18.8	)	$	7.1		$	(22.4	)
	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
			Ended March 31,
			2015				2014
	Gains (losses) on securities available for sale	Other income (expense)	$	(0.1	)		$	0.1
		Income tax benefit (expense)	—				—
	Gains (losses) on foreign currency forward contracts	Revenues	35				(4.7		)
		Income tax benefit (expense)	(0.2		)		0.4
	Total reclassifications, net of tax		$	34.7			$	(4.2	)

Earnings_per_Share

Earnings per Share	3 Months Ended
	Mar. 31, 2015
Earnings Per Share [Abstract]
Earnings per Share	Earnings per Share
	Basic and diluted earnings per share are calculated as follows:
		For the Three Months
		Ended March 31,
	(In millions)	2015		2014
	Numerator:
	Net income attributable to Biogen Inc.	$	822.5	$	480
	Denominator:
	Weighted average number of common shares outstanding	235		236.8
	Effect of dilutive securities:
	Stock options and employee stock purchase plan	0.1		0.1
	Time-vested restricted stock units	0.3		0.6
	Market stock units	0.2		0.3
	Dilutive potential common shares	0.6		1
	Shares used in calculating diluted earnings per share	235.6		237.8
	Amounts excluded from the calculation of net income per diluted share because their effects were anti-dilutive were insignificant.

Sharebased_Payments

Share-based Payments	3 Months Ended
	Mar. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-based Payments	Share-based Payments
	Share-based Compensation Expense
	The following table summarizes share-based compensation expense included within our condensed consolidated statements of income:
		For the Three Months
		Ended March 31,
	(In millions)	2015				2014
	Research and development	$	35.4			$	29.4
	Selling, general and administrative	55.6				47.2
	Subtotal	91				76.6
	Capitalized share-based compensation costs	(3.4		)		(2.6		)
	Share-based compensation expense included in total cost and expenses	87.6				74
	Income tax effect	(26.7		)		(22.3		)
	Share-based compensation expense included in net income attributable to Biogen Inc.	$	60.9			$	51.7
	The following table summarizes share-based compensation expense associated with each of our share-based compensation programs:
		For the Three Months
		Ended March 31,
	(In millions)	2015				2014
	Market stock units	$	17			$	14.9
	Time-vested restricted stock units	31.9				29.2
	Cash settled performance units	23.8				21.4
	Performance units	13.2				6.3
	Employee stock purchase plan	5.1				4.8
	Subtotal	91				76.6
	Capitalized share-based compensation costs	(3.4		)		(2.6		)
	Share-based compensation expense included in total cost and expenses	$	87.6			$	74
	Grants Under Share-based Compensation Plans
	The following table summarizes our equity grants to employees, officers and directors under our current stock plans:
		For the Three Months
		Ended March 31,
		2015			2014
	Market stock units	175,000			214,000
	Cash settled performance shares	112,000			172,000
	Performance units	89,000			50,000
	Time-vested restricted stock units	351,000			392,000
	In addition, for the three months ended March 31, 2015, approximately 68,000 shares were issued under our employee stock purchase plan compared to approximately 74,000 shares issued in the prior year comparative period.

Income_Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2015
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes
	For the three months ended March 31, 2015, our effective tax rate was 25.6%, as compared to 26.8%, in the prior year comparative period.
	A reconciliation between the U.S. federal statutory tax rate and our effective tax rate is summarized as follows:
		For the Three Months
		Ended March 31,
		2015		2014
	Statutory rate	35	%	35	%
	State taxes	(0.4	)	1.2
	Taxes on foreign earnings	(8.4	)	(8.4	)
	Credits and net operating loss utilization	(0.7	)	(1.3	)
	Purchased intangible assets	1		1.5
	Manufacturing deduction	(1.6	)	(2.0	)
	Other permanent items	0.6		0.3
	Other	0.1		0.5
	Effective tax rate	25.6	%	26.8	%
	For the three months ended March 31, 2015, compared to the same period in 2014, the decrease in our income tax rate was due to a benefit resulting from the remeasurement of one of our uncertain tax positions related to state income tax reporting, partially offset by a higher percentage of our income being earned in the U.S.
	Accounting for Uncertainty in Income Taxes
	We and our subsidiaries are routinely examined by various taxing authorities. We file income tax returns in the U.S. federal jurisdiction, various U.S. states, and foreign jurisdictions. With few exceptions, including the proposed disallowance we discuss below, 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 2004.
	In March 2015, we received a final assessment from the Danish Tax Authority (SKAT) for fiscal 2009, regarding withholding taxes and the treatment of certain intercompany transactions involving our Danish affiliate and another of our affiliates. The audits of our tax filings for 2010 through 2013 are not completed but have been prepared in a manner consistent with prior filings, with similar transactions, which may result in an assessment for those years. The total amount assessed for 2009 is $45.6 million, including interest. For all periods potentially under dispute, we believe that positions taken in our tax filings are valid and we are contesting the assessment vigorously.
	Federal Uncertain Tax Positions
	During the three months ended March 31, 2015, the net effect of adjustments to our uncertain tax positions was a net benefit of $16.4 million primarily related to the state impact of a federal uncertain tax item. It is reasonably possible that we will adjust the value of our uncertain tax positions related to our unconsolidated joint business and certain transfer pricing issues as we receive additional information from various taxing authorities, including reaching settlements with the authorities. In addition, the IRS 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.
	In October 2011, in conjunction with our examination, the IRS proposed a disallowance of approximately $130.0 million in deductions for tax years 2007, 2008 and 2009 related to payments for services provided by our wholly owned Danish subsidiary located in Hillerød, Denmark. We believe that these items represent valid deductible business expenses and are vigorously defending our position. We have initiated a mutual agreement procedure between the IRS and SKAT for the years 2001 through 2009, in an attempt to reach agreement on the issue. In addition, we have applied for a bilateral advanced pricing agreement for the years 2010 through 2014 to resolve similar issues for the subsequent years.

Other_Consolidated_Financial_S

Other Consolidated Financial Statement Detail	3 Months Ended
	Mar. 31, 2015
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
		Ended March 31,
	(In millions)	2015			2014
	Interest income	$	3.3		$	2.7
	Interest expense	(6.7		)	(7.6		)
	Gain (loss) on investments, net	(1.7		)	3
	Foreign exchange gains (losses), net	(13.0		)	(3.3		)
	Other, net	3.1			(0.4		)
	Total other income (expense), net	$	(15.0	)	$	(5.6	)
	Accrued Expenses and Other
	Accrued expenses and other consists of the following:
	(In millions)	As of			As of
		March 31,			December 31,
		2015			2014
	Revenue-related rebates	$	439.3		$	359.2
	Employee compensation and benefits	202.3			393.8
	Royalties and licensing fees	150.4			172.4
	Deferred revenue	124.9			120.9
	Current portion of contingent consideration obligations	64.4			265.5
	Other	403.5			507.5
	Total accrued expenses and other	$	1,384.80		$	1,819.30
	Other Long-Term Liabilities
	Other long-term liabilities consists of the following:
	(In millions)	As of			As of
		March 31,			December 31,
		2015			2014
	Contingent consideration obligation	$	397.5		$	200
	Employee compensation and benefits	213.3			200.7
	Other	281.7			249.4
	Total other long-term liabilities	$	892.5		$	650.1

Investments_in_Variable_Intere

Investments in Variable Interest Entities	3 Months Ended
	Mar. 31, 2015
Investments in Variable Interest Entities [Abstract]
Investments in Variable Interest Entities	Investments in Variable Interest Entities
	Consolidated Variable Interest Entities
	Our condensed consolidated financial statements include the financial results of variable interest entities in which we are the primary beneficiary.
	Neurimmune SubOne AG
	In 2007, we entered into a collaboration agreement with Neurimmune SubOne AG (Neurimmune), a subsidiary of Neurimmune AG, for the development and commercialization of antibodies for the treatment of Alzheimer’s disease. Neurimmune conducts research to identify potential therapeutic antibodies and we are responsible for the development, manufacturing and commercialization of all products. Our anti-amyloid beta antibody, aducanumab (BIIB037), for the treatment of Alzheimer’s disease resulted from this collaboration. In December 2014, we reported positive interim data from the Phase 1b trial of aducanumab. Based upon our current development plans, we may pay Neurimmune up to $335.0 million in remaining milestone payments, of which $60.0 million is payable upon the initiation of a late stage clinical trial. We may also pay royalties in the low-to-mid-teens on sales of any resulting commercial products.
	Amounts that are incurred by Neurimmune for research and development expenses in support of the collaboration that we reimburse are reflected in research and development expense in our condensed consolidated statements of income. Future milestone payments, if any, will be reflected within our condensed consolidated statements of income as a charge to noncontrolling interest, net of tax, when such milestones are achieved.
	For the three months ended March 31, 2015, the collaboration incurred expenses totaling $12.8 million, which is reflected as research and development expense within our condensed consolidated statements of income, as compared to $11.0 million, in the prior year comparative period.
	The assets and liabilities of Neurimmune are not significant to our financial position or results of operations as it is a research and development organization. We have provided no financing to Neurimmune other than contractually required amounts.
	Unconsolidated Variable Interest Entities
	We have relationships with other variable interest entities that we do not consolidate as we lack the power to direct the activities that significantly impact the economic success of these entities. These relationships include investments in certain biotechnology companies and research collaboration agreements.
	As of March 31, 2015 and December 31, 2014, the total carrying value of our investments in biotechnology companies totaled $8.5 million and $7.9 million, respectively. Our maximum exposure to losses related to these variable interest entities is limited to the carrying value of our investments.
	We have 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 within our condensed consolidated statements of income, as they are incurred. We have provided no financing to these variable interest entities other than previously contractually required amounts.
	For additional information related to our investments in variable interest entities, please read Note 19, Investments in Variable Interest Entities to our consolidated financial statements included within our 2014 Form 10-K.

Collaborative_and_Other_Relati

Collaborative and Other Relationships	3 Months Ended
	Mar. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaborative and Other Relationships	Collaborative and Other Relationships
	Samsung Bioepis
	Joint Venture Agreement
	In February 2012, we entered into a joint venture agreement with Samsung BioLogics Co. Ltd. (Samsung Biologics), establishing an entity, Samsung Bioepis, to develop, manufacture and market biosimilar pharmaceuticals. Samsung Biologics contributed 280.5 billion South Korean won (approximately $250.0 million) for an 85% stake in Samsung Bioepis and we contributed approximately 49.5 billion South Korean won (approximately $45.0 million) for the remaining 15% ownership interest. Under the joint venture agreement, we have no obligation to provide any additional funding and our ownership interest may be diluted due to financings in which we do not participate. As of March 31, 2015, our ownership interest is approximately 10%, which reflects our additional contribution of 6.3 billion South Korean won (approximately $5.7 million) in the first quarter of 2015 and the effect of an additional equity financing by Samsung Biologics in which we did not participate. We maintain an option to purchase additional stock in Samsung Bioepis that would allow us to increase our ownership percentage up to 49.9%. The exercise of this option is within our control and is based on paying for 49.9% of the total investment made by Samsung Biologics into Samsung Bioepis in excess of what we have already contributed during the agreement plus interest.
	As of March 31, 2015 and December 31, 2014, the carrying value of our investment in Samsung Bioepis totaled 13.6 billion and 9.1 billion South Korean won (approximately $12.4 million and $8.6 million), respectively, which is classified as a component of investments and other assets within our condensed consolidated balance sheets. Based on our level of influence over Samsung Bioepis, we account for this investment under the equity method of accounting and we recognize our share of the results of operations related to our investment in Samsung Bioepis one quarter in arrears when the results of the entity become available, which is reflected as equity in loss of investee, net of tax within our condensed consolidated statements of income. During the three months ended March 31, 2015, we recognized a loss on our investment of $0.8 million, as compared to $7.6 million, in the prior year comparative period.
	Commercial Agreement
	On December 17, 2013, pursuant to our rights under the joint venture agreement with Samsung Biologics, we entered into an agreement with Samsung Bioepis to commercialize anti-tumor necrosis factor (TNF) biosimilar product candidates in Europe and, in the case of one anti-TNF biosimilar candidate, Japan. Under the terms of this agreement, we paid $36.0 million, which was recorded as a research and development expense within our condensed consolidated statements of income as the programs they relate to had not achieved regulatory approval. Samsung Bioepis is eligible to receive an additional $75.0 million in additional milestones related to clinical development and regulatory approval of the product candidates. Upon commercialization, there will be a 50% profit share with Samsung Bioepis.
	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 months ended March 31, 2015, we recognized $18.9 million in other revenues in relation to these services, as compared to $24.1 million in the prior year comparative period, which is reflected as a component of other revenues within our condensed consolidated statement of income.
	For additional information related to our other significant collaboration arrangements, please read Note 20, Collaborative and Other Relationships to our consolidated financial statements included within our 2014 Form 10-K.

Litigation

Litigation	3 Months Ended
	Mar. 31, 2015
Loss Contingency, Information about Litigation Matters [Abstract]
Litigation	Litigation
	’755 Patent Litigation
	On May 28, 2010, Biogen MA Inc. (formerly Biogen Idec MA Inc.) filed a complaint in the U.S. District Court for the District of New Jersey alleging infringement by Bayer Healthcare Pharmaceuticals Inc. (Bayer) (manufacturer, marketer and seller of BETASERON and manufacturer of EXTAVIA), EMD Serono, Inc. (manufacturer, marketer and seller of REBIF), Pfizer, Inc. (co-marketer of REBIF), and Novartis Pharmaceuticals Corp. (marketer and seller of EXTAVIA) of our U.S. Patent No. 7,588,755 (’755 Patent), which claims the use of interferon beta for immunomodulation or treating a viral condition, viral disease, cancers or tumors. The complaint seeks monetary damages, including lost profits and royalties. Bayer had previously filed a complaint against us in the same court, on May 27, 2010, seeking a declaratory judgment that it does not infringe the ’755 Patent and that the patent is invalid, and seeking monetary relief in the form of attorneys' fees, costs and expenses. The court has consolidated the two lawsuits, and we refer to the two actions as the “Consolidated ’755 Patent Actions.”
	Bayer, Pfizer, Novartis and EMD Serono have all filed counterclaims in the Consolidated ’755 Patent Actions seeking declaratory judgments of patent invalidity and non-infringement, and seeking monetary relief in the form of costs and attorneys' fees, and EMD Serono and Bayer have each filed a counterclaim seeking a declaratory judgment that the ’755 Patent is unenforceable based on alleged inequitable conduct. Bayer has also amended its complaint to seek such a declaration. No trial date has been set.
	Italian National Medicines Agency
	In the fourth quarter of 2011, Biogen Italia SRL received notice from the Italian National Medicines Agency (Agenzia Italiana del Farmaco or AIFA) that sales of TYSABRI after mid-February 2009 exceeded a reimbursement limit established pursuant to a Price Determination Resolution (Price Resolution) granted by AIFA in December 2006. On December 23, 2011, we filed an appeal in the Regional Administrative Tribunal of Lazio (Il Tribunale Amministrativo Regionale per il Lazio) in Rome seeking a ruling that the reimbursement limit in the Price Resolution should apply as written to only “the first 24 months” of TYSABRI sales, which ended in mid-February 2009. The appeal is still pending. In June 2014, AIFA approved a resolution affirming that there is no reimbursement limit from and after February 2013. AIFA and Biogen Italia SRL are discussing a possible resolution for the period from mid-February 2009 through January 2013.
	Average Manufacturer Price Litigation
	In 2011, we and several other pharmaceutical companies were served with a complaint originally filed under seal on October 28, 2008 in the United States District Court for the Eastern District of Pennsylvania by Ronald Streck on behalf of himself and the United States, 24 states and the District of Columbia (collectively the “States”). The complaint alleges that Biogen violated the False Claims Act, 31 U.S.C. § 3729 et seq. and local statutory counterparts by under reporting Average Manufacturer Price (AMP) information to the Centers for Medicare and Medicaid Services. The United States and the States have declined to intervene. We have agreed to a settlement of the case. The federal government has consented to the settlement, and we are now seeking the consent of the States. As of March 31, 2015, we recorded the proposed settlement, which did not have a significant impact on our results of operations.
	Government Matters
	We have learned that state and federal governmental authorities are investigating our sales and promotional practices and have received related subpoenas. We have also received a subpoena from the federal government for documents relating to our relationship with certain pharmacy benefit managers. We are cooperating with the government in these matters.
	Qui Tam Litigation
	In August, 2012, we learned that a relator, on behalf of the United States and certain states, filed a suit under seal on February 17, 2011 against us, Elan Corporation, plc, and Elan Pharmaceuticals, Inc. in the United States District Court for the Western District of Virginia. We have neither seen nor been served with the complaint, but understand that it was filed under the Federal False Claims Act.
	Forward Pharma Litigation
	On November 18, 2014 Forward Pharma A/S (“Forward Pharma”) filed suit against us in the Regional Court of Dusseldorf, Germany alleging that TECFIDERA infringes German Utility Model DE 20 2005 022 112 U1, which was registered on April 24, 2014 and expires in October 2015. A hearing has been scheduled for early 2016.
	Forward Pharma seeks a declaration of infringement and a determination of damages. We believe that we have good and valid defenses to the complaint. We have not formed an opinion that an unfavorable outcome is either “probable” or “remote” and are unable to estimate the magnitude or range of any potential loss.
	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.

Commitments_and_Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies Disclosure	Commitments and Contingencies
	In 2006, we acquired Fumapharm AG. As part of this acquisition we acquired FUMADERM and TECFIDERA (together, Fumapharm Products). We paid $220.0 million upon closing of the transaction and agreed to pay an additional $15.0 million if a Fumapharm Product was approved for MS in the U.S. or E.U. In the second quarter of 2013, we paid this $15.0 million contingent payment as TECFIDERA was approved in the U.S. for MS by the U.S. Food and Drug Administration (FDA). We are also required to make additional contingent payments to former shareholders of Fumapharm AG or holders of their rights based on the attainment of certain cumulative sales levels of Fumapharm Products and the level of total net sales of Fumapharm Products in the prior twelve month period, as defined in the acquisition agreement.
	During the three months ended March 31, 2015, we paid a $250.0 million contingent payment as we reached the $4.0 billion cumulative sales level related to the Fumapharm Products in the fourth quarter of 2014.
	We will owe an additional $300.0 million contingent payment for every additional $1.0 billion in cumulative sales level of Fumapharm Products reached if the prior 12 months sales of the Fumapharm Products exceed $3.0 billion, until such time as the cumulative sales level reaches $20.0 billion, at which time no further contingent payments shall be due. These payments will be accounted for as an increase to goodwill as incurred, in accordance with the accounting standard applicable to business combinations when we acquired Fumapharm. Any portion of the payment which is tax deductible will be recorded as a reduction to goodwill. Payments are due within 60 days following the end of the quarter in which the applicable cumulative sales level has been reached.

Segment_Information

Segment Information	3 Months Ended
	Mar. 31, 2015
Segment Reporting [Abstract]
Segment Information	Segment Information
	We operate as one operating segment, which is focused on discovering, developing, manufacturing and delivering therapies for neurological, autoimmune and hematologic disorders, and, therefore, our chief operating decision-maker manages the operations of our company as a single operating segment.

Summary_of_Significant_Account1

Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Overview	Biogen is a global biopharmaceutical company focused on discovering, developing, manufacturing and delivering therapies for neurological, autoimmune and hematologic disorders. Our principal marketed products include AVONEX, PLEGRIDY, TECFIDERA, TYSABRI, and FAMPYRA for multiple sclerosis (MS), ALPROLIX for hemophilia B and ELOCTATE for hemophilia A. We also collaborate on the development and commercialization of RITUXAN for the treatment of non-Hodgkin's lymphoma, chronic lymphocytic leukemia and other conditions and share profits and losses for GAZYVA which is approved for the treatment of chronic lymphocytic leukemia.
Basis of presentation	In the opinion of management, the accompanying unaudited condensed consolidated financial statements include all adjustments, consisting of normal recurring accruals, necessary for a fair presentation of our financial statements for interim periods in accordance with accounting principles generally accepted in the United States (U.S. GAAP). The information included in this quarterly report on Form 10-Q should be read in conjunction with our consolidated financial statements and the accompanying notes included in our Annual Report on Form 10-K for the year ended December 31, 2014 (2014 Form 10-K). Our accounting policies are described in the “Notes to Consolidated Financial Statements” in our 2014 Form 10-K and updated, as necessary, in this Form 10-Q. The year-end condensed consolidated balance sheet data presented for comparative purposes was derived from our audited financial statements, but does not include all disclosures required by U.S. GAAP. The results of operations for the three months ended March 31, 2015, are not necessarily indicative of the operating results for the full year or for any other subsequent interim period.
Consolidation	Our condensed consolidated financial statements reflect our financial statements, those of our wholly-owned subsidiaries and those of certain variable interest entities where we are the primary beneficiary. For consolidated entities where we own or are exposed to less than 100% of the economics, we record net income (loss) attributable to noncontrolling interests in our condensed consolidated statements of income equal to the percentage of the economic or ownership interest retained in such entities by the respective noncontrolling parties. Intercompany balances and transactions are eliminated in consolidation.
	In determining whether we are the primary beneficiary of an entity and therefore required to consolidate, we apply a qualitative approach that determines whether we have both (1) the power to direct the economically significant activities of the entity and (2) the obligation to absorb losses of, or the right to receive benefits from, the entity that could potentially be significant to that entity. These considerations impact the way we account for our existing collaborative relationships and other arrangements. We continuously assess whether we are the primary beneficiary of a variable interest entity as changes to existing relationships or future transactions may result in us consolidating or deconsolidating one or more of our collaborators or partners.
Use of estimates	The preparation of our condensed consolidated financial statements requires us to make estimates, judgments, and assumptions that may affect the reported amounts of assets, liabilities, equity, revenues and expenses, and related disclosure of contingent assets and liabilities. On an on-going basis, we evaluate our estimates and judgments and methodologies. We base our estimates on historical experience and on various other assumptions that are believed to be reasonable, the results of which form the basis for making judgments about the carrying values of assets and liabilities. Actual results may differ from these estimates under different assumptions or conditions.
Property, plant and equipment	Property, plant and equipment are recorded at cost, net of accumulated depreciation. Accumulated depreciation on property, plant and equipment was $1,217.8 million and $1,186.4 million as of March 31, 2015 and December 31, 2014, respectively.

Acquisitions_Acquisitions_Tabl

Acquisitions Acquisitions (Tables)	3 Months Ended
	Mar. 31, 2015
Acquisitions [Abstract]
Purchase price schedule	The purchase price consists of the following:
	(In millions)
	Cash portion of consideration	$	200.1
	Contingent consideration	238.5
	Total purchase price	$	438.6
Estimated fair values of the separately identifiable assets acquired and liabilities assumed	The following table summarizes the estimated fair values of the separately identifiable assets acquired and liabilities assumed as of February 12, 2015:
	(In millions)
	In-process research and development	$	424.6
	Other intangible assets	7.6
	Goodwill	92.3
	Deferred tax liability	(84.9		)
	Other, net	(1.0		)
	Total purchase price	$	438.6

Accounts_Receivable_Tables

Accounts Receivable (Tables)	3 Months Ended
	Mar. 31, 2015
Receivables [Abstract]
Net accounts receivable balances from product sales in selected European countries	Our net accounts receivable balances from product sales in selected European countries are summarized as follows:
		As of March 31, 2015
	(In millions)	Current		Non-Current		Total
		Balance Included		Balance Included
		within Accounts		within Investments
		Receivable, net		and Other Assets
	Spain	$	71.1	$	2.2	$	73.3
	Portugal	$	12.9	$	7.7	$	20.6
		As of December 31, 2014
	(In millions)	Current		Non-Current		Total
		Balance Included		Balance Included
		within Accounts		within Investments
		Receivable, net		and Other Assets
	Spain	$	62.5	$	5	$	67.5
	Portugal	$	15.1	$	7.6	$	22.7

Reserves_for_Discounts_and_All1

Reserves for Discounts and Allowances (Tables)	3 Months Ended
	Mar. 31, 2015
Reserves for Discounts and Allowances [Abstract]
Analysis of the changes in reserves	An analysis of the change in reserves for discounts and allowances is summarized as follows:
	(In millions)	Discounts			Contractual			Returns			Total
					Adjustments
	Balance, as of December 31, 2014	$	47.6		$	387.1		$	49.1		$	483.8
	Current provisions relating to sales in current year	101.8			441.2			10.2			553.2
	Adjustments relating to prior years	(2.4		)	(10.2		)	(5.9		)	(18.5		)
	Payments/credits relating to sales in current year	(50.3		)	(166.7		)	(0.7		)	(217.7		)
	Payments/credits relating to sales in prior years	(41.3		)	(170.7		)	(4.6		)	(216.6		)
	Balance, as of March 31, 2015	$	55.4		$	480.7		$	48.1		$	584.2
Total reserves included in condensed consolidated balance sheets	The total reserves above, included in our condensed consolidated balance sheets, are summarized as follows:
	(In millions)	As of			As of
		March 31,			December 31,
		2015			2014
	Reduction of accounts receivable	$	144.9		$	124.6
	Component of accrued expenses and other	439.3			359.2
	Total reserves	$	584.2		$	483.8

Inventory_Tables

Inventory (Tables)	3 Months Ended
	Mar. 31, 2015
Inventory Disclosure [Abstract]
Components of inventory	The components of inventory are summarized as follows:
	(In millions)	As of		As of
		March 31,		December 31,
		2015		2014
	Raw materials	$	139.7	$	128.3
	Work in process	513.1		511.5
	Finished goods	172.6		164.2
	Total inventory	$	825.3	$	804

Intangible_Assets_and_Goodwill1

Intangible Assets and Goodwill (Tables)	3 Months Ended
	Mar. 31, 2015
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible assets	Intangible assets, net of accumulated amortization, impairment charges and adjustments, are summarized as follows:
				As of March 31, 2015							As of December 31, 2014
	(In millions)	Estimated		Cost		Accumulated			Net		Cost		Accumulated			Net
		Life				Amortization							Amortization
	Out-licensed patents	13-23 years		$	543.3	$	(487.8	)	$	55.5	$	543.3	$	(481.7	)	$	61.6
	Developed	15-23 years		3,005.30		(2,447.5		)	557.8		3,005.30		(2,396.8		)	608.5
	technology
	In-process research and development	Indefinite until commercialization		726.1		—			726.1		314.1		—			314.1
	Trademarks and	Indefinite		64		—			64		64		—			64
	tradenames
	Acquired and in-licensed rights	6-17 years		3,288.90		(339.2		)	2,949.70		3,280.40		(300.1		)	2,980.30
	and patents
	Total intangible assets			$	7,627.60	$	(3,274.5	)	$	4,353.10	$	7,207.10	$	(3,178.6	)	$	4,028.50
Estimated future amortization for acquired intangible assets	Based upon this recent analysis, the estimated future amortization for acquired intangible assets is expected to be as follows:
	(In millions)	As of
		March 31,
		2015
	2015 (remaining nine months)	$	269
	2016	313.3
	2017	286.1
	2018	281.6
	2019	270.4
	2020	263.4
	Total	$	1,683.80
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
		March 31,
		2015
	Goodwill, beginning of period	$	1,760.20
	Increase to goodwill	92.3
	Other	(2.7		)
	Goodwill, end of period	$	1,849.90

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2015
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 March 31, 2015 (In millions)	Total			Quoted Prices			Significant Other		Significant
					in Active			Observable Inputs		Unobservable
					Markets			(Level 2)		Inputs
					(Level 1)					(Level 3)
	Assets:
	Cash equivalents	$	752.5		$	—		$	752.5	$	—
	Marketable debt securities:
	Corporate debt securities	991			—			991		—
	Government securities	850.6			—			850.6		—
	Mortgage and other asset backed securities	209.6			—			209.6		—
	Marketable equity securities	6.2			6.2			—		—
	Venture capital investments	12.1			—			—		12.1
	Derivative contracts	168.2			—			168.2		—
	Plan assets for deferred compensation	44.1			—			44.1		—
	Total	$	3,034.30		$	6.2		$	3,016.00	$	12.1
	Liabilities:
	Derivative contracts	$	12.6		$	—		$	12.6	$	—
	Contingent consideration obligations	461.8			—			—		461.8
	Total	$	474.4		$	—		$	12.6	$	461.8
	As of December 31, 2014 (In millions)	Total			Quoted Prices			Significant		Significant
					in Active			Other		Unobservable
					Markets			Observable		Inputs
					(Level 1)			Inputs		(Level 3)
								(Level 2)
	Assets:
	Cash equivalents	$	716.3		$	—		$	716.3	$	—
	Marketable debt securities:
	Corporate debt securities	1,063.00			—			1,063.00		—
	Government securities	849.8			—			849.8		—
	Mortgage and other asset backed securities	198.3			—			198.3		—
	Marketable equity securities	6.9			6.9			—		—
	Venture capital investments	14.5			—			—		14.5
	Derivative contracts	72.7			—			72.7		—
	Plan assets for deferred compensation	36.9			—			36.9		—
	Total	$	2,958.40		$	6.9		$	2,937.00	$	14.5
	Liabilities:
	Derivative contracts	$	5.4		$	—		$	5.4	$	—
	Contingent consideration obligations	215.5			—			—		215.5
	Total	$	220.9		$	—		$	5.4	$	215.5
Fair value of venture capital investments	The following table provides a roll forward of the fair value of our venture capital investments, which includes Level 3 measurements:
		For the Three Months
		Ended March 31,
	(In millions)	2015			2014
	Fair value, beginning of period	$	14.5		$	21.9
	Unrealized gains included in earnings	—			2.9
	Unrealized losses included in earnings	(2.4		)	(1.2		)
	Fair value, end of period	$	12.1		$	23.6
Summary of fair and carrying value of debt instruments	The fair and carrying values of our debt instruments, which are Level 2 liabilities, are summarized as follows:
		As of March 31, 2015						As of December 31, 2014
	(In millions)	Fair			Carrying			Fair		Carrying
		Value			Value			Value		Value
	Notes payable to Fumedica	$	12.9		$	12.1		$	12.6	$	11.7
	6.875% Senior Notes due March 1, 2018	633.9			571.8			634.6		573.5
	Total	$	646.8		$	583.9		$	647.2	$	585.2
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
		Ended March 31,
	(In millions)	2015			2014
	Fair value, beginning of period	$	215.5		$	280.9
	Additions	238.5			—
	Changes in fair value	7.8			(0.8		)
	Payments	—			(5.0		)
	Fair value, end of period	$	461.8		$	275.1

Financial_Instruments_Tables

Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2015
Investments, All Other Investments [Abstract]
Marketable debt and equity securities	The following tables summarize our marketable debt and equity securities:
	As of March 31, 2015 (In millions)	Fair			Gross			Gross			Amortized
		Value			Unrealized			Unrealized			Cost
					Gains			Losses
	Available-for-sale:
	Corporate debt securities
	Current	$	385.3		$	0.1		$	(0.1	)	$	385.3
	Non-current	605.7			0.8			(0.4		)	605.3
	Government securities
	Current	288.5			—			(0.1		)	288.6
	Non-current	562.1			0.6			(0.1		)	561.6
	Mortgage and other asset backed securities
	Current	0.1			—			—			0.1
	Non-current	209.5			0.4			(0.1		)	209.2
	Total marketable debt securities	$	2,051.20		$	1.9		$	(0.8	)	$	2,050.10
	Marketable equity securities, non-current	$	6.2		$	0.3		$	—		$	5.9
	As of December 31, 2014 (In millions)	Fair			Gross			Gross			Amortized
		Value			Unrealized			Unrealized			Cost
					Gains			Losses
	Available-for-sale:
	Corporate debt securities
	Current	$	370.4		$	—		$	(0.2	)	$	370.6
	Non-current	692.6			0.2			(1.5		)	693.9
	Government securities
	Current	269.9			—			(0.1		)	270
	Non-current	579.9			0.3			(0.4		)	580
	Mortgage and other asset backed securities
	Current	0.2			—			—			0.2
	Non-current	198.1			0.2			(0.2		)	198.1
	Total marketable debt securities	$	2,111.10		$	0.7		$	(2.4	)	$	2,112.80
	Marketable equity securities, non-current	$	6.9		$	1.2		$	(0.2	)	$	5.9
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 within cash and cash equivalents on the accompanying condensed consolidated balance sheet:
	(In millions)	As of			As of
		March 31,			December 31,
		2015			2014
	Commercial paper	$	10.3		$	54.2
	Overnight reverse repurchase agreements	308.6			305
	Money market funds	427.2			321.2
	Short-term debt securities	6.4			35.9
	Total	$	752.5		$	716.3
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 March 31, 2015						As of December 31, 2014
	(In millions)	Estimated			Amortized			Estimated			Amortized
		Fair Value			Cost			Fair Value			Cost
	Due in one year or less	$	673.9		$	674		$	640.5		$	640.8
	Due after one year through five years	1,252.00			1,251.00			1,343.70			1,345.20
	Due after five years	125.3			125.1			126.9			126.8
	Total available-for-sale securities	$	2,051.20		$	2,050.10		$	2,111.10		$	2,112.80
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
		Ended March 31,
	(In millions)	2015			2014
	Proceeds from maturities and sales	$	373.6		$	757.5
	Realized gains	$	0.2		$	0.2
	Realized losses	$	(0.3	)	$	(0.1	)

Derivative_Instruments_Tables

Derivative Instruments (Tables)	3 Months Ended
	Mar. 31, 2015
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 is summarized as follows:
		Notional Amount
	Foreign Currency: (In millions)	As of			As of
		March 31,			December 31,
		2015			2014
	Euro	$	1,482.30		$	1,174.60
	Canadian dollar	45.3			56.7
	British pound sterling	29.2			34.5
	Japanese yen	16.6			16.6
	Australian dollar	15.1			19.9
	Total foreign currency forward contracts	$	1,588.50		$	1,302.30
Summary of the effect of derivatives designated as hedging instruments on the condensed consolidated statements of income	The following table summarizes the effect of derivatives designated as hedging instruments on our condensed consolidated statements of income:
	For the Three Months Ended March 31,
	Net Gains/(Losses)								Net Gains/(Losses)
	Reclassified from AOCI into Operating Income								Recognized into Net Income
	(Effective Portion)								(Ineffective Portion)
	Location		2015			2014			Location	2015		2014
	Revenue		$	35		$	(4.7	)	Other income (expense)	$	2.2	$	(0.2	)
Summary of fair value and presentation of derivatives	The following table summarizes the fair value and presentation in our condensed consolidated balance sheets for our outstanding derivatives including those designated as hedging instruments:
	(In millions)	Balance Sheet Location	Fair Value As of March 31, 2015
	Hedging Instruments:
	Asset derivatives	Other current assets	$	160.1
		Investments and other assets	$	6.2
	Liability derivatives	Accrued expenses and other	$	1.3
		Other long-term liabilities	$	3.7
	Other Derivatives:
	Asset derivatives	Other current assets	$	1.9
	Liability derivatives	Accrued expenses and other	$	7.6
	(In millions)	Balance Sheet Location	Fair Value As of December 31, 2014
	Hedging Instruments:
	Asset derivatives	Other current assets	$	69.5
		Investments and other assets	$	1.9
	Other Derivatives:
	Asset derivatives	Other current assets	$	1.3
	Liability derivatives	Accrued expenses and other	$	5.4

Equity_Tables

Equity (Tables)	3 Months Ended
	Mar. 31, 2015
Equity [Abstract]
Reconciliation of equity attributable to noncontrolling interests	The following table reconciles equity attributable to noncontrolling interests (NCI):
		For the Three Months
		Ended March 31,
	(In millions)	2015			2014
	NCI, beginning of period	$	5		$	0.6
	Net income (loss) attributable to NCI, net of tax	(2.4		)	(0.2		)
	Fair value of net assets and liabilities acquired and assigned to NCI	—			4
	Translation adjustment and other	0.1			—
	NCI, end of period	$	2.7		$	4.4

Accumulated_Other_Comprehensiv1

Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Tables)	3 Months Ended
	Mar. 31, 2015
Equity [Abstract]
Schedule of accumulated other comprehensive income (loss)	The following table summarizes the changes in accumulated other comprehensive income (loss), net of tax by component:
	(In millions)	Unrealized Gains (Losses) on Securities Available for Sale				Unrealized Gains (Losses) on Foreign Currency Forward Contracts				Unfunded Status of Postretirement Benefit Plans			Translation Adjustments			Total
	Balance, as of December 31, 2014	$	(0.4	)		$	71.7			$	(31.6	)	$	(99.2	)	$	(59.5	)
	Other comprehensive income (loss) before reclassifications	1.2				122.1				1.3			(100.9		)	23.7
	Amounts reclassified from accumulated other comprehensive income (loss)	0.1				(34.8		)		—			—			(34.7		)
	Net current period other comprehensive income (loss)	1.3				87.3				1.3			(100.9		)	(11.1		)
	Balance, as of March 31, 2015	$	0.9			$	159			$	(30.3	)	$	(200.1	)	$	(70.5	)
	(In millions)	Unrealized Gains (Losses) on Securities Available for Sale				Unrealized Gains (Losses) on Foreign Currency Forward Contracts				Unfunded Status of Postretirement Benefit Plans			Translation Adjustments			Total
	Balance, as of December 31, 2013	$	5.6			$	(23.7	)		$	(19.6	)	$	10		$	(27.7	)
	Other comprehensive income (loss) before reclassifications	1.8				1.5				0.8			(2.9		)	1.2
	Amounts reclassified from accumulated other comprehensive income (loss)	(0.1		)		4.3				—			—			4.2
	Net current period other comprehensive income (loss)	1.7				5.8				0.8			(2.9		)	5.4
	Balance, as of March 31, 2014	$	7.3			$	(17.9	)		$	(18.8	)	$	7.1		$	(22.4	)
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
			Ended March 31,
			2015				2014
	Gains (losses) on securities available for sale	Other income (expense)	$	(0.1	)		$	0.1
		Income tax benefit (expense)	—				—
	Gains (losses) on foreign currency forward contracts	Revenues	35				(4.7		)
		Income tax benefit (expense)	(0.2		)		0.4
	Total reclassifications, net of tax		$	34.7			$	(4.2	)

Earnings_per_Share_Tables

Earnings per Share (Tables)	3 Months Ended
	Mar. 31, 2015
Earnings Per Share [Abstract]
Basic and diluted earnings per share	Basic and diluted earnings per share are calculated as follows:
		For the Three Months
		Ended March 31,
	(In millions)	2015		2014
	Numerator:
	Net income attributable to Biogen Inc.	$	822.5	$	480
	Denominator:
	Weighted average number of common shares outstanding	235		236.8
	Effect of dilutive securities:
	Stock options and employee stock purchase plan	0.1		0.1
	Time-vested restricted stock units	0.3		0.6
	Market stock units	0.2		0.3
	Dilutive potential common shares	0.6		1
	Shares used in calculating diluted earnings per share	235.6		237.8

ShareBased_Payments_Tables

Share-Based Payments (Tables)	3 Months Ended
	Mar. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-based compensation expense included in condensed consolidated statements of income	The following table summarizes share-based compensation expense included within our condensed consolidated statements of income:
		For the Three Months
		Ended March 31,
	(In millions)	2015				2014
	Research and development	$	35.4			$	29.4
	Selling, general and administrative	55.6				47.2
	Subtotal	91				76.6
	Capitalized share-based compensation costs	(3.4		)		(2.6		)
	Share-based compensation expense included in total cost and expenses	87.6				74
	Income tax effect	(26.7		)		(22.3		)
	Share-based compensation expense included in net income attributable to Biogen Inc.	$	60.9			$	51.7
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
		Ended March 31,
	(In millions)	2015				2014
	Market stock units	$	17			$	14.9
	Time-vested restricted stock units	31.9				29.2
	Cash settled performance units	23.8				21.4
	Performance units	13.2				6.3
	Employee stock purchase plan	5.1				4.8
	Subtotal	91				76.6
	Capitalized share-based compensation costs	(3.4		)		(2.6		)
	Share-based compensation expense included in total cost and expenses	$	87.6			$	74
Summary of equity grants to employees, officers and directors under our current stock plans	The following table summarizes our equity grants to employees, officers and directors under our current stock plans:
		For the Three Months
		Ended March 31,
		2015			2014
	Market stock units	175,000			214,000
	Cash settled performance shares	112,000			172,000
	Performance units	89,000			50,000
	Time-vested restricted stock units	351,000			392,000

Income_Taxes_Tables

Income Taxes (Tables)	3 Months Ended
	Mar. 31, 2015
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
		Ended March 31,
		2015		2014
	Statutory rate	35	%	35	%
	State taxes	(0.4	)	1.2
	Taxes on foreign earnings	(8.4	)	(8.4	)
	Credits and net operating loss utilization	(0.7	)	(1.3	)
	Purchased intangible assets	1		1.5
	Manufacturing deduction	(1.6	)	(2.0	)
	Other permanent items	0.6		0.3
	Other	0.1		0.5
	Effective tax rate	25.6	%	26.8	%

Other_Consolidated_Financial_S1

Other Consolidated Financial Statement Detail (Tables)	3 Months Ended
	Mar. 31, 2015
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
		Ended March 31,
	(In millions)	2015			2014
	Interest income	$	3.3		$	2.7
	Interest expense	(6.7		)	(7.6		)
	Gain (loss) on investments, net	(1.7		)	3
	Foreign exchange gains (losses), net	(13.0		)	(3.3		)
	Other, net	3.1			(0.4		)
	Total other income (expense), net	$	(15.0	)	$	(5.6	)
Accrued expenses and other	Accrued expenses and other consists of the following:
	(In millions)	As of			As of
		March 31,			December 31,
		2015			2014
	Revenue-related rebates	$	439.3		$	359.2
	Employee compensation and benefits	202.3			393.8
	Royalties and licensing fees	150.4			172.4
	Deferred revenue	124.9			120.9
	Current portion of contingent consideration obligations	64.4			265.5
	Other	403.5			507.5
	Total accrued expenses and other	$	1,384.80		$	1,819.30
Other long-term liabilities	Other long-term liabilities consists of the following:
	(In millions)	As of			As of
		March 31,			December 31,
		2015			2014
	Contingent consideration obligation	$	397.5		$	200
	Employee compensation and benefits	213.3			200.7
	Other	281.7			249.4
	Total other long-term liabilities	$	892.5		$	650.1

Summary_of_Significant_Account2

Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Business (Textual) [Abstract]
Interest in subsidiary (less than given percentage)	100.00%
Accumulated depreciation	$1,217.80	$1,186.40

Acquisitions_Acquisitions_Deta

Acquisitions Acquisitions (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Business Acquisition [Line Items]
Contingent consideration	$238.50	$0
Convergence Pharmaceuticals
Business Acquisition [Line Items]
Cash portion of consideration	200.1
Contingent consideration	238.5
Total purchase price	$438.60

Acquisitions_Acquisitions_Deta1

Acquisitions Acquisitions (Details 1) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Feb. 12, 2015
Business Acquisition [Line Items]
Goodwill	$1,849,852,000	$1,760,249,000
Convergence Pharmaceuticals
Business Acquisition [Line Items]
In-process research and development and other intangible assets			424,600,000
Other intangible assets			7,600,000
Goodwill			92,300,000
Deferred tax liability			-84,900,000
Other, net			-1,000,000
Total purchase price			$438,600,000

Acquisitions_Acquisitions_Deta2

Acquisitions Acquisitions (Details Textual) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Feb. 12, 2015
Business Acquisition [Line Items]
Contingent consideration	$238.50	$0
Convergence Pharmaceuticals
Business Acquisition [Line Items]
Cash portion of consideration	200.1
Potential future milestone payments			450
Contingent consideration	238.5
Discount rate	2.00%
In-process research and development and other intangible assets			424.6
Convergence Pharmaceuticals | CNV1014802
Business Acquisition [Line Items]
Potential future milestone payments			350
Discount rate	11.00%
Remaining cost to complete CNV1014802			143
In-process research and development and other intangible assets			170
Convergence Pharmaceuticals | Neuropathic pain indications
Business Acquisition [Line Items]
Remaining cost to complete CNV1014802			$446

Accounts_Receivable_Details

Accounts Receivable (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Accounts receivable balances on product sales from different countries
Current balance included within accounts receivable, net	$1,389,995,000	$1,292,445,000
Spain
Accounts receivable balances on product sales from different countries
Current balance included within accounts receivable, net	71,100,000	62,500,000
Non-current balance included within investments and other assets	2,200,000	5,000,000
Total	73,300,000	67,500,000
Portugal
Accounts receivable balances on product sales from different countries
Current balance included within accounts receivable, net	12,900,000	15,100,000
Non-current balance included within investments and other assets	7,700,000	7,600,000
Total	$20,600,000	$22,700,000

Accounts_Receivable_Details_Te

Accounts Receivable (Details Textual)	3 Months Ended			3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2011	Mar. 31, 2015	Dec. 31, 2014
	USD ($)	USD ($)	USD ($)	Italy	Italy	Italy	Italy
				USD ($)		EUR (€)	EUR (€)
Accounts Receivable (Textual) [Abstract]
Payment terms of accounts receivable arising from product sales	30 to 90 days
Minimum term for accounts receivable	30 days
Maximum term for accounts receivable	90 days
Net accounts receivable balances overdue more than one year	$11,400,000		$11,100,000
Reimbursement limit stated in the resolution					24 months
Product, net	2,172,322,000	1,742,765,000		53,500,000
Proposed settlement							35,600,000
Amount reserved for this matter						€ 75,000,000

Reserves_for_Discounts_and_All2

Reserves for Discounts and Allowances (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015
Analysis of the amount of, and change in, reserves
Balance as of December 31, 2014	$483.80
Current provisions relating to sales in current year	553.2
Adjustments relating to prior years	-18.5
Payments/credits relating to sales in current year	-217.7
Payments/credits relating to sales in prior years	-216.6
Balance as of March 31, 2015	584.2
Discounts
Analysis of the amount of, and change in, reserves
Balance as of December 31, 2014	47.6
Current provisions relating to sales in current year	101.8
Adjustments relating to prior years	-2.4
Payments/credits relating to sales in current year	-50.3
Payments/credits relating to sales in prior years	-41.3
Balance as of March 31, 2015	55.4
Contractual Adjustments
Analysis of the amount of, and change in, reserves
Balance as of December 31, 2014	387.1
Current provisions relating to sales in current year	441.2
Adjustments relating to prior years	-10.2
Payments/credits relating to sales in current year	-166.7
Payments/credits relating to sales in prior years	-170.7
Balance as of March 31, 2015	480.7
Returns
Analysis of the amount of, and change in, reserves
Balance as of December 31, 2014	49.1
Current provisions relating to sales in current year	10.2
Adjustments relating to prior years	-5.9
Payments/credits relating to sales in current year	-0.7
Payments/credits relating to sales in prior years	-4.6
Balance as of March 31, 2015	$48.10

Reserves_for_Discounts_and_All3

Reserves for Discounts and Allowances (Details 1) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Summary of total product revenue reserves included in consolidated balance sheets
Total reserves	$584.20	$483.80
Reduction of accounts receivable
Summary of total product revenue reserves included in consolidated balance sheets
Total reserves	144.9	124.6
Component of accrued expenses and other
Summary of total product revenue reserves included in consolidated balance sheets
Total reserves	$439.30	$359.20

Inventory_Details

Inventory (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Components of inventories
Raw materials	$139,700,000	$128,300,000
Work in process	513,100,000	511,500,000
Finished goods	172,600,000	164,200,000
Total inventory	$825,349,000	$804,022,000

Intangible_Assets_and_Goodwill2

Intangible Assets and Goodwill (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Feb. 12, 2015
Intangible assets
Total intangible assets, gross	$7,627,600,000		$7,207,100,000
Accumulated Amortization	-3,274,500,000		-3,178,600,000
Intangible assets, net	4,353,123,000		4,028,507,000
Intangible Assets and Goodwill (Textual) [Abstract]
Amortization of acquired intangible assets	95,903,000	143,258,000
Accumulated impairment losses related to goodwill	0
Intangible Assets And Goodwill (Additional Textual) [Abstract]
Expected future amortization expense, 2015 (remaining nine months)	269,000,000
Expected future amortization expense, 2016	313,300,000
Expected future amortization expense, 2017	286,100,000
Expected future amortization expense, 2018	281,600,000
Expected future amortization expense, 2019	270,400,000
Expected future amortization expense, 2020	263,400,000
Total	1,683,800,000
Out-licensed patents
Intangible assets
Cost	543,300,000		543,300,000
Accumulated Amortization	-487,800,000		-481,700,000
Net	55,500,000		61,600,000
Impairment of out-licensed patent		34,700,000
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	3,005,300,000		3,005,300,000
Accumulated Amortization	-2,447,500,000		-2,396,800,000
Net	557,800,000		608,500,000
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	548,700,000
Acquired and in-licensed rights and patents
Intangible assets
Cost	3,288,900,000		3,280,400,000
Accumulated Amortization	-339,200,000		-300,100,000
Net	2,949,700,000		2,980,300,000
Acquired and in-licensed rights and patents | Minimum
Intangible assets
Estimated life, (in years)	6 years
Acquired and in-licensed rights and patents | Maximum
Intangible assets
Estimated life, (in years)	17 years
In-process research and development
Intangible assets
Indefinite lived intangible assets useful life	Indefinite until commercialization
Cost and Net	726,100,000		314,100,000
Accumulated Amortization	0		0
Trademarks and trade names
Intangible assets
Indefinite lived intangible assets useful life	Indefinite
Cost and Net	64,000,000		64,000,000
Accumulated Amortization	0		0
Convergence Pharmaceuticals
Intangible assets
In-process research and development and other intangible assets				424,600,000
TYSABRI product | Acquired and in-licensed rights and patents
Intangible assets
Net	$2,886,600,000

Intangible_Assets_and_Goodwill3

Intangible Assets and Goodwill (Details 1) (USD $)	3 Months Ended
	Mar. 31, 2015
Summary of roll forward of the changes in goodwill
Goodwill, beginning of period	$1,760,249,000
Increase to goodwill	92,300,000
Other	-2,700,000
Goodwill, end of period	$1,849,852,000

Fair_Value_Measurements_Detail

Fair Value Measurements (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Assets:
Marketable debt securities	$2,051.20	$2,111.10
Fair Value, Measurements, Recurring
Assets:
Cash equivalents	752.5	716.3
Derivative contracts	168.2	72.7
Plan assets for deferred compensation	44.1	36.9
Total	3,034.30	2,958.40
Liabilities:
Derivative contracts	12.6	5.4
Contingent consideration obligations	461.8	215.5	275.1	280.9
Total	474.4	220.9
Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	991	1,063
Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	850.6	849.8
Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	209.6	198.3
Fair Value, Measurements, Recurring | Marketable equity securities
Assets:
Investments	6.2	6.9
Fair Value, Measurements, Recurring | Venture capital investments
Assets:
Investments	12.1	14.5	23.6	21.9
Quoted Prices in Active Markets, Level 1 | Fair Value, Measurements, Recurring
Assets:
Cash equivalents	0	0
Derivative contracts	0	0
Plan assets for deferred compensation	0	0
Total	6.2	6.9
Liabilities:
Derivative contracts	0	0
Contingent consideration obligations	0	0
Total	0	0
Quoted Prices in Active Markets, Level 1 | Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	0	0
Quoted Prices in Active Markets, Level 1 | Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	0	0
Quoted Prices in Active Markets, Level 1 | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	0	0
Quoted Prices in Active Markets, Level 1 | Fair Value, Measurements, Recurring | Marketable equity securities
Assets:
Investments	6.2	6.9
Quoted Prices in Active Markets, Level 1 | Fair Value, Measurements, Recurring | Venture capital investments
Assets:
Investments	0	0
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Assets:
Cash equivalents	752.5	716.3
Derivative contracts	168.2	72.7
Plan assets for deferred compensation	44.1	36.9
Total	3,016	2,937
Liabilities:
Derivative contracts	12.6	5.4
Contingent consideration obligations	0	0
Total	12.6	5.4
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	991	1,063
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	850.6	849.8
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	209.6	198.3
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Marketable equity securities
Assets:
Investments	0	0
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Venture capital investments
Assets:
Investments	0	0
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Assets:
Cash equivalents	0	0
Derivative contracts	0	0
Plan assets for deferred compensation	0	0
Total	12.1	14.5
Liabilities:
Derivative contracts	0	0
Contingent consideration obligations	461.8	215.5
Total	461.8	215.5
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
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Marketable equity securities
Assets:
Investments	0	0
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Venture capital investments
Assets:
Investments	$12.10	$14.50

Fair_Value_Measurements_Detail1

Fair Value Measurements (Details 1) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unrealized gains included in earnings	$0	$2.90
Unrealized losses included in earnings	-2.4	-1.2
Fair Value, Measurements, Recurring | Venture capital investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, beginning of period			14.5	21.9
Fair value, end of period	$12.10	$23.60	$14.50	$21.90

Fair_Value_Measurements_Detail2

Fair Value Measurements (Details 2) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Summary of fair and carrying value of debt instruments
Notes payable, carrying value	$583.90	$585.20
Debt instruments, fair value	646.8	647.2
Notes Payable to Fumedica
Summary of fair and carrying value of debt instruments
Notes payable, fair value	12.9	12.6
Notes payable, carrying value	12.1	11.7
6.875% Senior Notes due 2018
Summary of fair and carrying value of debt instruments
Notes payable, fair value	633.9	634.6
Notes payable, carrying value	$571.80	$573.50

Fair_Value_Measurements_Detail3

Fair Value Measurements (Details 3) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013
Business Acquisition, Contingent Consideration [Line Items]
Additions	$238,500,000	$0
Changes in fair value	7,844,000	-799,000
Payments	0	-5,000,000
Fair Value, Measurements, Recurring
Business Acquisition, Contingent Consideration [Line Items]
Fair value, beginning of period			215,500,000	280,900,000
Fair value, end of period	$461,800,000	$275,100,000	$215,500,000	$280,900,000

Fair_Value_Measurements_Detail4

Fair Value Measurements (Details Textual) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Feb. 12, 2015
Business Acquisition, Contingent Consideration [Line Items]
Asset impairment charges	$0
Changes in valuation techniques	0
Contingent consideration obligations	397.5		200
Contingent consideration	238.5	0
6.875% Senior Notes due 2018
Business Acquisition, Contingent Consideration [Line Items]
Stated rate on debt	6.88%
Convergence Pharmaceuticals
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration	238.5
Potential future milestone payments				450
Discount rate	2.00%
In-process research and development and other intangible assets				$424.60

Financial_Instruments_Details

Financial Instruments (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Marketable Debt and Equity Securities
Fair Value	$2,051.20	$2,111.10
Gross Unrealized Gains	1.9	0.7
Gross Unrealized Losses	-0.8	-2.4
Amortized Cost	2,050.10	2,112.80
Corporate debt securities Current
Marketable Debt and Equity Securities
Fair Value	385.3	370.4
Gross Unrealized Gains	0.1	0
Gross Unrealized Losses	-0.1	-0.2
Amortized Cost	385.3	370.6
Corporate debt securities Non-current
Marketable Debt and Equity Securities
Fair Value	605.7	692.6
Gross Unrealized Gains	0.8	0.2
Gross Unrealized Losses	-0.4	-1.5
Amortized Cost	605.3	693.9
Government securities Current
Marketable Debt and Equity Securities
Fair Value	288.5	269.9
Gross Unrealized Gains	0	0
Gross Unrealized Losses	-0.1	-0.1
Amortized Cost	288.6	270
Government securities Non-current
Marketable Debt and Equity Securities
Fair Value	562.1	579.9
Gross Unrealized Gains	0.6	0.3
Gross Unrealized Losses	-0.1	-0.4
Amortized Cost	561.6	580
Mortgage and other asset backed securities Current
Marketable Debt and Equity Securities
Fair Value	0.1	0.2
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Amortized Cost	0.1	0.2
Mortgage and other asset backed securities Non-current
Marketable Debt and Equity Securities
Fair Value	209.5	198.1
Gross Unrealized Gains	0.4	0.2
Gross Unrealized Losses	-0.1	-0.2
Amortized Cost	209.2	198.1
Marketable equity securities
Marketable Debt and Equity Securities
Fair Value	6.2	6.9
Gross Unrealized Gains	0.3	1.2
Gross Unrealized Losses	0	-0.2
Amortized Cost	$5.90	$5.90

Financial_Instruments_Details_

Financial Instruments (Details 1) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$752.50	$716.30
Commercial paper
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	10.3	54.2
Overnight reverse repurchase agreements
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	308.6	305
Money market funds
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	427.2	321.2
Short-term debt securities
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$6.40	$35.90

Financial_Instruments_Details_1

Financial Instruments (Details 2) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Summary of Contractual Maturities: Available-for-Sale Securities
Due in one year or less, estimated fair value	$673.90	$640.50
Due in one year or less, amortized cost	674	640.8
Due after one year through five years, Estimated Fair Value	1,252	1,343.70
Due after one year through five years, Amortized Cost	1,251	1,345.20
Due after five years, estimated fair value	125.3	126.9
Due after five years, amortized cost	125.1	126.8
Total available-for-sale securities, Fair Value	2,051.20	2,111.10
Total available-for-sale securities, Amortized Cost	$2,050.10	$2,112.80

Financial_Instruments_Details_2

Financial Instruments (Details 3) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Proceeds from Marketable Debt Securities
Proceeds from maturities and sales	$373,563,000	$757,512,000
Realized gains	200,000	200,000
Realized losses	($300,000)	($100,000)

Financial_Instruments_Details_3

Financial Instruments (Details Textual)	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Investments, All Other Investments [Abstract]
Original maturities of commercial paper and short-term debt securities	less than 90 days
Average maturity of marketable securities, months	15 months	15 months

Financial_Instruments_Financia

Financial Instruments Financial Instruments (Details Textual 2) (Strategic Investments, USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Strategic Investments
Business Acquisition [Line Items]
Strategic investment portfolio	$45.30	$47.80
Venture capital investments	18.3	21.4
Cost basis of equity securities of certain privately-owned companies and venture capital funds	$27	$26.40

Derivative_Instruments_Details

Derivative Instruments (Details) (USD $)	3 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Derivative Instruments (Textual) [Abstract]
Lower range of durations of foreign currency forward contracts	1 month		1 month
Higher range of durations of foreign currency forward contracts	21 months		15 months
Gain/Loss on fair value of foreign currency forward contracts	$159.80		$72.10
Expected settlement time for contracts, in months	21 months
Net gains (losses) of other income (expense) related to foreign currency forward contracts	9.7	1.4
Designated as hedging instrument
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	1,588.50		1,302.30
Not designated as hedging instrument
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	494.2		365.2
Foreign Exchange Contract | Other current assets | Designated as hedging instrument
Summary of Derivatives designated as Hedging Instruments
Derivative asset, fair value, net	160.1		69.5
Foreign Exchange Contract | Other current assets | Not designated as hedging instrument
Summary of Derivatives designated as Hedging Instruments
Derivative asset, fair value, net	1.9		1.3
Foreign Exchange Contract | Investments and other assets | Designated as hedging instrument
Summary of Derivatives designated as Hedging Instruments
Derivative asset, fair value, net	6.2		1.9
Foreign Exchange Contract | Accrued expenses and other | Designated as hedging instrument
Summary of Derivatives designated as Hedging Instruments
Derivative liability, fair value, net	1.3
Foreign Exchange Contract | Accrued expenses and other | Not designated as hedging instrument
Summary of Derivatives designated as Hedging Instruments
Derivative liability, fair value, net	7.6		5.4
Foreign Exchange Contract | Other long-term liabilities | Designated as hedging instrument
Summary of Derivatives designated as Hedging Instruments
Derivative liability, fair value, net	3.7
Revenue | Foreign Exchange Contract
Derivative Instruments (Textual) [Abstract]
Net gains (losses) in product revenue for the settlement of certain effective cash flow hedge instruments	35	-4.7
Other income (expense) | Foreign Exchange Contract
Derivative Instruments (Textual) [Abstract]
Net gains (losses) in earnings of Foreign currency forward contracts due to hedge ineffectiveness	2.2	-0.2
Euro | Designated as hedging instrument
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	1,482.30		1,174.60
Canadian Dollar | Designated as hedging instrument
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	45.3		56.7
British Pound Sterling | Designated as hedging instrument
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	29.2		34.5
Japanese Yen | Designated as hedging instrument
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	16.6		16.6
Australian Dollar | Designated as hedging instrument
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	$15.10		$19.90

Equity_Details

Equity (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Reconciliation of equity attributable to non-controlling interests
NCI, beginning of period	$5,030,000	$600,000
Net income (loss) attributable to NCI, net of tax	-2,367,000	-228,000
Fair value of net assets and liabilities acquired and assigned to NCI	0	4,000,000
Translation adjustment and other	100,000	0
NCI, end of period	$2,687,000	$4,400,000

Equity_Details_Textual

Equity (Details Textual) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Feb. 28, 2011
Equity (Textual) [Abstract]
Equity increase during the period	$855,400,000
Net income attributable to Biogen Inc	822,541,000	479,956,000
Adjustments to additional paid in capital, share-based compensation and exercise of stock options	$46,300,000
Common stock shares authorized for repurchase	1,300,000		20,000,000

Accumulated_Other_Comprehensiv2

Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013
Accumulated other comprehensive income [Line Items]
Accumulated other comprehensive income (loss), net of tax	($70,538,000)	($22,400,000)	($59,488,000)	($27,700,000)
Other comprehensive income loss before reclassifications	23,700,000	1,200,000
Amounts reclassified from accumulated other comprehensive income loss	-34,700,000	4,200,000
Net current period other comprehensive income loss	-11,050,000	5,389,000
Accumulated Net Unrealized Investment Gain (Loss)
Accumulated other comprehensive income [Line Items]
Accumulated other comprehensive income (loss), net of tax	900,000	7,300,000	-400,000	5,600,000
Other comprehensive income loss before reclassifications	1,200,000	1,800,000
Amounts reclassified from accumulated other comprehensive income loss	100,000	-100,000
Net current period other comprehensive income loss	1,300,000	1,700,000
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated other comprehensive income [Line Items]
Accumulated other comprehensive income (loss), net of tax	159,000,000	-17,900,000	71,700,000	-23,700,000
Other comprehensive income loss before reclassifications	122,100,000	1,500,000
Amounts reclassified from accumulated other comprehensive income loss	-34,800,000	4,300,000
Net current period other comprehensive income loss	87,300,000	5,800,000
Accumulated Defined Benefit Plans Adjustment
Accumulated other comprehensive income [Line Items]
Accumulated other comprehensive income (loss), net of tax	-30,300,000	-18,800,000	-31,600,000	-19,600,000
Other comprehensive income loss before reclassifications	1,300,000	800,000
Amounts reclassified from accumulated other comprehensive income loss	0	0
Net current period other comprehensive income loss	1,300,000	800,000
Accumulated Translation Adjustment
Accumulated other comprehensive income [Line Items]
Accumulated other comprehensive income (loss), net of tax	-200,100,000	7,100,000	-99,200,000	10,000,000
Other comprehensive income loss before reclassifications	-100,900,000	-2,900,000
Amounts reclassified from accumulated other comprehensive income loss	0	0
Net current period other comprehensive income loss	($100,900,000)	($2,900,000)

Accumulated_Other_Comprehensiv3

Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Details 1) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (expense)	($14,986)	($5,601)
Income tax benefit (expense)	281,881	178,414
Product, net	2,172,322	1,742,765
Net income attributable to Biogen Inc.	822,541	479,956
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income attributable to Biogen Inc.	34,700	-4,200
Accumulated Net Unrealized Investment Gain (Loss) | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (expense)	-100	100
Income tax benefit (expense)	0	0
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax benefit (expense)	-200	400
Product, net	$35,000	($4,700)

Earnings_per_Share_Details

Earnings per Share (Details) (USD $)	3 Months Ended
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Numerator:
Net income attributable to Biogen Inc	$822,541	$479,956
Denominator:
Weighted average number of common shares outstanding	234,995,000	236,786,000
Effect of dilutive securities:
Dilutive potential common shares	600,000	1,000,000
Shares used in calculating diluted earnings per share	235,630,000	237,849,000
Stock options and employee stock purchase plan
Effect of dilutive securities:
Stock units	100,000	100,000
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	300,000	600,000
Market stock units
Effect of dilutive securities:
Stock units	200,000	300,000

ShareBased_Payments_Details

Share-Based Payments (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Share-based Compensation Expense included in consolidated statements of income
Subtotal	$91	$76.60
Capitalized share-based compensation costs	-3.4	-2.6
Share-based compensation expense included in total costs and expenses	87.6	74
Income tax effect	-26.7	-22.3
Research and development
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	35.4	29.4
Selling, general and administrative
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	55.6	47.2
Total share-based compensation expense, net of tax
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	$60.90	$51.70

ShareBased_Payments_Details_1

Share-Based Payments (Details 1) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Summary of share based compensation expense associated with different programs
Subtotal	$91	$76.60
Capitalized share-based compensation costs	-3.4	-2.6
Share-based compensation expense included in total costs and expenses	87.6	74
Market stock units
Summary of share based compensation expense associated with different programs
Share-based compensation expense	17	14.9
Time-vested restricted stock units
Summary of share based compensation expense associated with different programs
Share-based compensation expense	31.9	29.2
Cash settled performance shares
Summary of share based compensation expense associated with different programs
Share-based compensation expense	23.8	21.4
Performance units
Summary of share based compensation expense associated with different programs
Share-based compensation expense	13.2	6.3
Employee stock purchase plan
Summary of share based compensation expense associated with different programs
Share-based compensation expense	$5.10	$4.80

ShareBased_Payments_Details_2

Share-Based Payments (Details 2)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Market stock units
Schedule of stock option grants
Stock units	175,000	214,000
Cash settled performance shares
Schedule of stock option grants
Stock units	112,000	172,000
Performance units
Schedule of stock option grants
Stock units	89,000	50,000
Time-vested restricted stock units
Schedule of stock option grants
Stock units	351,000	392,000

ShareBased_Payments_Details_Te

Share-Based Payments (Details Textual)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Shares issued under ESPP	68,000	74,000

Income_Taxes_Details

Income Taxes (Details)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Reconciliation between the U.S. federal statutory tax rate and effective tax rate
Statutory rate	35.00%	35.00%
State taxes	-0.40%	1.20%
Taxes on foreign earnings	-8.40%	-8.40%
Credits and net operating loss utilization	0.70%	1.30%
Purchased intangible assets	1.00%	1.50%
Manufacturing deduction	-1.60%	-2.00%
Permanent items	0.60%	0.30%
Other	0.10%	0.50%
Effective tax rate	25.60%	26.80%

Income_Taxes_Details_Textual

Income Taxes (Details Textual) (USD $)	3 Months Ended		1 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Oct. 31, 2011
Income Tax Contingency [Line Items]
Effective tax rate	25.60%	26.80%
Notice of assessment of corporate withholding tax including penalties and interest	$45.60
Net benefit for a previously unrecognized position	16.4
Tax Year 2007
Income Tax Contingency [Line Items]
Proposed deduction disallowance by tax authorities			130
Tax Year 2008
Income Tax Contingency [Line Items]
Proposed deduction disallowance by tax authorities			130
Tax Year 2009
Income Tax Contingency [Line Items]
Proposed deduction disallowance by tax authorities			$130

Other_Consolidated_Financial_S2

Other Consolidated Financial Statement Detail (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Accrued Expenses and Other
Revenue-related rebates	$584,200,000		$483,800,000
Employee compensation and benefits	202,300,000		393,800,000
Royalties and licensing fees	150,400,000		172,400,000
Deferred revenue	124,900,000		120,900,000
Current portion of contingent consideration obligations	64,400,000		265,500,000
Other	403,500,000		507,500,000
Total accrued expenses and other	1,384,774,000		1,819,334,000
Other Income (Expense), Net
Interest income	3,300,000	2,700,000
Interest expense	-6,700,000	-7,600,000
Gain (loss) on sales of investments, net	-1,700,000	3,000,000
Foreign exchange gains (losses), net	-13,000,000	-3,300,000
Other, net	3,100,000	-400,000
Total other income (expense), net	-14,986,000	-5,601,000
Liabilities, Noncurrent
Contingent consideration obligations	397,500,000		200,000,000
Employee related liabilities noncurrent	213,300,000		200,700,000
Other	281,700,000		249,400,000
Total other long-term liabilities	892,529,000		650,096,000
Component of accrued expenses and other
Accrued Expenses and Other
Revenue-related rebates	$439,300,000		$359,200,000

Investments_in_Variable_Intere1

Investments in Variable Interest Entities (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Investment in Variable Interest Entities (Textual) [Abstract]
Research and development	$460,549,000	$528,884,000
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities	8,500,000		7,900,000
Neurimmune
Investment in Variable Interest Entities (Textual) [Abstract]
Remaining potential development milestone payments and royalties on commercial sales under the terms of collaboration agreement	335,000,000
Expected milestone payment for initiation of a late stage clinical trial	60,000,000
Research and development	$12,800,000	$11,000,000

Collaborative_and_Other_Relati1

Collaborative and Other Relationships Collaborative and Other Relationships - Equity Method Investments (Details)	3 Months Ended		1 Months Ended		3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Feb. 29, 2012	Feb. 29, 2012	Mar. 31, 2015	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2014
	USD ($)	USD ($)	Samsung Biosimilar Agreement [Member]	Samsung Biosimilar Agreement [Member]	Samsung Biosimilar Agreement [Member]	Samsung Biosimilar Agreement [Member]	Samsung Biosimilar Agreement [Member]	Samsung Biosimilar Agreement [Member]	Samsung Biosimilar Agreement [Member]	Samsung Biosimilar Agreement [Member]
			USD ($)	KRW	USD ($)	KRW	USD ($)	USD ($)	USD ($)	KRW
Schedule of Equity Method Investments [Line Items]
Contribution from Samsung to develop, manufacture and market biosimilar pharmaceuticals			$250,000,000	280,500,000,000
Variable interest entity, qualitative or quantitative information, ownership percentage			85.00%	85.00%
Contribution from Biogen to develop, manufacture and market biosimilar pharmaceuticals			45,000,000	49,500,000,000	12,400,000	13,600,000,000			8,600,000	9,100,000,000
Percentage of stake in entity minimum			15.00%	15.00%	10.00%	10.00%
Additional contribution made to Samsung Bioepis					5,700,000	6,300,000,000
Percentage of stake in entity maximum			49.90%	49.90%
Recognized loss	-834,000	-7,605,000			-800,000		-7,600,000
Research and development	460,549,000	528,884,000						36,000,000
Additional milestone payment								75,000,000
Expected profit share with Samsung Bioepis								50.00%
Revenue related to technical development and technology transfer services	$52,030,000	$90,101,000			$18,900,000		$24,100,000

Litigation_Details

Litigation (Details) (Italy)	3 Months Ended
	Dec. 31, 2011
Italy
Loss Contingencies [Line Items]
Reimbursement limit stated in the resolution	24 months

Commitments_and_Contingences_D

Commitments and Contingences (Details) (Fumapharm AG, USD $)	12 Months Ended	3 Months Ended
	Dec. 31, 2006	Mar. 31, 2015	Jun. 30, 2013
Business Acquisition, Contingent Consideration [Line Items]
Cash portion of consideration	$220,000,000
TECFIDERA
Business Acquisition, Contingent Consideration [Line Items]
Maximum contingent consideration in the form of development and approval milestones			15,000,000
Four billion
Business Acquisition, Contingent Consideration [Line Items]
Cash portion of consideration		250,000,000
Cumulative sales level		4,000,000,000
Each additional one billion up to twenty billion
Business Acquisition, Contingent Consideration [Line Items]
Cumulative sales level		20,000,000,000
Payment of contingent payment for acquisition		300,000,000
One billion
Business Acquisition, Contingent Consideration [Line Items]
Cumulative sales level		1,000,000,000
Three billion
Business Acquisition, Contingent Consideration [Line Items]
Cumulative sales level		$3,000,000,000

Segment_Information_Details

Segment Information (Details)	3 Months Ended
	Mar. 31, 2015
	segment
Segment Information (Textual) [Abstract]
Number of reportable segment	1