Biogen (BIIB) 10-Q 28 Oct 13 2013 Q3 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	9 Months Ended
	Sep. 30, 2013	Oct. 21, 2013
Document and Entity Information [Abstract]
Entity Registrant Name	BIOGEN IDEC 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	30-Sep-13
Document Fiscal Year Focus	2013
Document Fiscal Period Focus	Q3
Amendment Flag	FALSE
Entity Common Stock, Shares Outstanding		236,212,930

Condensed_Consolidated_Stateme

Condensed Consolidated Statements of Income (Unaudited) (USD $)	3 Months Ended		9 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Revenues:
Product, net	$1,453,554	$1,039,110	$3,935,251	$3,091,398
Unconsolidated joint business	303,210	287,792	856,601	856,975
Other	71,016	58,652	174,497	150,147
Total revenues	1,827,780	1,385,554	4,966,349	4,098,520
Cost and expenses:
Cost of sales, excluding amortization of acquired intangible assets	234,696	139,358	599,173	411,666
Research and development	410,017	304,217	1,021,820	989,738
Selling, general and administrative	405,584	299,631	1,189,194	901,488
Amortization of acquired intangible assets	99,998	53,013	233,524	151,256
Collaboration profit sharing	0	75,545	85,357	239,951
(Gain) loss on fair value remeasurement of contingent consideration	-97	9,456	-2,983	23,573
Restructuring charges	0	803	0	2,225
Total cost and expenses	1,150,198	882,023	3,126,085	2,719,897
Gain on sale of rights	6,949	31,719	17,319	31,719
Income from operations	684,531	535,250	1,857,583	1,410,342
Other income (expense), net	-4,640	-4,548	-29,525	13,546
Income before income tax expense and equity in loss of investee, net of tax	679,891	530,702	1,828,058	1,423,888
Income tax expense	186,105	131,044	410,753	334,213
Equity in loss of investee, net of tax	6,170	1,258	12,270	1,769
Net income	487,616	398,400	1,405,035	1,087,906
Net income attributable to noncontrolling interests, net of tax	0	0	0	0
Net income attributable to Biogen Idec Inc.	$487,616	$398,400	$1,405,035	$1,087,906
Net income per share:
Basic earnings per share attributable to Biogen Idec Inc.	$2.06	$1.68	$5.93	$4.56
Diluted earnings per share attributable to Biogen Idec Inc.	$2.05	$1.67	$5.89	$4.53
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Idec Inc.	237,070	236,474	237,131	238,331
Diluted earnings per share attributable to Biogen Idec Inc.	238,349	238,125	238,508	240,137

Condensed_Consolidated_Stateme1

Condensed Consolidated Statements of Comprehensive Income (Unaudited) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Statement of Comprehensive Income [Abstract]
Net income	$487,616	$398,400	$1,405,035	$1,087,906
Other comprehensive income:
Unrealized gains on securities available for sale, net of tax of $3,639 and $883 for the three months ended September 30, 2013 and 2012, respectively; and $6,554 and $1,958 for the nine months ended September 30, 2013 and 2012, respectively	6,211	1,503	11,171	3,331
Unrealized losses on foreign currency forward contracts, net of tax of $298 and $3,140 for the three months ended September 30, 2013 and 2012, respectively; and $1,182 and $3,118 for the nine months ended September 30, 2013 and 2012, respectively	-14,847	-27,354	-5,549	-27,457
Unrealized gains on pension benefit obligation	892	198	3,167	590
Currency translation adjustment	33,564	25,093	17,201	-980
Total other comprehensive income (loss), net of tax	25,820	-560	25,990	-24,516
Comprehensive income	513,436	397,840	1,431,025	1,063,390
Comprehensive income attributable to noncontrolling interests, net of tax	0	0	0	65
Comprehensive income attributable to Biogen Idec Inc	$513,436	$397,840	$1,431,025	$1,063,325

Condensed_Consolidated_Stateme2

Condensed Consolidated Statements of Comprehensive Income (Unaudited) (Parenthetical) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Statement of Comprehensive Income [Abstract]
Tax effect on net unrealized gains recognized on securities available for sale	$3,639	$883	$6,554	$1,958
Tax effect of net unrealized losses recognized on foreign currency forward contracts	$298	$3,140	$1,182	$3,118

Condensed_Consolidated_Balance

Condensed Consolidated Balance Sheets (Unaudited) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Current assets:
Cash and cash equivalents	$427,811	$570,721
Marketable securities	240,882	1,134,989
Accounts receivable, net	891,396	686,848
Due from unconsolidated joint business, net	263,465	268,395
Inventory	614,483	447,373
Other current assets	205,635	136,011
Total current assets	2,643,672	3,244,337
Marketable securities	373,558	2,036,658
Property, plant and equipment, net	1,806,074	1,742,226
Intangible assets, net	4,580,199	1,631,547
Goodwill	1,210,718	1,201,296
Investments and other assets	636,028	274,054
Total assets	11,250,249	10,130,118
Current liabilities:
Current portion of notes payable and line of credit	3,385	453,379
Taxes payable	144,180	20,066
Accounts payable	166,709	203,999
Accrued expenses and other	1,204,437	979,945
Total current liabilities	1,518,711	1,657,389
Notes payable and other financing arrangements	694,894	687,396
Long-term deferred tax liability	302,483	217,272
Other long-term liabilities	608,473	604,266
Total liabilities	3,124,561	3,166,323
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	128	127
Additional paid-in capital	3,987,377	3,854,525
Accumulated other comprehensive loss	-29,315	-55,305
Retained earnings	5,891,829	4,486,794
Treasury stock, at cost	-1,724,927	-1,324,618
Total Biogen Idec Inc. shareholders' equity	8,125,092	6,961,523
Noncontrolling interests	596	2,272
Total equity	8,125,688	6,963,795
Total liabilities and equity	$11,250,249	$10,130,118

Condensed_Consolidated_Balance1

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) (USD $)	Sep. 30, 2013	Dec. 31, 2012
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 $)	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Cash flows from operating activities:
Net income	$1,405,035	$1,087,906
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization	373,357	268,772
Share-based compensation	104,851	88,378
Deferred income taxes	-166,187	-86,858
Other	-36,577	6,043
Changes in operating assets and liabilities, net:
Accounts receivable	-219,860	18,486
Inventory	-182,814	-82,423
Other changes in operating assets and liabilities, net	198,268	71,686
Net cash flows provided by operating activities	1,476,073	1,371,990
Cash flows from investing activities:
Proceeds from sales and maturities of marketable securities	5,025,218	1,913,381
Purchases of marketable securities	-2,473,609	-2,192,343
Acquisition of TYSABRI rights	-3,262,719	0
Acquisitions of business, net of cash acquired	0	-72,401
Purchases of property, plant and equipment	-167,628	-185,511
Other	-15,954	-38,014
Net cash flows used in investing activities	-894,692	-574,888
Cash flows from financing activities:
Purchase of treasury stock	-400,308	-963,171
Proceeds from issuance of stock for share-based compensation arrangements	56,367	58,278
Repayments of borrowings under senior notes	-452,340	0
Other	68,572	42,939
Net cash flows used in financing activities	-727,709	-861,954
Net decrease in cash and cash equivalents	-146,328	-64,852
Effect of exchange rate changes on cash and cash equivalents	3,418	2,033
Cash and cash equivalents, beginning of the period	570,721	514,542
Cash and cash equivalents, end of the period	$427,811	$451,723

Business

Business	9 Months Ended
	Sep. 30, 2013
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Business	Business
	Overview
	Biogen Idec is a global biotechnology company focused on discovering, developing, manufacturing and marketing therapies for the treatment of multiple sclerosis and other autoimmune disorders, neurodegenerative diseases and hemophilia. We also collaborate on the development and commercialization of RITUXAN and anti-CD20 product candidates for the treatment of non-Hodgkin's lymphoma and other conditions.
	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, 2012 (2012 Form 10-K). Our accounting policies are described in the “Notes to Consolidated Financial Statements” in our 2012 Form 10-K and updated, as necessary, in this Form 10-Q. The year-end condensed consolidated balance sheet data presented for comparative purposes was derived from our audited financial statements, but does not include all disclosures required by U.S. GAAP. The results of operations for the three and nine months ended September 30, 2013 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 our partner(s) to collaborations and other arrangements.
	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.

Acquisitions_Acquisitions

Acquisitions Acquisitions	9 Months Ended
	Sep. 30, 2013
Business Combinations [Abstract]
Acquisitions	Acquisitions
	TYSABRI
	On April 2, 2013, we acquired full ownership of, and strategic, commercial and decision-making rights to, TYSABRI from Elan Pharma International, Ltd (Elan), an affiliate of Elan Corporation, plc. Upon the closing of the transaction, we made an upfront payment of $3.25 billion to Elan, which was funded from our existing cash, and our collaboration agreement with Elan was terminated.
	We are accounting for this transaction as the acquisition of an asset as we did not acquire any employees from Elan nor did we acquire any significant processes that we did not previously perform or manage under the collaboration agreement. Under the collaboration agreement, we manufactured TYSABRI and collaborated with Elan on the product's marketing, commercial, regulatory, distribution and ongoing development activities. The collaboration agreement was designed to effect an equal sharing of worldwide profits and losses generated by the activities of the collaboration. For additional information related to this collaboration, please read Note 21, Collaborative and Other Relationships to our consolidated financial statements included within our 2012 Form 10-K.
	The $3.25 billion upfront payment was capitalized in the second quarter of 2013 as an intangible asset within our condensed consolidated balance sheet as TYSABRI has reached technological feasibility. We adjusted the value of this intangible asset by $84.4 million related to deferred revenue from two sales-based milestones previously paid by Elan as well as transaction costs. The net intangible asset capitalized was $3,178.3 million. Commencing in the second quarter of 2013, we began amortizing this intangible asset over the estimated useful life of 17 years using an economic consumption method based on actual and expected revenue generated from the sales of our TYSABRI product.
	Following the April 2, 2013 closing of the transaction, we began recording 100% of U.S. revenues, cost of sales and operating expenses related to TYSABRI within our condensed consolidated statements of income. Under the terms of the acquisition agreement, we continued to share TYSABRI profits with Elan on an equal basis until April 30, 2013. We recorded this profit split for the month ended April 30, 2013 as cost of sales within our condensed consolidated statements of income as we controlled TYSABRI effective April 2, 2013. Commencing May 1, 2013 and for the first twelve months thereafter, we will make contingent payments to Elan of 12% on worldwide net sales of TYSABRI and, thereafter, 18% on annual worldwide net sales up to $2.0 billion and 25% on annual worldwide net sales that exceed $2.0 billion. In 2014, the $2.0 billion threshold will be pro-rated for the portion of 2014 remaining after the first 12 months expires. Royalty payments to Elan will be recognized as cost of sales within our condensed consolidated statements of income.

Accounts_Receivable

Accounts Receivable	9 Months Ended
	Sep. 30, 2013
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 large 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 write-offs of accounts receivable have not been significant.
	The credit and economic conditions within Italy, Spain and Portugal, among other members of the European Union, remain uncertain. Uncertain credit and economic conditions have generally led to a lengthening of time to collect our accounts receivable in some of these countries. In some regions in these countries where our collections have slowed and a significant portion of these receivables are routinely being collected 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 long-term 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 September 30, 2013
	(In millions)	Current		Non-Current		Total
		Balance Included		Balance Included
		within Accounts		within Investments
		Receivable, net		and Other Assets
	Spain	$	94.1	$	18.2	$	112.3
	Italy	$	89.2	$	2.5	$	91.7
	Portugal	$	12.5	$	18.1	$	30.6
		As of December 31, 2012
	(In millions)	Current		Non-Current		Total
		Balance Included		Balance Included
		within Accounts		within Investments
		Receivable, net		and Other Assets
	Spain	$	78.9	$	—	$	78.9
	Italy	$	94.4	$	10.2	$	104.6
	Portugal	$	16.6	$	7.4	$	24
	Approximately $40.8 million and $11.8 million of the total net accounts receivable balances for these countries were overdue more than one year as of September 30, 2013 and December 31, 2012, respectively.
	Pricing of TYSABRI in Italy - AIFA
	In the fourth quarter of 2011, Biogen Idec SRL, our Italian subsidiary, received a notice from the Italian National Medicines Agency (AIFA) stating that sales of TYSABRI for the period from February 2009 through February 2011 exceeded by EUR30.7 million a reimbursement limit established pursuant to a Price Determination Resolution (Price Resolution) granted by AIFA in December 2006. In December 2011, based on our interpretation that the Price Resolution by its terms only applied to the first 24 months of TYSABRI sales (which began in February 2007), we filed an appeal against AIFA in administrative court seeking a ruling that the reimbursement limit does not apply and that the position of AIFA is unenforceable. That appeal is pending.
	In June 2013, Biogen Idec SRL received an additional notice from AIFA, stating that sales of TYSABRI from February 2011 through February 2013 also exceeded the same reimbursement limit in the Price Resolution. We dispute that the reimbursement limit applies to this period for the same reason that we dispute its application to the February 2009 through February 2011 period.
	In July 2013, we reached an agreement in principle with the Price and Reimbursement Committee of AIFA to settle all of AIFA's existing claims relating to sales of TYSABRI in excess of the reimbursement limit for the periods between February 2009 through February 2013 for an aggregate repayment of EUR33.3 million. As part of this settlement, we also agreed that the reimbursement limit in the Pricing Resolution will no longer be in effect as of February 2013. The settlement is pending approval by the Italy Avvocatura Generale dello Stato and the board of directors of AIFA. Upon this approval and the execution of the settlement, we will dismiss our appeal.
	As a result of this agreement, we recorded a liability and reduction to revenue of EUR15.4 million. That adjustment approximates 50% of the claim related to the period from February 2009 through February 2011 for which we had not recorded any amounts. We recorded the adjustment as of June 30, 2013 as the likelihood of making a payment to settle AIFA's claims for this period was now probable and the amount could be estimated.
	Since being notified in the fourth quarter of 2011 that AIFA believed a reimbursement limit was in effect, we have deferred revenue on sales of TYSABRI as if the reimbursement limit were in effect. As of September 30, 2013, we have deferred an aggregate amount of $120.7 million, of which $12.7 million and $39.9 million were deferred during the three and nine months ended September 30, 2013, respectively. We will continue to defer revenue until the settlement is approved. Upon approval of the settlement, any deferred revenue related to the periods subsequent to February 2011 that is in excess of the settlement will be recognized as revenue. At the time of sale, our net accounts receivable balances from product sales in Italy include the amount of deferred revenue discussed above as our customers pay the invoice price of the product. For additional information, please read Note 20, Litigation to these condensed consolidated financial statements.

Reserves_for_Discounts_and_All

Reserves for Discounts and Allowances	9 Months Ended
	Sep. 30, 2013
Reserves for Discounts and Allowances [Abstract]
Reserves for discounts and allowances	Reserves for Discounts and Allowances
	As a result of our acquisition of TYSABRI rights from Elan, we began recognizing reserves for discounts and allowances for U.S. TYSABRI revenue in the second quarter of 2013. Prior periods included reserves for discounts and allowances for rest of world TYSABRI revenue and worldwide AVONEX revenue only. In addition, following our start of U.S. commercial sales of TECFIDERA in the second quarter of 2013, we began recognizing reserves for discounts and allowances related to U.S. TECFIDERA revenue.
	An analysis of the change in reserves is summarized as follows:
	(In millions)	Discounts			Contractual			Returns			Total
					Adjustments
	Balance, as of December 31, 2012	$	15.5		$	194.8		$	26.8		$	237.1
	Current provisions relating to sales in current year	202.9			549.8			16			768.7
	Adjustments relating to prior years	(0.8		)	(7.8		)	0.9			(7.7		)
	Payments/returns relating to sales in current year	(145.3		)	(306.1		)	—			(451.4		)
	Payments/returns relating to sales in prior years	(13.8		)	(127.8		)	(13.9		)	(155.5		)
	Balance, as of September 30, 2013	$	58.5		$	302.9		$	29.8		$	391.2
	The total reserves above, included in our condensed consolidated balance sheets, are summarized as follows:
	(In millions)	As of September 30,			As of December 31,
		2013			2012
	Reduction of accounts receivable	$	143.8		$	46.1
	Component of accrued expenses and other	247.4			191
	Total reserves	$	391.2		$	237.1

Inventory

Inventory	9 Months Ended
	Sep. 30, 2013
Inventory Disclosure [Abstract]
Inventory	Inventory
	The components of inventory are summarized as follows:
	(In millions)	As of		As of
		September 30,		December 31,
		2013		2012
	Raw materials	$	112.2	$	101.8
	Work in process	389.8		230.5
	Finished goods	112.5		115.1
	Total inventory	$	614.5	$	447.4
	As of September 30, 2013, our inventory includes $82.4 million associated with our ELOCTATE, ALPROLIX, Serum-Free AVONEX and PLEGRIDY programs, which have been capitalized in advance of regulatory approval.

Intangible_Assets_and_Goodwill

Intangible Assets and Goodwill	9 Months Ended
	Sep. 30, 2013
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible Assets and Goodwill	Intangible Assets and Goodwill
	Intangible Assets
	Intangible assets, net of accumulated amortization, impairment charges and adjustments, are summarized as follows:
				As of September 30, 2013									As of December 31, 2012
	(In millions)	Estimated		Cost				Accumulated			Net		Cost		Accumulated			Net
		Life						Amortization							Amortization
	Out-licensed patents	13-23 years		$	578			$	(443.3	)	$	134.7	$	578	$	(421.0	)	$	157
	AVONEX Core developed	15-23 years		3,005.30				(2,107.0		)	898.3		3,005.30		(1,965.7		)	1,039.60
	technology
	In-process research and development	Up to 15 years upon		327.4				—			327.4		330.1		—			330.1
		commercialization
	Trademarks and	Indefinite		64				—			64		64		—			64
	tradenames
	Acquired and in-licensed rights	6-17 years		3,236.10				(80.3		)	3,155.80		53.7		(12.9		)	40.8
	and patents
	Total intangible assets			$	7,210.80			$	(2,630.6	)	$	4,580.20	$	4,031.10	$	(2,399.6	)	$	1,631.50
	For the three and nine months ended September 30, 2013, amortization of acquired intangible assets totaled $100.0 million and $233.5 million, respectively, as compared to $53.0 million and $151.3 million, respectively, in the prior year comparative periods. The increase in amortization for the three and nine months ended September 30, 2013 was primarily driven by amortization recorded in relation to the intangible asset recorded upon our acquisition of TYSABRI rights and the amount of amortization recorded in relation to our AVONEX core technology asset. Amortization of acquired intangible assets for the nine months ended September 30, 2013 also includes a charge of $2.6 million related to a write down in carrying value of one of our in-process research and development assets to reflect a change in its estimated fair value.
	AVONEX Core Developed Technology
	Core developed technology primarily relates to our AVONEX product, which was recorded in connection with the merger of Biogen, Inc. and IDEC Pharmaceuticals Corporation in 2003. The net book value of this asset as of September 30, 2013 was $887.9 million. We amortize this intangible asset using the economic consumption method based on revenue generated from our AVONEX product. An analysis of the anticipated lifetime revenue of AVONEX is performed annually during our long range planning cycle each year. This analysis serves as the basis for the calculation of economic consumption for the AVONEX core technology asset.
	Acquired and In-licensed Rights and Patents
	The increase in acquired and in-licensed rights and patents during the three and nine months ended September 30, 2013, as compared to the same periods in 2012, was primarily related to the $3,178.3 million intangible asset capitalized in connection with our acquisition of TYSABRI rights from Elan on April 2, 2013. In the second quarter of 2013, we began amortizing this intangible asset over the estimated useful life using an economic consumption method based on actual and expected revenue generated from the sales of our TYSABRI product. For a more detailed description of this transaction, please read Note 2, Acquisitions to these condensed consolidated financial statements.
	Estimated Future Amortization of Intangible Assets
	Our most recent long range planning cycle was completed in the third quarter of 2013, which reflected a decrease in the expected future product revenues of AVONEX and TYSABRI, resulting in an increase in amortization expense as compared to prior quarters. The results of our analysis were most significantly impacted by changes in the estimated impact of TECFIDERA, as well as other oral and alternative formulations including PLEGRIDY that may compete with AVONEX and TYSABRI. Based upon this more recent analysis, the estimated future amortization for acquired intangible assets is expected to be as follows:
	(In millions)	As of September 30, 2013
	2013 (remaining three months)	$	113.2
	2014	418.9
	2015	337.4
	2016	323.4
	2017	328.1
	2018	330.7
	Total	$	1,851.70
	Goodwill
	The following table provides a roll-forward of the changes in our goodwill balance:
	(In millions)	As of				As of
		September 30,				December 31,
		2013				2012
	Goodwill, beginning of period	$	1,201.30			$	1,146.30
	Goodwill acquired during the period	13.5				48.2
	Other	(4.1		)		6.8
	Goodwill, end of period	$	1,210.70			$	1,201.30
	The increase in goodwill during the nine months ended September 30, 2013 was primarily related to the $15.0 million contingent payment made to former shareholders of Fumapharm AG (net of $1.5 million tax benefit), which became payable upon the approval of TECFIDERA in the U.S. For additional information related to future contingent payments, please read Note 21, Commitments and Contingencies to these condensed consolidated financial statements.
	For the nine months ended September 30, 2013, we adjusted goodwill to establish a deferred tax asset related to our Stromedix Inc. (Stromedix) transaction. For additional information related to our transaction with Stromedix, please read Note 2, Acquisitions to our consolidated financial statements included within our 2012 Form 10-K.
	As of September 30, 2013, we had no accumulated impairment losses related to goodwill.

Fair_Value_Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2013
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:
	(In millions)	As of			Quoted Prices			Significant Other			Significant
		September 30,			in Active			Observable Inputs			Unobservable
		2013			Markets			(Level 2)			Inputs
					(Level 1)						(Level 3)
	Assets:
	Cash equivalents	$	287.2		$	—		$	287.2		$	—
	Marketable debt securities:
	Corporate debt securities	287			—			287			—
	Government securities	267.2			—			267.2			—
	Mortgage and other asset backed securities	60.2			—			60.2			—
	Marketable equity securities	40.3			40.3			—			—
	Venture capital investments	27.6			—			—			27.6
	Derivative contracts	1.8			—			1.8			—
	Plan assets for deferred compensation	21.1			—			21.1			—
	Total	$	992.4		$	40.3		$	924.5		$	27.6
	Liabilities:
	Derivative contracts	$	17.1		$	—		$	17.1		$	—
	Contingent consideration obligations	281			—			—			281
	Total	$	298.1		$	—		$	17.1		$	281
	(In millions)	As of			Quoted Prices			Significant			Significant
		December 31,			in Active			Other			Unobservable
		2012			Markets			Observable			Inputs
					(Level 1)			Inputs			(Level 3)
								(Level 2)
	Assets:
	Cash equivalents	$	439.4		$	—		$	439.4		$	—
	Marketable debt securities:
	Corporate debt securities	1,001.00			—			1,001.00			—
	Government securities	1,657.80			—			1,657.80			—
	Mortgage and other asset backed securities	512.9			—			512.9			—
	Marketable equity securities	9			9			—			—
	Venture capital investments	20.3			—			—			20.3
	Derivative contracts	1.8			—			1.8			—
	Plan assets for deferred compensation	14.3			—			14.3			—
	Total	$	3,656.50		$	9		$	3,627.20		$	20.3
	Liabilities:
	Derivative contracts	$	14.4		$	—		$	14.4		$	—
	Contingent consideration obligations	293.9			—			—			293.9
	Total	$	308.3		$	—		$	14.4		$	293.9
	There have been no impairments of our assets measured and carried at fair value during the three and nine months ended September 30, 2013. In addition, there were no changes in valuation techniques or inputs utilized or transfers between fair value measurement levels during the three and nine months ended September 30, 2013. The fair value of Level 2 instruments classified as cash equivalents and marketable debt securities were determined through valuation models of 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 2012 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 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. These venture capital investments represented approximately 0.2% of total assets as of September 30, 2013 and December 31, 2012, respectively.
	The following table provides a roll-forward of the fair value of our venture capital investments that are Level 3 assets:
		For the Three Months						For the Nine Months
		Ended September 30,						Ended September 30,
	(In millions)	2013			2012			2013			2012
	Fair value, beginning of period	$	23.5		$	25.4		$	20.3		$	23.5
	Unrealized gains included in earnings	3.8			1.4			10.5			4.9
	Unrealized losses included in earnings	—			(1.6		)	(2.0		)	(3.6		)
	Purchases	0.3			—			0.3			0.4
	Settlements	—			—			(1.5		)	—
	Fair value, end of period	$	27.6		$	25.2		$	27.6		$	25.2
	Debt Instruments
	The fair and carrying values of our debt instruments, which are Level 2 liabilities, are summarized as follows:
		As of September 30, 2013						As of December 31, 2012
	(In millions)	Fair			Carrying			Fair			Carrying
		Value			Value			Value			Value
	Notes payable to Fumedica	$	16.8		$	15.3		$	20		$	17.9
	Credit facility	—			—			—			—
	6.0% Senior Notes due March 1, 2013	—			—			453.7			450
	6.875% Senior Notes due March 1, 2018	649.5			581.7			681.6			586.4
	Total	$	666.3		$	597		$	1,155.30		$	1,054.30
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 11, Indebtedness to these condensed consolidated financial statements.
Contingent Consideration Obligations
The following table provides a roll-forward of the fair values of our contingent consideration obligations which includes Level 3 measurements:
	For the Three Months						For the Nine Months
	Ended September 30,						Ended September 30,
(In millions)	2013			2012			2013			2012
Fair value, beginning of period	$	281		$	280.9		$	293.9		$	151
Additions	—			—			—			122.2
Changes in fair value	(0.1		)	9.5			(3.0		)	23.6
Payments	0.1			—			(9.9		)	(6.5		)
Fair value, end of period	$	281		$	290.3		$	281		$	290.3
As of September 30, 2013 and December 31, 2012, approximately $251.9 million and $271.5 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.

Financial_Instruments

Financial Instruments	9 Months Ended
	Sep. 30, 2013
Investments, All Other Investments [Abstract]
Financial Instruments	Financial Instruments
	On April 2, 2013, we used $3.25 billion to fund the upfront payment we made to Elan in connection with our acquisition of TYSABRI rights.
	Marketable Securities
	The following tables summarize our marketable debt and equity securities:
	As of September 30, 2013 (In millions)	Fair		Gross			Gross			Amortized
		Value		Unrealized			Unrealized			Cost
				Gains			Losses
	Available-for-sale:
	Corporate debt securities
	Current	$	76.7	$	—		$	—		$	76.7
	Non-current	210.3		0.3			(0.1		)	210.1
	Government securities
	Current	164.2		—			—			164.2
	Non-current	103		—			(0.1		)	103.1
	Mortgage and other asset backed securities
	Current	—		—			—			—
	Non-current	60.2		—			(0.1		)	60.3
	Total marketable debt securities	$	614.4	$	0.3		$	(0.3	)	$	614.4
	Marketable equity securities, non-current	$	40.3	$	24.3		$	—		$	16
	As of December 31, 2012 (In millions)	Fair		Gross			Gross			Amortized
		Value		Unrealized			Unrealized			Cost
				Gains			Losses
	Available-for-sale:
	Corporate debt securities
	Current	$	346.9	$	0.3		$	—		$	346.6
	Non-current	654.1		2.8			(0.6		)	651.9
	Government securities
	Current	783.4		0.3			—			783.1
	Non-current	874.4		0.8			—			873.6
	Mortgage and other asset backed securities
	Current	4.8		—			—			4.8
	Non-current	508.1		1.4			(1.3		)	508
	Total marketable debt securities	$	3,171.70	$	5.6		$	(1.9	)	$	3,168.00
	Marketable equity securities, non-current	$	9	$	3		$	—		$	6
	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
		September 30,		December 31,
		2013		2012
	Commercial paper	$	12	$	40.7
	Overnight repurchase agreements	35.5		67.4
	Short-term debt securities	239.7		331.3
	Total	$	287.2	$	439.4
	The carrying values of our commercial paper, including accrued interest, overnight repurchase agreements, and our short-term debt securities approximate fair value.
	Summary of Contractual Maturities: Available-for-Sale Securities
	The estimated fair value and amortized cost of our marketable debt securities available-for-sale by contractual maturity are summarized as follows:
		As of September 30, 2013					As of December 31, 2012
	(In millions)	Estimated		Amortized			Estimated			Amortized
		Fair Value		Cost			Fair Value			Cost
	Due in one year or less	$	240.9	$	240.9		$	1,135.00		$	1,134.50
	Due after one year through five years	351.8		351.6			1,744.30			1,741.20
	Due after five years	21.7		21.9			292.4			292.3
	Total available-for-sale securities	$	614.4	$	614.4		$	3,171.70		$	3,168.00
	The average maturity of our marketable debt securities available-for-sale as of September 30, 2013 and December 31, 2012 was 14 months, respectively.
	Proceeds from Marketable Debt Securities
	The proceeds from maturities and sales of marketable debt securities and resulting realized gains and losses are summarized as follows:
		For the Three Months					For the Nine Months
		Ended September 30,					Ended September 30,
	(In millions)	2013		2012			2013			2012
	Proceeds from maturities and sales	$	617.5	$	491.3		$	5,025.20		$	1,913.40
	Realized gains	$	0.2	$	0.4		$	6.6		$	1.7
	Realized losses	$	—	$	(0.8	)	$	(2.1	)	$	(2.7	)
	Strategic Investments
	As of September 30, 2013 and December 31, 2012, our strategic investment portfolio was comprised of investments totaling $93.0 million and $64.2 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 $68.0 million and $29.3 million as of September 30, 2013 and December 31, 2012, 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 $25.0 million and $34.9 million as of September 30, 2013 and December 31, 2012, respectively.
Net Gains, Impairments and Changes to Fair Value
During the three and nine months ended September 30, 2013, we realized net gains, impairments and changes to fair value recorded through income, of $3.8 million and $7.9 million, respectively, on our strategic investment portfolio as compared to net losses of $1.8 million and net gains of $11.7 million, respectively, in the prior year comparative periods. The net gains recognized during the prior year nine months included a gain of $9.0 million recognized upon our acquisition of Stromedix in which we previously held an equity interest. For a more detailed description of this transaction, please read Note 2, Acquisitions to our consolidated financial statements included within our 2012 Form 10-K.
Impairments
For the three and nine months ended September 30, 2013, we recognized impairment charges on our marketable equity securities of certain biotechnology companies, investments in venture capital funds accounted for under the cost method and investments in privately-held companies totaling $0.0 million and $1.7 million, respectively, as compared to $3.5 million and $4.8 million, respectively, in the prior year comparative periods.

Derivative_Instruments

Derivative Instruments	9 Months Ended
	Sep. 30, 2013
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 September 30, 2013 and December 31, 2012 had durations of 1 to 15 months and 1 to 12 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
		September 30,				December 31,
		2013				2012
	Euro	$	570			$	492.2
	Canadian dollar	10.6				31.8
	Total foreign currency forward contracts	$	580.6			$	524
	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 losses of $16.2 million and $11.8 million as of September 30, 2013 and December 31, 2012, respectively. We expect all contracts to be settled over the next 15 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 September 30, 2013 and December 31, 2012, respectively, credit risk did not materially 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 September 30,
	Net Gains/(Losses)								Net Gains/(Losses)
	Reclassified from AOCI into Net Income								Recognized into Net Income
	(Effective Portion)								(Ineffective Portion)
	Location		2013				2012		Location	2013			2012
	Revenue		$	(7.3	)		$	12	Other income (expense)	$	(0.1	)	$	0.8
	For the Nine Months Ended September 30,
	Net Gains/(Losses)								Net Gains/(Losses)
	Reclassified from AOCI into Net Income								Recognized into Net Income
	(Effective Portion)								(Ineffective Portion)
	Location		2013				2012		Location	2013			2012
	Revenue		$	(6.1	)		$	31	Other income (expense)	$	0.2		$	4
	We recognized in product revenue net losses of $7.3 million and $6.1 million for the settlement of certain effective cash flow hedge instruments for the three and nine months ended September 30, 2013, respectively, as compared to net gains of $12.0 million and $31.0 million, respectively, in the prior year comparative periods. These settlements were recorded in the same period as the related revenues were generated.
	In relation to our foreign currency forward contracts, due to hedge ineffectiveness, we recognized in other income (expense) a net loss of $0.1 million and a net gain of $0.2 million for the three and nine months ended September 30, 2013, respectively, as compared to net gains of $0.8 million and $4.0 million, respectively, in the prior year comparative periods.
	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 other outstanding foreign currency contracts was $176.7 million and $243.2 million as of September 30, 2013 and December 31, 2012, respectively. A net loss of $0.2 million and a net gain of $1.3 million related to these contracts were recognized as a component of other income (expense), net, for the three and nine months ended September 30, 2013, respectively, as compared to a net loss of $5.7 million and a net gain of $5.6 million, respectively, in the prior year comparative periods.
	Summary of Derivatives
	While certain of our derivatives are subject to netting arrangements with our counterparties, we do not offset derivative assets and liabilities 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 September 30, 2013
	Hedging Instruments:
	Asset derivatives	Other current assets	$	0.4
	Liability derivatives	Accrued expenses and other	$	15.4
	Other Derivatives:
	Asset derivatives	Other current assets	$	1.4
	Liability derivatives	Accrued expenses and other	$	1.7
	(In millions)	Balance Sheet Location	Fair Value As of December 31, 2012
	Hedging Instruments:
	Asset derivatives	Other current assets	$	0.6
	Liability derivatives	Accrued expenses and other	$	11.5
	Other Derivatives:
	Asset derivatives	Other current assets	$	1.2
	Liability derivatives	Accrued expenses and other	$	2.9

Property_Plant_and_Equipment

Property, Plant and Equipment	9 Months Ended
	Sep. 30, 2013
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	Property, Plant and Equipment
	Property, plant and equipment are recorded at historical cost, net of accumulated depreciation. Accumulated depreciation on property, plant and equipment was $1,074.0 million and $941.1 million as of September 30, 2013 and December 31, 2012, respectively.
	For the three and nine months ended September 30, 2013, we capitalized interest costs related to construction in progress totaling approximately $2.2 million and $5.7 million, respectively, as compared to $6.6 million and $23.4 million, respectively, in the prior year comparative periods.
	Cambridge Leases
	In July 2011, we executed leases for two office buildings to be constructed in Cambridge, Massachusetts with a planned occupancy during the second half of 2013. Construction of these facilities began in late 2011. In accordance with accounting guidance applicable to entities involved with the construction of an asset that will be leased when the construction is completed, we are considered the owner of these properties during the construction period. Accordingly, we recorded an asset along with a corresponding financing obligation on our condensed consolidated balance sheet for the amount of total project costs incurred related to the construction in progress for these buildings.
	In July 2013, the construction of one of the two buildings was completed and we started leasing the facility. Upon completion of the construction of the building, we determined as part of a normal process that we are no longer considered the owner of that building because we do not have any unusual or significant continuing involvement. Consequently, we derecognized the building and its associated financing obligation of approximately $47.7 million from our condensed consolidated balance sheet. The derecognition of the building and its associated financing obligation constitute non-cash investing and financing transactions in the amount of $47.7 million. Upon completion of the second building, we will perform a similar assessment and determine if the remaining asset and corresponding financing obligation should also be derecognized. As of September 30, 2013 and December 31, 2012, the amounts recorded within our condensed consolidated balance sheet as property, plant and equipment and financing obligation totaled approximately $101.3 million and $86.5 million, respectively.
	As a result of our decision to relocate our corporate headquarters to Cambridge, Massachusetts, we expect to vacate part of our Weston, Massachusetts facility in the fourth quarter of 2013. We expect to incur a charge of approximately $25.0 million in connection with this process. This estimate represents our remaining lease obligation for the vacated portion of our Weston facility, net of sublease income to be received. We will begin to sublease the vacated portion of our Weston facility in the first half of 2014 for the remaining term of our lease agreement.
	Hillerød, Denmark Facility
	As of September 2012, our large-scale biologics manufacturing facility in Hillerød, Denmark was ready for its intended use as we began the process of manufacturing clinical products for sale to third parties. As a result, we transferred $465.9 million from construction in progress to various fixed asset accounts. We ceased capitalizing a majority of the interest expense and began recording depreciation on the various assets during the third quarter of 2012. The average estimated useful life for the facility and its assets is 20 years. In July 2013, the facility was approved by the U.S. Food and Drug Administration (FDA) and European Medicines Agency (EMA) for the manufacture of commercial TYSABRI.
	Research Triangle Park (RTP) Lease
	In December 2012, we entered into an arrangement with Eisai, Inc. (Eisai) to lease a portion of their facility in RTP to manufacture our and Eisai's oral solid dose products and for Eisai to provide us with vial-filling services for biologic therapies and packaging services for oral solid dose products. The 10 year operating lease agreement, which is cancellable after 5 years, became effective in February 2013 and gives us the option to purchase the facility.

Indebtedness

Indebtedness	9 Months Ended
	Sep. 30, 2013
Debt Disclosure [Abstract]
Indebtedness	Indebtedness
	Credit Facility
	In March 2013, we entered into a $750.0 million senior unsecured revolving credit facility, which we may choose to use for working capital and general corporate purposes. The terms of this revolving credit facility include a financial covenant that requires us to not exceed a maximum debt to EBITDA ratio. This facility terminates in March 2014. As of September 30, 2013, we had no outstanding borrowings and were in compliance with all covenants under this facility.
	Senior Notes
	On March 1, 2013, we repaid the $450.0 million principal amount of our 6.0% Senior Notes.

Equity

Equity	9 Months Ended
	Sep. 30, 2013
Equity [Abstract]
Equity	Equity
	Total equity as of September 30, 2013 increased $1,161.9 million compared to December 31, 2012. This increase was primarily driven by net income attributable to Biogen Idec Inc. of $1,405.0 million and an increase in additional paid in capital resulting from our share-based compensation arrangements totaling $132.9 million, partially offset by repurchases of our common stock totaling $400.3 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 nine months ended September 30, 2013, we repurchased approximately 2.0 million shares at a cost of $400.3 million. Of those shares, 1.7 million were repurchased during the three months ended September 30, 2013 at a cost of $359.3 million. During the nine months ended September 30, 2012, we repurchased approximately 7.7 million shares at a cost of approximately $963.2 million.
	Approximately 4.2 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:
		For the Three Months				For the Nine Months
		Ended September 30,				Ended September 30,
	(In millions)	2013		2012		2013			2012
	Noncontrolling interests, beginning of period	$	0.6	$	2.4	$	2.3		$	1.5
	Net income attributable to noncontrolling interests, net of tax	—		—		—			—
	Currency translation adjustment	—		—		—			0.1
	Deconsolidation of noncontrolling interest	—		—		(1.7		)	(0.5		)
	Distributions to noncontrolling interests	—		—		—			1.3
	Noncontrolling interests, end of period	$	0.6	$	2.4	$	0.6		$	2.4
	For additional information related to our deconsolidation of noncontrolling interests related to Knopp Neurosciences, Inc. in March 2013, please read Note 18, Investments in Variable Interest Entities to these condensed consolidated financial statements.

Accumulated_Other_Comprehensiv

Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss)	9 Months Ended
	Sep. 30, 2013
Equity [Abstract]
Comprehensive Income (Loss) Note	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, 2012	$	4.2			$	(10.7	)		$	(21.7	)		$	(27.1	)		$	(55.3	)
	Other comprehensive income (loss) before reclassifications	14.1				(12.0		)		3.2				17.2				22.5
	Amounts reclassified from accumulated other comprehensive income (loss)	(2.9		)		6.4				—				—				3.5
	Net current period other comprehensive income (loss)	11.2				(5.6		)		3.2				17.2				26
	Balance, as of September 30, 2013	$	15.4			$	(16.3	)		$	(18.5	)		$	(9.9	)		$	(29.3	)
	The following table summarizes the amounts reclassified from accumulated other comprehensive income:
	(In millions)	Income Statement Location	Amounts Reclassified from Accumulated Other Comprehensive Income
			For the Three Months								For the Nine Months
			Ended September 30,								Ended September 30,
			2013				2012				2013				2012
	Gains (losses) on securities available for sale	Other income (expense)	$	0.2			$	(0.3	)		$	4.5			$	(1.1	)
		Income tax expense	(0.1		)		0.1				(1.6		)		0.4
	Gains (losses) on foreign currency forward contracts	Revenues	(7.3		)		12				(6.1		)		31
		Income tax expense	—				—				(0.3		)		(1.9		)
	Total reclassifications, net of tax		$	(7.2	)		$	11.8			$	(3.5	)		$	28.4
	Securities Available for Sale
	Balances included within accumulated other comprehensive income (loss) related to unrealized gains (losses) on securities available for sale are shown net of tax of $9.0 million and $2.5 million as of September 30, 2013 and December 31, 2012, respectively. Other comprehensive income (loss) before reclassifications recognized during the nine months ended September 30, 2013 are shown net of tax of $6.6 million.
	Foreign Currency Forward Contracts
	Balances included within accumulated other comprehensive income (loss) related to unrealized gains (losses) on foreign currency forward contracts are shown net of tax of $0.1 million and $1.1 million as of September 30, 2013 and December 31, 2012, respectively. Other comprehensive income (loss) before reclassifications recognized during the nine months ended September 30, 2013 are shown net of tax of $1.2 million.
	Postretirement Benefit Plans
	Tax amounts related to the unfunded status of pension and retirement benefit plans were immaterial for all amounts presented.

Earnings_per_Share

Earnings per Share	9 Months Ended
	Sep. 30, 2013
Earnings Per Share [Abstract]
Earnings per Share	Earnings per Share
	Basic and diluted earnings per share are calculated as follows:
		For the Three Months				For the Nine Months
		Ended September 30,				Ended September 30,
	(In millions)	2013		2012		2013		2012
	Numerator:
	Net income attributable to Biogen Idec Inc.	$	487.6	$	398.4	$	1,405.00	$	1,087.90
	Denominator:
	Weighted average number of common shares outstanding	237.1		236.5		237.1		238.3
	Effect of dilutive securities:
	Stock options and employee stock purchase plan	0.2		0.4		0.4		0.5
	Time-vested restricted stock units	0.7		0.9		0.7		1
	Market stock units	0.3		0.3		0.3		0.3
	Dilutive potential common shares	1.2		1.6		1.4		1.8
	Shares used in calculating diluted earnings per share	238.3		238.1		238.5		240.1
	Amounts excluded from the calculation of net income per diluted share because their effects were anti-dilutive were insignificant.

Sharebased_Payments

Share-based Payments	9 Months Ended
	Sep. 30, 2013
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							For the Nine Months
		Ended September 30,							Ended September 30,
	(In millions)	2013				2012			2013			2012
	Research and development	$	20.7			$	18		$	69.6		$	55.8
	Selling, general and administrative	39.9				27.7			119.5			81.5
	Subtotal	60.6				45.7			189.1			137.3
	Capitalized share-based compensation costs	(2.4		)		(1.5		)	(7.4		)	(4.0		)
	Share-based compensation expense included in total cost and expenses	58.2				44.2			181.7			133.3
	Income tax effect	(17.4		)		(13.0		)	(54.3		)	(40.1		)
	Share-based compensation expense included in net income attributable to Biogen Idec Inc.	$	40.8			$	31.2		$	127.4		$	93.2
	The following table summarizes share-based compensation expense associated with each of our share-based compensation programs:
		For the Three Months							For the Nine Months
		Ended September 30,							Ended September 30,
	(In millions)	2013				2012			2013			2012
	Stock options	$	0.1			$	0.7		$	0.5		$	1.6
	Market stock units	12.2				5.6			27.2			17.2
	Time-vested restricted stock units	25.3				21.4			77.2			69.5
	Performance-vested restricted stock units settled in shares	—				—			—			0.1
	Cash settled performance shares	20.6				16.4			76.8			45.1
	Employee stock purchase plan	2.4				1.6			7.4			3.8
	Subtotal	60.6				45.7			189.1			137.3
	Capitalized share-based compensation costs	(2.4		)		(1.5		)	(7.4		)	(4.0		)
	Share-based compensation expense included in total cost and expenses	$	58.2			$	44.2		$	181.7		$	133.3
	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 Nine Months
		Ended September 30,
		2013			2012
	Market stock units	268,000			312,000
	Cash settled performance shares	273,000			327,000
	Time-vested restricted stock units	708,000			902,000
	No performance-vested restricted stock units or stock options were granted during the nine months ended September 30, 2013 and 2012. In addition, for the nine months ended September 30, 2013, approximately 208,000 shares were issued under our employee stock purchase plan (ESPP) compared to approximately 225,000 shares issued in the prior year comparative period.

Income_Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2013
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes
	For the three and nine months ended September 30, 2013, our effective tax rate was 27.4% and 22.5%, respectively, as compared to 24.7% and 23.5%, respectively, in the prior year comparative periods.
	A reconciliation between the U.S. federal statutory tax rate and our effective tax rate is summarized as follows:
		For the Three Months				For the Nine Months
		Ended September 30,				Ended September 30,
		2013		2012		2013		2012
	Statutory rate	35	%	35	%	35	%	35	%
	State taxes	2.5		0.8		3.7		0.8
	Taxes on foreign earnings	(6.6	)	(7.1	)	(7.7	)	(7.6	)
	Credits and net operating loss utilization	(2.2	)	(3.4	)	(3.0	)	(3.7	)
	Purchased intangible assets	1.9		1.3		1.6		1.2
	Manufacturing deduction	(2.5	)	(2.5	)	(8.0	)	(2.3	)
	Other permanent items	(0.3	)	0.4		1		(0.4	)
	Other	(0.4	)	0.2		(0.1	)	0.5
	Effective tax rate	27.4	%	24.7	%	22.5	%	23.5	%
	The increase in our tax rate for the three months ended September 30, 2013 compared to the same period in 2012, is primarily attributable to the commercial launch of TECFIDERA in the United States, which has resulted in a higher relative percentage of our earnings being recognized in the U.S., a higher tax jurisdiction. In addition, in 2013 we are recognizing reduced expenditures eligible for the orphan drug credit and higher state taxes. These factors were partially offset by lower intercompany royalties owed by a foreign wholly owned subsidiary to a U.S. wholly owned subsidiary on the international sales of one of our products.
	For the nine months ended September 30, 2013, the reduction in our income tax rate compared to the same period in 2012, was primarily the result of a change in our uncertain tax position related to our U.S. federal manufacturing deduction, described below, lower intercompany royalties owed by a foreign wholly owned subsidiary to a U.S. wholly owned subsidiary on the international sales of one of our products and the reinstatement of the federal research and development tax credit. These favorable items were partially offset by higher relative earnings in the United States from the commercial launch of TECFIDERA, lower orphan drug credits due to reduced expenditures in eligible clinical trials and higher state taxes.
	The change in the state taxes, manufacturing deduction and other permanent items of the effective tax rate reconciliation for the periods disclosed in the table above is primarily related to changes in the valuation of our federal uncertain tax positions, as discussed below under "Accounting for Uncertainty in Income Taxes".
	During the nine months ended September 30, 2013, we recorded a deferred charge of $203.7 million in connection with an intercompany transfer of the intellectual property for Daclizumab. The deferred charge will be amortized to income tax expense over the economic life of the Daclizumab program. If the Daclizumab program were to be discontinued, we will accelerate the amortization of this deferred charge and record an expense equal to its remaining net book value.
	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 2010 or state, local, or non-U.S. income tax examinations for years before 2004.
	Federal Uncertain Tax Positions
	During the three months ended March 31, 2013, we received updated technical guidance from the IRS concerning our current and prior year filings and calculation of our U.S. federal manufacturing deduction related to our unconsolidated joint business. Based on this guidance we reevaluated the level of our unrecognized benefits related to uncertain tax positions, and recorded a $33.0 million benefit, which is net of ancillary federal and state tax effects. This benefit is for a previously unrecognized position and relates to years 2005 through 2012 and is net of a $10.0 million expense for non-income based state taxes, which is recorded in other income (expense) within our condensed consolidated statements of income. Subsequent to establishing the uncertain tax position, we have remeasured the uncertain tax position upon receiving new information and recorded a net benefit of approximately $7.0 million.
	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.
	It is reasonably possible that we will adjust the value of our uncertain tax positions related to the manufacturing deduction as we receive additional information from various taxing authorities. In addition, the IRS routinely examines our intercompany transfer pricing with respect to intellectual property related transactions and it is possible that the IRS may disagree with one or more positions we have taken with respect to such valuations. We do not anticipate any other significant changes in our positions in the next twelve months other than expected settlements which have been classified as current liabilities within the accompanying balance sheet.
	State Uncertain Tax Positions
	On June 8, 2010, we received Notices of Assessment from the Massachusetts Department of Revenue (DOR) against Biogen Idec MA Inc. (BIMA), one of our wholly-owned subsidiaries, for $103.5 million of corporate excise tax, including associated interest and penalties, related to our 2004, 2005 and 2006 state tax filings. The assessment asserted that the portion of sales attributable to Massachusetts, the computation of BIMA's research and development credits and certain deductions claimed by BIMA were not appropriate. On February 3, 2013, we filed a petition with the Massachusetts Appellate Tax Board (Massachusetts ATB) appealing the denial of our application for abatement. On October 15, 2013 we reached a settlement with the Massachusetts DOR related to our 2004, 2005 and 2006 state tax filings under which we agreed to pay $5.0 million in taxes and interest and agreed on the nature and amount of tax credits carried forward into 2007. This resolution did not have a significant impact on our results of operations.
	The audit fieldwork associated with our state tax filings for 2007 and 2008 has been completed without a significant proposed adjustment.

Other_Consolidated_Financial_S

Other Consolidated Financial Statement Detail	9 Months Ended
	Sep. 30, 2013
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Other Consolidated Financial Statement Detail	Other Consolidated Financial Statement Detail
	Other Income (Expense), Net
	Components of other income (expense), net, are summarized as follows:
		For the Three Months						For the Nine Months
		Ended September 30,						Ended September 30,
	(In millions)	2013			2012			2013			2012
	Interest income	$	1.5		$	5.9		$	6.7		$	22.6
	Interest expense	(6.6		)	(8.7		)	(25.5		)	(23.1		)
	Impairments of investments	—			(3.5		)	(1.7		)	(4.8		)
	Gain (loss) on investments, net	3.9			1.3			14			15.6
	Foreign exchange gains (losses), net	(3.1		)	0.1			(11.0		)	0.2
	Other, net	(0.3		)	0.4			(12.0		)	3.1
	Total other income (expense), net	$	(4.6	)	$	(4.5	)	$	(29.5	)	$	13.5
	Accrued Expenses and Other
	Accrued expenses and other consists of the following:
	(In millions)	As of			As of
		September 30,			December 31,
		2013			2012
	Employee compensation and benefits	261.5			248.5
	Revenue-related rebates	247.4			191
	Deferred revenue	240.7			148
	Royalties and licensing fees	117.3			45.2
	Clinical development expenses	57.1			51.6
	Production expenses	43.6			45.6
	Current portion of contingent consideration obligations	29.1			22.4
	Construction in progress	21.9			12.3
	Other	185.8			215.3
	Total accrued expenses and other	$	1,204.40		$	979.9

Investments_in_Variable_Intere

Investments in Variable Interest Entities	9 Months Ended
	Sep. 30, 2013
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.
	Knopp
	During the three months ended March 31, 2013, we terminated our license agreement with Knopp Neurosciences, Inc. (Knopp), a subsidiary of Knopp Holdings, LLC, for the development, manufacture and commercialization of dexpramipexole in people with amyotrophic lateral sclerosis (ALS). We expect to exercise our put option on the 30.0% of the Class B common shares to Knopp in the fourth quarter.
	We had previously determined that we were the primary beneficiary of Knopp because we had the power through the license agreement to direct the activities that most significantly impacted Knopp’s economic performance and were required to fund 100% of the research and development costs incurred in support of the collaboration agreement. As such, we consolidated the results of Knopp. In March 2013, we deconsolidated the results of Knopp upon termination of the license agreement. The assets and liabilities of Knopp were not significant to our financial position or results of operations. We have not provided any financing to Knopp other than contractually required amounts previously disclosed in Note 20, Investments in Variable Interest Entities included within our 2012 Form 10-K.
	For the three and nine months ended September 30, 2013, the collaboration did not incur any development expenses. For the three and nine months ended September 30, 2012, the collaboration incurred development expenses totaling $15.7 million and $58.9 million, respectively, which were reflected as research and development expense within our condensed consolidated statements of income.
	For additional information, please read Note 20, Litigation to these condensed consolidated financial statements.
	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 under the collaboration. Based upon our current development plans, we may pay Neurimmune up to $345.0 million in remaining milestone payments, as well as royalties on sales of any resulting commercial products.
	We determined that we are the primary beneficiary of Neurimmune because we have the power through the collaboration to direct the activities that most significantly impact the entity’s economic performance and are required to fund 100% of the research and development costs incurred in support of the collaboration agreement. Accordingly, we consolidate the results of Neurimmune.
	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 will be reflected within our condensed consolidated statements of income as a charge to the noncontrolling interest, net of tax, when such milestones are achieved.
	For the three and nine months ended September 30, 2013, the collaboration incurred development expenses totaling $6.1 million and $16.8 million, respectively, which is reflected as research and development expense within our condensed consolidated statements of income, as compared to $3.4 million and $8.5 million, respectively, in the prior year comparative periods.
	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 not provided any financing to Neurimmune other than previously contractually required amounts.
	Unconsolidated Variable Interest Entities
	We have relationships with other variable interest entities that we do not consolidate as we lack the power to direct the activities that significantly impact the economic success of these entities. These relationships include investments in certain biotechnology companies and research collaboration agreements.
	As of September 30, 2013 and December 31, 2012, the total carrying value of our investments in biotechnology companies that we have determined to be variable interest entities, but do not consolidate as we do not have the power to direct their activities, totaled $6.8 million and $9.4 million, respectively. Our maximum exposure to loss related to these variable interest entities is limited to the carrying value of our investments.
	We have entered into research collaborations 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 20, Investments in Variable Interest Entities to our consolidated financial statements included within our 2012 Form 10-K.

Collaborative_and_Other_Relati

Collaborative and Other Relationships	9 Months Ended
	Sep. 30, 2013
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaborative and Other Relationships	Collaborative and Other Relationships
	Samsung Biosimilar Agreement
	In February 2012, we finalized an agreement with Samsung BioLogics Co. Ltd. (Samsung Biologics) that established an entity, Samsung Bioepis, to develop, manufacture and market biosimilar pharmaceuticals. Under the terms of the agreement, Samsung Biologics will contribute 280.5 billion South Korean won (approximately $250.0 million) for an 85 percent stake in Samsung Bioepis and we will contribute approximately 49.5 billion South Korean won (approximately $45.0 million) for the remaining 15 percent ownership interest. Our investment is limited to this contribution as we have no obligation to provide any additional funding; however, we maintain an option to purchase additional stock in Samsung Bioepis that would allow us to increase our ownership percentage up to 49.9 percent. The exercise of this option is within our control.
	During the three months ended September 30, 2013, we contributed the remaining 6.5 billion South Korean won (approximately $5.8 million) to Samsung Bioepis to complete our obligation to contribute an aggregate of approximately 49.5 billion South Korean won (approximately $45.0 million) for our 15 percent ownership interest. As of September 30, 2013 and December 31, 2012, the carrying value of our investment in Samsung Bioepis totaled 28.5 billion and 29.7 billion South Korean won (approximately $26.5 million and $27.8 million), respectively, which is classified as a component of investments and other assets within our condensed consolidated balance sheets. 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 and nine months ended September 30, 2013, we recognized a loss on our investment of $6.2 million and $12.3 million, respectively, as compared to $1.3 million and $1.8 million, respectively, in the prior year comparative periods.
	Simultaneous with the formation of Samsung Bioepis, we entered into a license agreement and technical development and manufacturing services agreements with Samsung Bioepis. For the three and nine months ended September 30, 2013, we recognized $4.3 million and $18.5 million, respectively, in other revenues in relation to these services, as compared to $4.3 million and $9.9 million, respectively, in the prior year comparative periods, which is reflected as a component of other revenues within our condensed consolidated statements of income.
	For additional information related to our other significant collaboration arrangements, please read Note 21, Collaborative and Other Relationships to our consolidated financial statements included within our 2012 Form 10-K.
	Isis Pharmaceuticals Inc. (Isis)
	In September 2013, we entered into a six year research collaboration with Isis Pharmaceuticals Inc. (Isis) under which both companies will perform discovery level research and then develop and commercialize antisense or other therapeutics for the treatment of neurological disorders. Under the collaboration, Isis will perform research on a set of neurological targets identified within the agreement. Once the research has reached a specific stage of development we will make the determination whether antisense is the preferred approach to develop a therapeutic candidate or whether another modality is preferred. If antisense is selected, Isis will continue development and identify a product candidate. If another modality is used, we will assume the responsibility for identifying a product candidate and developing it.
	Under the terms of this agreement, we paid Isis an upfront amount of $100.0 million. Of this payment, we recorded prepaid research and discovery services of approximately $25.0 million, representing the value of the Isis full time equivalent employee resources which are required by the collaboration to provide research and discovery services to us over the next six years. The remaining $75.0 million of the upfront payment was recorded as research and development expense in the third quarter of 2013, the period in which we entered into the collaboration, as it represents the purchase of intellectual property that has not reached technological feasibility.
	Isis is also eligible to receive milestone payments, license fees and royalty payments for all treatments developed through this collaboration, with the specific amount dependent upon the modality of the molecule advanced by us.
	For antisense product candidates, Isis will be responsible for global development through the completion of a Phase 2 trial and we will provide advice on the clinical trial design and regulatory strategy. We have an option to license the product candidate until completion of the Phase 2 trial. If we exercise our option, we will pay Isis up to a $70.0 million license fee and assume global development, regulatory and commercialization responsibilities. Isis could receive additional milestone payments upon the achievement of certain regulatory milestones of up to $150.0 million, plus additional amounts related to the cost of clinical trials conducted by Isis under the collaboration, and royalties on future sales if we successfully develop the product candidate after option exercise.
	For product candidates using a different modality, we will be responsible for global development through all stages and will owe Isis up to $90.0 million upon the achievement of certain regulatory milestones.
	Other Research and Discovery Arrangements
	During the three months ended September 30, 2013, we entered into research and discovery arrangements that resulted in $35.0 million recorded as investments and other assets within our condensed consolidated balance sheet and $2.0 million recorded as research and development expense within our condensed consolidated statements of income. These additional arrangements include the potential for future milestone payments based on clinical and commercial development over a period of several years.

Litigation

Litigation	9 Months Ended
	Sep. 30, 2013
Loss Contingency, Information about Litigation Matters [Abstract]
Litigation	Litigation
	Massachusetts Department of Revenue
	On June 8, 2010, we received Notices of Assessment from the Massachusetts DOR against BIMA for the payment of additional corporate excise tax, including associated interest and penalties, related to our 2004, 2005 and 2006 state tax filings. On February 3, 2011, we filed a petition with the Massachusetts ATB appealing the denial of our application for abatement, and on October 15, 2013 we reached a settlement resolving the dispute.
	Hoechst — Genentech Arbitration
	On October 24, 2008, Hoechst GmbH (Hoechst), an affiliate of Sanofi-Aventis Deutschland GmbH (Sanofi), filed a request for arbitration with the ICC International Court of Arbitration (Paris) claiming that Genentech breached a license agreement, now terminated, under which Hoechst claims rights (the Hoechst License). The Hoechst License granted Genentech certain rights with respect to U.S. Patents 5,849,522 (’522 patent) and 6,218,140 (’140 patent). In 2012, the arbitrator ruled that Genentech is liable to Hoechst for license royalties with respect to sales of RITUXAN, and in February 2013 he awarded Hoechst royalties of EUR108 million together with interest (estimated by Hoechst to be approximately EUR54 million as of the date of the award). Genentech has filed Declarations of Appeal in the Paris Court of Appeal to vacate the arbitral awards in their entirety and those annulment proceedings are pending. Genentech nonetheless paid Hoechst the amounts that Hoechst claims that it is owed, while reserving all legal rights, including the right to challenge the awards and the right to claim reimbursement.
	Although we are not a party to the arbitration, we expect that amounts paid by Genentech and not ultimately recovered from Hoechst may be a cost charged to our collaboration with Genentech. Our revenues from unconsolidated joint business were reduced by approximately $50.0 million in the second quarter of 2011 and by approximately $41.5 million in the first quarter of 2013 to reflect our share of the royalties awarded to Hoechst and the interest Hoechst claims. Our share may vary from these amounts if Genentech is successful in challenging the awards.
	Sanofi ’522 and ’140 Patent Litigation
	On October 27, 2008, Sanofi filed suit against Genentech and Biogen Idec in the United States District Court for the Eastern District of Texas claiming that RITUXAN infringes the ’522 patent and the ’140 patent. That action was transferred and consolidated with an action filed on the same day by Genentech and Biogen Idec against Sanofi in the United States District for the Northern District of California seeking declaratory judgments that RITUXAN does not infringe the patents and that the asserted patent claims are invalid and unenforceable. On April 21, 2011, the federal court in California entered a separate and final judgment of non-infringement, and the United States Court of Appeals for the Federal Circuit affirmed on March 22, 2012. The federal court in California has stayed further proceedings relating to Biogen Idec's and Genentech's claims for a declaration that the asserted patent claims are invalid and unenforceable, and no trial date has been set on those claims.
	’755 Patent Litigation
	On September 15, 2009, we were issued 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. This patent, which expires in September 2026, covers, among other things, the treatment of MS with our product AVONEX. On May 27, 2010, Bayer Healthcare Pharmaceuticals Inc. (Bayer) filed a lawsuit against us in the U.S. District Court for the District of New Jersey seeking a declaratory judgment of patent invalidity and non-infringement and seeking monetary relief in the form of attorneys' fees, costs and expenses. On May 28, 2010, BIMA filed a lawsuit in the U.S. District Court for the District of New Jersey alleging infringement of the ’755 Patent by EMD Serono, Inc. (manufacturer, marketer and seller of REBIF), Pfizer, Inc. (co-marketer of REBIF), Bayer (manufacturer, marketer and seller of BETASERON and manufacturer of EXTAVIA), and Novartis Pharmaceuticals Corp. (marketer and seller of EXTAVIA) and seeking monetary damages, including lost profits and royalties. 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.
	Novartis V&D ’688 Patent Litigation
	On January 26, 2011, Novartis Vaccines and Diagnostics, Inc. (Novartis V&D) filed suit against us in the United States District Court for the District of Delaware, alleging that TYSABRI infringes U.S. Patent No. 5,688,688 “Vector for Expression of a Polypeptide in a Mammalian Cell” (’688 Patent), which was granted in November 1997 and expires in November 2014. Novartis V&D seeks a declaration of infringement, a finding of willful infringement, compensatory damages in the form of a royalty and lost profits, treble damages, interest, costs and attorneys' fees. On July 18, 2012, the court granted Novartis V&D leave to add Novartis Pharma AG, an alleged exclusive licensee of the ’688 Patent, as co-plaintiff. 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. We believe that we have good and valid defenses to the complaint and will vigorously defend against it. A trial on liability has been set for January 2014.
	Italian National Medicines Agency
	In the fourth quarter of 2011, Biogen Idec SRL received a notice from the Italian National Medicines Agency (Agenzia Italiana del Farmaco or AIFA) stating that sales of TYSABRI for the period from February 2009 through February 2011 exceeded by EUR30.7 million a reimbursement limit established pursuant to a Price Determination Resolution (Price Resolution) granted by AIFA in December 2006. The Price Resolution set the initial price for the sale of TYSABRI in Italy and limited the amount of government reimbursement “for the first 24 months” of TYSABRI sales. As the basis for the claim, the AIFA notice referred to a 2001 Decree that provides for an automatic 24-month renewal of the terms of all Price Resolutions that are not renegotiated prior to the expiration of their term.
	On November 17, 2011, Biogen Idec SRL responded to AIFA that the reimbursement limit in the Price Resolution by its terms applied only to the first 24 months of TYSABRI sales, which began in February 2007. On December 23, 2011, we filed an appeal in the Regional Administrative Tribunal of Lazio (Il Tribunale Amministrativo Regionale per il Lazio) in Rome against AIFA, seeking a ruling that our interpretation of the Price Resolution is valid and that the position of AIFA is unenforceable, and the appeal is pending. On November 21, 2012, the tribunal ruled that the reimbursement limit would not automatically renew for another 24-month term pending resolution of the dispute.
	In June 2013, the Price and Reimbursement Committee of AIFA informed Biogen Idec SRL that sales of TYSABRI for February 2011 through February 2013 exceeded the reimbursement limit in the Price Resolution. We dispute that the reimbursement limit applies to this period for the same reason that we dispute its application to the February 2009 through February 2011 period.
	In July 2013, we reached an agreement in principle with the Price and Reimbursement Committee of AIFA to settle all of AIFA's claims relating to sales of TYSABRI in excess of the reimbursement limit for the periods between February 2009 through February 2013 for an aggregate repayment of EUR33.3 million. As part of this settlement, we also agreed that the reimbursement limit will no longer be in effect as of February 2013. The settlement is pending approval by the Italy Avvocatura Generale dello Stato and the board of directors of AIFA. Upon this approval and the execution of the settlement, we will dismiss our appeal.
	Average Manufacturer Price Litigation
	On September 6, 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 (the relator) on behalf of himself and the United States, and the states of New Jersey, California, Rhode Island, Michigan, Montana, Wisconsin, Massachusetts, Tennessee, Oklahoma, Texas, Indiana, New Hampshire, North Carolina, Florida, Georgia, New Mexico, Illinois, New York, Virginia, Delaware, Hawaii, Louisiana, Connecticut, and Nevada (collectively, the States), and the District of Columbia, alleging violations of the False Claims Act, 31 U.S.C. § 3729 et seq. and state and District of Columbia statutory counterparts. The United States and the States have declined to intervene, and the District of Columbia has not intervened. The complaint as amended alleges that Biogen Idec and other defendants underreport Average Manufacturer Price (AMP) information to the Centers for Medicare and Medicaid Services, thereby causing Biogen Idec and the other defendants to underpay rebates under the Medicaid Drug Rebate Program. The relator alleges that the underreporting has occurred because Biogen Idec and other defendants improperly consider various payments that they make to drug wholesalers to be discounts under applicable federal law. On July 3, 2012, the court dismissed all state and federal claims as to AMP submissions before January 1, 2007, and dismissed the remaining state-law claims in whole as to claims under New Mexico law and in part as to claims under the laws of Delaware, New Hampshire, Texas, Connecticut, Georgia, Indiana, Montana, New York, Oklahoma, and Rhode Island. The court denied our motion to dismiss federal law claims as to AMP submissions after January 1, 2007. A trial has been set for September 2014. We have not formed an opinion that an unfavorable outcome under the remaining claims is either “probable” or “remote,” and are unable at this stage of the litigation to form an opinion as to the magnitude or range of any potential loss. We believe that we have good and valid defenses and intend to vigorously defend against the allegations.
	Government Review of Sales and Promotional Practices
	We have learned that state and federal governmental authorities are investigating our sales and promotional practices. We are cooperating with the government.
	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.
	Canada Lease Dispute
	On April 18, 2008, First Real Properties Limited filed suit against Biogen Idec Canada Inc. (BI Canada) in the Superior Court of Justice in London, Ontario alleging breach of an offer for lease of property signed by BI Canada in 2007 and an unsigned proposed lease for the same property. The plaintiff's complaint seeks $7.0 million in damages, but the plaintiff submitted an expert report estimating the plaintiff's damages to be approximately $2.5 million after mitigation. The plaintiff also seeks costs of approximately $0.4 million and interest. A trial occurred in February 2013 and the decision is pending.
	Knopp Neurosciences Dispute
	On February 25, 2013, Knopp filed suit against Biogen Idec International Holding Ltd. in the United States District Court for the District of Massachusetts, alleging that Biogen Idec wrongfully terminated its license agreement with Knopp for the development of a product for ALS. Knopp seeks damages and injunctive relief. Knopp had also sought the transfer of certain biosamples, but the court dismissed that claim with prejudice in June 2013. No trial date has been set for Knopp's remaining claims. We have not formed an opinion that an unfavorable outcome is either “probable” or “remote,” and are unable at this stage of the litigation to form an opinion as to the magnitude or range of any potential loss. We believe that we have good and valid defenses and are vigorously defending against the claims.
	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	9 Months Ended
	Sep. 30, 2013
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 is 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 FDA. We are also required to make the following additional milestone payments to former shareholders of Fumapharm AG 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:
		Cumulative Sales Level
	Prior 12 Month Sales	$500M		$1.0B		$2.0B		$3.0B		Each additional $1.0B up to $20.0B
		Payment Amount (In millions)
	< $500 million	$	—	$	—	$	—	$	—	$	—
	$500 million - $1.0 billion	22		25		50		50		50
	$1.0 billion - $1.5 billion	—		50		100		100		100
	$1.5 billion - $2.0 billion	—		—		150		150		150
	$2.0 billion - $2.5 billion	—		—		200		200		200
	$2.5 billion - $3.0 billion	—		—		—		250		250
	> $3.0 billion	—		—		—		—		300
	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	9 Months Ended
	Sep. 30, 2013
Segment Reporting [Abstract]
Segment Information	Segment Information
	We operate as one business segment, which is the business of discovering, developing, manufacturing and marketing therapies for the treatment of multiple sclerosis and other autoimmune disorders, neurodegenerative diseases and hemophilia and, therefore, our chief operating decision-maker manages the operations of our company as a single operating segment.

Business_Policies

Business (Policies)	9 Months Ended
	Sep. 30, 2013
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Overview	Biogen Idec is a global biotechnology company focused on discovering, developing, manufacturing and marketing therapies for the treatment of multiple sclerosis and other autoimmune disorders, neurodegenerative diseases and hemophilia. We also collaborate on the development and commercialization of RITUXAN and anti-CD20 product candidates for the treatment of non-Hodgkin's lymphoma and other conditions.
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, 2012 (2012 Form 10-K). Our accounting policies are described in the “Notes to Consolidated Financial Statements” in our 2012 Form 10-K and updated, as necessary, in this Form 10-Q. The year-end condensed consolidated balance sheet data presented for comparative purposes was derived from our audited financial statements, but does not include all disclosures required by U.S. GAAP. The results of operations for the three and nine months ended September 30, 2013 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 our partner(s) to collaborations and other arrangements.
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.

Accounts_Receivable_Tables

Accounts Receivable (Tables)	9 Months Ended
	Sep. 30, 2013
Receivables [Abstract]
Net accounts receivable balances from product sales in selected European countries
		As of September 30, 2013
	(In millions)	Current		Non-Current		Total
		Balance Included		Balance Included
		within Accounts		within Investments
		Receivable, net		and Other Assets
	Spain	$	94.1	$	18.2	$	112.3
	Italy	$	89.2	$	2.5	$	91.7
	Portugal	$	12.5	$	18.1	$	30.6
		As of December 31, 2012
	(In millions)	Current		Non-Current		Total
		Balance Included		Balance Included
		within Accounts		within Investments
		Receivable, net		and Other Assets
	Spain	$	78.9	$	—	$	78.9
	Italy	$	94.4	$	10.2	$	104.6
	Portugal	$	16.6	$	7.4	$	24

Reserves_for_Discounts_and_All1

Reserves for Discounts and Allowances (Tables)	9 Months Ended
	Sep. 30, 2013
Reserves for Discounts and Allowances [Abstract]
Analysis of the amount of, and change in, reserves
	(In millions)	Discounts			Contractual			Returns			Total
					Adjustments
	Balance, as of December 31, 2012	$	15.5		$	194.8		$	26.8		$	237.1
	Current provisions relating to sales in current year	202.9			549.8			16			768.7
	Adjustments relating to prior years	(0.8		)	(7.8		)	0.9			(7.7		)
	Payments/returns relating to sales in current year	(145.3		)	(306.1		)	—			(451.4		)
	Payments/returns relating to sales in prior years	(13.8		)	(127.8		)	(13.9		)	(155.5		)
	Balance, as of September 30, 2013	$	58.5		$	302.9		$	29.8		$	391.2
Total reserves, included in consolidated balance sheets
	(In millions)	As of September 30,			As of December 31,
		2013			2012
	Reduction of accounts receivable	$	143.8		$	46.1
	Component of accrued expenses and other	247.4			191
	Total reserves	$	391.2		$	237.1

Inventory_Tables

Inventory (Tables)	9 Months Ended
	Sep. 30, 2013
Inventory Disclosure [Abstract]
Components of inventories
	(In millions)	As of		As of
		September 30,		December 31,
		2013		2012
	Raw materials	$	112.2	$	101.8
	Work in process	389.8		230.5
	Finished goods	112.5		115.1
	Total inventory	$	614.5	$	447.4

Intangible_Assets_and_Goodwill1

Intangible Assets and Goodwill (Tables)	9 Months Ended
	Sep. 30, 2013
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible assets
				As of September 30, 2013									As of December 31, 2012
	(In millions)	Estimated		Cost				Accumulated			Net		Cost		Accumulated			Net
		Life						Amortization							Amortization
	Out-licensed patents	13-23 years		$	578			$	(443.3	)	$	134.7	$	578	$	(421.0	)	$	157
	AVONEX Core developed	15-23 years		3,005.30				(2,107.0		)	898.3		3,005.30		(1,965.7		)	1,039.60
	technology
	In-process research and development	Up to 15 years upon		327.4				—			327.4		330.1		—			330.1
		commercialization
	Trademarks and	Indefinite		64				—			64		64		—			64
	tradenames
	Acquired and in-licensed rights	6-17 years		3,236.10				(80.3		)	3,155.80		53.7		(12.9		)	40.8
	and patents
	Total intangible assets			$	7,210.80			$	(2,630.6	)	$	4,580.20	$	4,031.10	$	(2,399.6	)	$	1,631.50
Estimated future amortization for acquired intangible assets
	(In millions)	As of September 30, 2013
	2013 (remaining three months)	$	113.2
	2014	418.9
	2015	337.4
	2016	323.4
	2017	328.1
	2018	330.7
	Total	$	1,851.70
Summary of roll forward of the changes in goodwill
	(In millions)	As of				As of
		September 30,				December 31,
		2013				2012
	Goodwill, beginning of period	$	1,201.30			$	1,146.30
	Goodwill acquired during the period	13.5				48.2
	Other	(4.1		)		6.8
	Goodwill, end of period	$	1,210.70			$	1,201.30

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2013
Fair Value Disclosures [Abstract]
Summary of Assets and Liabilities Recorded at Fair Value
	(In millions)	As of			Quoted Prices			Significant			Significant
		December 31,			in Active			Other			Unobservable
		2012			Markets			Observable			Inputs
					(Level 1)			Inputs			(Level 3)
								(Level 2)
	Assets:
	Cash equivalents	$	439.4		$	—		$	439.4		$	—
	Marketable debt securities:
	Corporate debt securities	1,001.00			—			1,001.00			—
	Government securities	1,657.80			—			1,657.80			—
	Mortgage and other asset backed securities	512.9			—			512.9			—
	Marketable equity securities	9			9			—			—
	Venture capital investments	20.3			—			—			20.3
	Derivative contracts	1.8			—			1.8			—
	Plan assets for deferred compensation	14.3			—			14.3			—
	Total	$	3,656.50		$	9		$	3,627.20		$	20.3
	Liabilities:
	Derivative contracts	$	14.4		$	—		$	14.4		$	—
	Contingent consideration obligations	293.9			—			—			293.9
	Total	$	308.3		$	—		$	14.4		$	293.9
	(In millions)	As of			Quoted Prices			Significant Other			Significant
		September 30,			in Active			Observable Inputs			Unobservable
		2013			Markets			(Level 2)			Inputs
					(Level 1)						(Level 3)
	Assets:
	Cash equivalents	$	287.2		$	—		$	287.2		$	—
	Marketable debt securities:
	Corporate debt securities	287			—			287			—
	Government securities	267.2			—			267.2			—
	Mortgage and other asset backed securities	60.2			—			60.2			—
	Marketable equity securities	40.3			40.3			—			—
	Venture capital investments	27.6			—			—			27.6
	Derivative contracts	1.8			—			1.8			—
	Plan assets for deferred compensation	21.1			—			21.1			—
	Total	$	992.4		$	40.3		$	924.5		$	27.6
	Liabilities:
	Derivative contracts	$	17.1		$	—		$	17.1		$	—
	Contingent consideration obligations	281			—			—			281
	Total	$	298.1		$	—		$	17.1		$	281
Fair value of venture capital investments
		For the Three Months						For the Nine Months
		Ended September 30,						Ended September 30,
	(In millions)	2013			2012			2013			2012
	Fair value, beginning of period	$	23.5		$	25.4		$	20.3		$	23.5
	Unrealized gains included in earnings	3.8			1.4			10.5			4.9
	Unrealized losses included in earnings	—			(1.6		)	(2.0		)	(3.6		)
	Purchases	0.3			—			0.3			0.4
	Settlements	—			—			(1.5		)	—
	Fair value, end of period	$	27.6		$	25.2		$	27.6		$	25.2
Summary of fair and carrying value of debt instruments
		As of September 30, 2013						As of December 31, 2012
	(In millions)	Fair			Carrying			Fair			Carrying
		Value			Value			Value			Value
	Notes payable to Fumedica	$	16.8		$	15.3		$	20		$	17.9
	Credit facility	—			—			—			—
	6.0% Senior Notes due March 1, 2013	—			—			453.7			450
	6.875% Senior Notes due March 1, 2018	649.5			581.7			681.6			586.4
	Total	$	666.3		$	597		$	1,155.30		$	1,054.30
Fair value of contingent consideration obligations
		For the Three Months						For the Nine Months
		Ended September 30,						Ended September 30,
	(In millions)	2013			2012			2013			2012
	Fair value, beginning of period	$	281		$	280.9		$	293.9		$	151
	Additions	—			—			—			122.2
	Changes in fair value	(0.1		)	9.5			(3.0		)	23.6
	Payments	0.1			—			(9.9		)	(6.5		)
	Fair value, end of period	$	281		$	290.3		$	281		$	290.3

Financial_Instruments_Tables

Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2013
Investments, All Other Investments [Abstract]
Marketable Debt and Equity Securities
	As of September 30, 2013 (In millions)	Fair		Gross			Gross			Amortized
		Value		Unrealized			Unrealized			Cost
				Gains			Losses
	Available-for-sale:
	Corporate debt securities
	Current	$	76.7	$	—		$	—		$	76.7
	Non-current	210.3		0.3			(0.1		)	210.1
	Government securities
	Current	164.2		—			—			164.2
	Non-current	103		—			(0.1		)	103.1
	Mortgage and other asset backed securities
	Current	—		—			—			—
	Non-current	60.2		—			(0.1		)	60.3
	Total marketable debt securities	$	614.4	$	0.3		$	(0.3	)	$	614.4
	Marketable equity securities, non-current	$	40.3	$	24.3		$	—		$	16
	As of December 31, 2012 (In millions)	Fair		Gross			Gross			Amortized
		Value		Unrealized			Unrealized			Cost
				Gains			Losses
	Available-for-sale:
	Corporate debt securities
	Current	$	346.9	$	0.3		$	—		$	346.6
	Non-current	654.1		2.8			(0.6		)	651.9
	Government securities
	Current	783.4		0.3			—			783.1
	Non-current	874.4		0.8			—			873.6
	Mortgage and other asset backed securities
	Current	4.8		—			—			4.8
	Non-current	508.1		1.4			(1.3		)	508
	Total marketable debt securities	$	3,171.70	$	5.6		$	(1.9	)	$	3,168.00
	Marketable equity securities, non-current	$	9	$	3		$	—		$	6
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
	(In millions)	As of		As of
		September 30,		December 31,
		2013		2012
	Commercial paper	$	12	$	40.7
	Overnight repurchase agreements	35.5		67.4
	Short-term debt securities	239.7		331.3
	Total	$	287.2	$	439.4
Summary of Contractual Maturities: Available-for-Sale Securities
		As of September 30, 2013					As of December 31, 2012
	(In millions)	Estimated		Amortized			Estimated			Amortized
		Fair Value		Cost			Fair Value			Cost
	Due in one year or less	$	240.9	$	240.9		$	1,135.00		$	1,134.50
	Due after one year through five years	351.8		351.6			1,744.30			1,741.20
	Due after five years	21.7		21.9			292.4			292.3
	Total available-for-sale securities	$	614.4	$	614.4		$	3,171.70		$	3,168.00
Proceeds from Marketable Debt Securities
		For the Three Months					For the Nine Months
		Ended September 30,					Ended September 30,
	(In millions)	2013		2012			2013			2012
	Proceeds from maturities and sales	$	617.5	$	491.3		$	5,025.20		$	1,913.40
	Realized gains	$	0.2	$	0.4		$	6.6		$	1.7
	Realized losses	$	—	$	(0.8	)	$	(2.1	)	$	(2.7	)

Derivative_Instruments_Tables

Derivative Instruments (Tables)	9 Months Ended
	Sep. 30, 2013
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Foreign currency forward contracts that were entered into to hedge forecasted revenue
		Notional Amount
	Foreign Currency: (In millions)	As of				As of
		September 30,				December 31,
		2013				2012
	Euro	$	570			$	492.2
	Canadian dollar	10.6				31.8
	Total foreign currency forward contracts	$	580.6			$	524
Summary of the effect of derivatives designated as hedging instruments on the consolidated statements of income
	For the Three Months Ended September 30,
	Net Gains/(Losses)								Net Gains/(Losses)
	Reclassified from AOCI into Net Income								Recognized into Net Income
	(Effective Portion)								(Ineffective Portion)
	Location		2013				2012		Location	2013			2012
	Revenue		$	(7.3	)		$	12	Other income (expense)	$	(0.1	)	$	0.8
	For the Nine Months Ended September 30,
	Net Gains/(Losses)								Net Gains/(Losses)
	Reclassified from AOCI into Net Income								Recognized into Net Income
	(Effective Portion)								(Ineffective Portion)
	Location		2013				2012		Location	2013			2012
	Revenue		$	(6.1	)		$	31	Other income (expense)	$	0.2		$	4
Summary of Derivatives designated as Hedging Instruments
	(In millions)	Balance Sheet Location	Fair Value As of September 30, 2013
	Hedging Instruments:
	Asset derivatives	Other current assets	$	0.4
	Liability derivatives	Accrued expenses and other	$	15.4
	Other Derivatives:
	Asset derivatives	Other current assets	$	1.4
	Liability derivatives	Accrued expenses and other	$	1.7
	(In millions)	Balance Sheet Location	Fair Value As of December 31, 2012
	Hedging Instruments:
	Asset derivatives	Other current assets	$	0.6
	Liability derivatives	Accrued expenses and other	$	11.5
	Other Derivatives:
	Asset derivatives	Other current assets	$	1.2
	Liability derivatives	Accrued expenses and other	$	2.9

Equity_Tables

Equity (Tables)	9 Months Ended
	Sep. 30, 2013
Equity [Abstract]
Reconciliation of equity attributable to noncontrolling interests
		For the Three Months				For the Nine Months
		Ended September 30,				Ended September 30,
	(In millions)	2013		2012		2013			2012
	Noncontrolling interests, beginning of period	$	0.6	$	2.4	$	2.3		$	1.5
	Net income attributable to noncontrolling interests, net of tax	—		—		—			—
	Currency translation adjustment	—		—		—			0.1
	Deconsolidation of noncontrolling interest	—		—		(1.7		)	(0.5		)
	Distributions to noncontrolling interests	—		—		—			1.3
	Noncontrolling interests, end of period	$	0.6	$	2.4	$	0.6		$	2.4

Accumulated_Other_Comprehensiv1

Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Tables)	9 Months Ended
	Sep. 30, 2013
Equity [Abstract]
Schedule of Accumulated Other Comprehensive Income (Loss)
	(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, 2012	$	4.2			$	(10.7	)		$	(21.7	)		$	(27.1	)		$	(55.3	)
	Other comprehensive income (loss) before reclassifications	14.1				(12.0		)		3.2				17.2				22.5
	Amounts reclassified from accumulated other comprehensive income (loss)	(2.9		)		6.4				—				—				3.5
	Net current period other comprehensive income (loss)	11.2				(5.6		)		3.2				17.2				26
	Balance, as of September 30, 2013	$	15.4			$	(16.3	)		$	(18.5	)		$	(9.9	)		$	(29.3	)
Reclassification out of Accumulated Other Comprehensive Income
	(In millions)	Income Statement Location	Amounts Reclassified from Accumulated Other Comprehensive Income
			For the Three Months								For the Nine Months
			Ended September 30,								Ended September 30,
			2013				2012				2013				2012
	Gains (losses) on securities available for sale	Other income (expense)	$	0.2			$	(0.3	)		$	4.5			$	(1.1	)
		Income tax expense	(0.1		)		0.1				(1.6		)		0.4
	Gains (losses) on foreign currency forward contracts	Revenues	(7.3		)		12				(6.1		)		31
		Income tax expense	—				—				(0.3		)		(1.9		)
	Total reclassifications, net of tax		$	(7.2	)		$	11.8			$	(3.5	)		$	28.4

Earnings_per_Share_Tables

Earnings per Share (Tables)	9 Months Ended
	Sep. 30, 2013
Earnings Per Share [Abstract]
Basic and diluted earnings per share
		For the Three Months				For the Nine Months
		Ended September 30,				Ended September 30,
	(In millions)	2013		2012		2013		2012
	Numerator:
	Net income attributable to Biogen Idec Inc.	$	487.6	$	398.4	$	1,405.00	$	1,087.90
	Denominator:
	Weighted average number of common shares outstanding	237.1		236.5		237.1		238.3
	Effect of dilutive securities:
	Stock options and employee stock purchase plan	0.2		0.4		0.4		0.5
	Time-vested restricted stock units	0.7		0.9		0.7		1
	Market stock units	0.3		0.3		0.3		0.3
	Dilutive potential common shares	1.2		1.6		1.4		1.8
	Shares used in calculating diluted earnings per share	238.3		238.1		238.5		240.1

ShareBased_Payments_Tables

Share-Based Payments (Tables)	9 Months Ended
	Sep. 30, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-based compensation expense included in consolidated statements of income
		For the Three Months							For the Nine Months
		Ended September 30,							Ended September 30,
	(In millions)	2013				2012			2013			2012
	Research and development	$	20.7			$	18		$	69.6		$	55.8
	Selling, general and administrative	39.9				27.7			119.5			81.5
	Subtotal	60.6				45.7			189.1			137.3
	Capitalized share-based compensation costs	(2.4		)		(1.5		)	(7.4		)	(4.0		)
	Share-based compensation expense included in total cost and expenses	58.2				44.2			181.7			133.3
	Income tax effect	(17.4		)		(13.0		)	(54.3		)	(40.1		)
	Share-based compensation expense included in net income attributable to Biogen Idec Inc.	$	40.8			$	31.2		$	127.4		$	93.2
Summary of share-based compensation expense associated with each of our share-based compensating programs
		For the Three Months							For the Nine Months
		Ended September 30,							Ended September 30,
	(In millions)	2013				2012			2013			2012
	Stock options	$	0.1			$	0.7		$	0.5		$	1.6
	Market stock units	12.2				5.6			27.2			17.2
	Time-vested restricted stock units	25.3				21.4			77.2			69.5
	Performance-vested restricted stock units settled in shares	—				—			—			0.1
	Cash settled performance shares	20.6				16.4			76.8			45.1
	Employee stock purchase plan	2.4				1.6			7.4			3.8
	Subtotal	60.6				45.7			189.1			137.3
	Capitalized share-based compensation costs	(2.4		)		(1.5		)	(7.4		)	(4.0		)
	Share-based compensation expense included in total cost and expenses	$	58.2			$	44.2		$	181.7		$	133.3
Summary of equity grants to employees, officers and directors under our current stock plans
		For the Nine Months
		Ended September 30,
		2013			2012
	Market stock units	268,000			312,000
	Cash settled performance shares	273,000			327,000
	Time-vested restricted stock units	708,000			902,000

Income_Taxes_Tables

Income Taxes (Tables)	9 Months Ended
	Sep. 30, 2013
Income Tax Disclosure [Abstract]
Reconciliation between the U.S. federal statutory tax rate and effective tax rate
		For the Three Months				For the Nine Months
		Ended September 30,				Ended September 30,
		2013		2012		2013		2012
	Statutory rate	35	%	35	%	35	%	35	%
	State taxes	2.5		0.8		3.7		0.8
	Taxes on foreign earnings	(6.6	)	(7.1	)	(7.7	)	(7.6	)
	Credits and net operating loss utilization	(2.2	)	(3.4	)	(3.0	)	(3.7	)
	Purchased intangible assets	1.9		1.3		1.6		1.2
	Manufacturing deduction	(2.5	)	(2.5	)	(8.0	)	(2.3	)
	Other permanent items	(0.3	)	0.4		1		(0.4	)
	Other	(0.4	)	0.2		(0.1	)	0.5
	Effective tax rate	27.4	%	24.7	%	22.5	%	23.5	%

Other_Consolidated_Financial_S1

Other Consolidated Financial Statement Detail (Tables)	9 Months Ended
	Sep. 30, 2013
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Other income (expense), net
		For the Three Months						For the Nine Months
		Ended September 30,						Ended September 30,
	(In millions)	2013			2012			2013			2012
	Interest income	$	1.5		$	5.9		$	6.7		$	22.6
	Interest expense	(6.6		)	(8.7		)	(25.5		)	(23.1		)
	Impairments of investments	—			(3.5		)	(1.7		)	(4.8		)
	Gain (loss) on investments, net	3.9			1.3			14			15.6
	Foreign exchange gains (losses), net	(3.1		)	0.1			(11.0		)	0.2
	Other, net	(0.3		)	0.4			(12.0		)	3.1
	Total other income (expense), net	$	(4.6	)	$	(4.5	)	$	(29.5	)	$	13.5
Accrued Expenses and Other
	(In millions)	As of			As of
		September 30,			December 31,
		2013			2012
	Employee compensation and benefits	261.5			248.5
	Revenue-related rebates	247.4			191
	Deferred revenue	240.7			148
	Royalties and licensing fees	117.3			45.2
	Clinical development expenses	57.1			51.6
	Production expenses	43.6			45.6
	Current portion of contingent consideration obligations	29.1			22.4
	Construction in progress	21.9			12.3
	Other	185.8			215.3
	Total accrued expenses and other	$	1,204.40		$	979.9

Commitments_and_Contingencies_

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2013
Commitments and Contingencies Disclosure [Abstract]
Schedule of milestone payments based on the attainment of certain sales levels
		Cumulative Sales Level
	Prior 12 Month Sales	$500M		$1.0B		$2.0B		$3.0B		Each additional $1.0B up to $20.0B
		Payment Amount (In millions)
	< $500 million	$	—	$	—	$	—	$	—	$	—
	$500 million - $1.0 billion	22		25		50		50		50
	$1.0 billion - $1.5 billion	—		50		100		100		100
	$1.5 billion - $2.0 billion	—		—		150		150		150
	$2.0 billion - $2.5 billion	—		—		200		200		200
	$2.5 billion - $3.0 billion	—		—		—		250		250
	> $3.0 billion	—		—		—		—		300

Business_Details

Business (Details)	9 Months Ended
	Sep. 30, 2013
Business (Textual) [Abstract]
Interest in subsidiary (less than given percentage)	100.00%

Acquisitions_Acquisitions_Deta

Acquisitions Acquisitions (Details) (USD $)	9 Months Ended			9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013	Apr. 02, 2013
				TYSABRI product [Member]	TYSABRI product [Member]
Business Acquisition [Line Items]
Acquisition of TYSABRI rights	$3,262,719,000	$0		$3,250,000,000
Deferred revenue	240,700,000		148,000,000		84,400,000
Net intangible asset					3,178,300,000
Estimated life, (in years)				17 years
Future contingent payment for the first 12 months					12.00%
Future contingent payment for annual worldwide net sales up to $2.0 billion					18.00%
Future contingent payment threshold					$2,000,000,000
Future contingent payment for annual worldwide net sales that exceed $2.0 billion					25.00%

Accounts_Receivable_Details

Accounts Receivable (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Accounts receivable balances on product sales from different countries
Current balance included within accounts receivable, net	$891,396,000	$686,848,000
Spain [Member]
Accounts receivable balances on product sales from different countries
Current balance included within accounts receivable, net	94,100,000	78,900,000
Non-current balance included within accounts receivable, net	18,200,000	0
Total	112,300,000	78,900,000
Italy [Member]
Accounts receivable balances on product sales from different countries
Current balance included within accounts receivable, net	89,200,000	94,400,000
Non-current balance included within accounts receivable, net	2,500,000	10,200,000
Total	91,700,000	104,600,000
Portugal [Member]
Accounts receivable balances on product sales from different countries
Current balance included within accounts receivable, net	12,500,000	16,600,000
Non-current balance included within accounts receivable, net	18,100,000	7,400,000
Total	$30,600,000	$24,000,000

Accounts_Receivable_Details_Te

Accounts Receivable (Details Textual)	9 Months Ended		3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013	Dec. 31, 2011	Sep. 30, 2013	Sep. 30, 2013
	USD ($)	USD ($)	Italy [Member]	Italy [Member]	Italy [Member]	Italy [Member]
			USD ($)	EUR (€)	USD ($)	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
Accounts receivable outstanding for greater than one year	$40.80	$11.80
Payment in case of any unfavorable determination received				30.7
Reimbursement Limit Stated In Resolution				24 months
Loss contingency, expected settlement						33.3
Loss contingency accrual						15.4
Cumulative deferred revenue on sales			120.7		120.7
Amount of revenue deferred during the period			$12.70		$39.90

Reserves_for_Discounts_and_All2

Reserves for Discounts and Allowances (Details) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013
Analysis of the amount of, and change in, reserves
Balance, as of December 31, 2012	$237.10
Current provisions relating to sales in current year	768.7
Adjustments relating to prior years	-7.7
Payments/returns relating to sales in current year	-451.4
Payments/returns relating to sales in prior years	-155.5
Balance, as of September 30, 2013	391.2
Discounts [Member]
Analysis of the amount of, and change in, reserves
Balance, as of December 31, 2012	15.5
Current provisions relating to sales in current year	202.9
Adjustments relating to prior years	-0.8
Payments/returns relating to sales in current year	-145.3
Payments/returns relating to sales in prior years	-13.8
Balance, as of September 30, 2013	58.5
Contractual Adjustments [Member]
Analysis of the amount of, and change in, reserves
Balance, as of December 31, 2012	194.8
Current provisions relating to sales in current year	549.8
Adjustments relating to prior years	-7.8
Payments/returns relating to sales in current year	-306.1
Payments/returns relating to sales in prior years	-127.8
Balance, as of September 30, 2013	302.9
Returns [Member]
Analysis of the amount of, and change in, reserves
Balance, as of December 31, 2012	26.8
Current provisions relating to sales in current year	16
Adjustments relating to prior years	0.9
Payments/returns relating to sales in current year	0
Payments/returns relating to sales in prior years	-13.9
Balance, as of September 30, 2013	$29.80

Reserves_for_Discounts_and_All3

Reserves for Discounts and Allowances (Details 1) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Summary of total product revenue reserves included in consolidated balance sheets
Reduction of accounts receivable	$143.80	$46.10
Component of accrued expenses and other	247.4	191
Total reserves	$391.20	$237.10

Inventory_Details

Inventory (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Components of inventories
Raw materials	$112,200,000	$101,800,000
Work in process	389,800,000	230,500,000
Finished goods	112,500,000	115,100,000
Total inventory	614,483,000	447,373,000
Inventory, Various Programs, Gross	$82,400,000

Intangible_Assets_and_Goodwill2

Intangible Assets and Goodwill (Details) (USD $)

3 Months Ended

9 Months Ended

9 Months Ended

9 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

9 Months Ended

9 Months Ended

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Apr. 02, 2013

Out-licensed patents [Member]

Out-licensed patents [Member]

Out-licensed patents [Member]

Out-licensed patents [Member]

AVONEX core developed technology [Member]

AVONEX core developed technology [Member]

AVONEX core developed technology [Member]

AVONEX core developed technology [Member]

AVONEX core developed technology [Member]

Acquired and in-licensed rights and patents [Member]

Acquired and in-licensed rights and patents [Member]

Acquired and in-licensed rights and patents [Member]

Acquired and in-licensed rights and patents [Member]

In-process research and development [Member]

In-process research and development [Member]

In-process research and development [Member]

Trademarks and trade names [Member]

Trademarks and trade names [Member]

TYSABRI product [Member]

TYSABRI product [Member]

Maximum [Member]

Minimum [Member]

Maximum [Member]

Minimum [Member]

AVONEX [Member]

Maximum [Member]

Minimum [Member]

Intangible assets

Estimated Life, (In Years)

23 years

13 years

23 years

15 years

17 years

6 years

15 years

17 years

Indefinite Lived Intangible Assets Useful Life

Indefinite

Cost

$578,000,000

$578,000,000

$3,005,300,000

$3,005,300,000

$3,236,100,000

$53,700,000

Cost and Net

327,400,000

327,400,000

330,100,000

64,000,000

64,000,000

Total intangible assets, gross

7,210,800,000

7,210,800,000

4,031,100,000

Accumulated Amortization

-2,630,600,000

-2,630,600,000

-2,399,600,000

-443,300,000

-421,000,000

-2,107,000,000

-1,965,700,000

-80,300,000

-12,900,000

0

0

0

0

0

Net

134,700,000

157,000,000

898,300,000

1,039,600,000

887,900,000

3,155,800,000

40,800,000

Intangible assets, net

4,580,199,000

4,580,199,000

1,631,547,000

Write down in carrying value of in-process research and development asset

2,600,000

Intangible Assets and Goodwill (Additional Textual) [Abstract]

Expected future amortization expense, 2013

113,200,000

113,200,000

Expected future amortization expense, 2014

418,900,000

418,900,000

Expected future amortization expense, 2015

337,400,000

337,400,000

Expected future amortization expense, 2016

323,400,000

323,400,000

Expected future amortization expense, 2017

328,100,000

328,100,000

Expected futured amortization expense, 2018

330,700,000

330,700,000

FiniteLivedIntangibleAssetsFutureAmortization

1,851,700,000

1,851,700,000

Intangible Assets and Goodwill (Textual) [Abstract]

Amortization of acquired intangible assets

99,998,000

53,013,000

233,524,000

151,256,000

Net intangible asset

3,178,300,000

Accumulated impairment losses related to goodwill

$0

$0

Intangible_Assets_and_Goodwill3

Intangible Assets and Goodwill (Details 1) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Goodwill [Line Items]
Income tax expense	$186,105,000	$131,044,000	$410,753,000	$334,213,000
Goodwill acquired during the period			13,500,000		48,200,000
Summary of roll forward of the changes in goodwill
Goodwill, beginning of period			1,201,296,000	1,146,300,000	1,146,300,000
Goodwill acquired during the period			13,500,000		48,200,000
Other Goodwill Adjustments			-4,100,000		6,800,000
Goodwill, end of period	1,210,718,000		1,210,718,000		1,201,296,000
TECFIDERA [Member]
Goodwill [Line Items]
Income tax expense			1,500,000
Goodwill acquired during the period			15,000,000
Summary of roll forward of the changes in goodwill
Goodwill acquired during the period			$15,000,000

Fair_Value_Measurements_Detail

Fair Value Measurements (Details) (USD $)	Sep. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Mar. 31, 2012	Dec. 31, 2011
In Millions, unless otherwise specified
Assets:
Marketable debt securities	$614.40		$3,171.70
Fair Value, Measurements, Recurring [Member]
Assets:
Cash equivalents	287.2		439.4
Derivative contracts	1.8		1.8
Plan assets for deferred compensation	21.1		14.3
Total	992.4		3,656.50
Liabilities:
Derivative contracts	17.1		14.4
Contingent consideration obligations	281	281	293.9	290.3	280.9	151
Total	298.1		308.3
Fair Value, Measurements, Recurring [Member] | Corporate debt securities [Member]
Assets:
Marketable debt securities	287		1,001
Fair Value, Measurements, Recurring [Member] | Government securities [Member]
Assets:
Marketable debt securities	267.2		1,657.80
Fair Value, Measurements, Recurring [Member] | Mortgage and other asset backed securities [Member]
Assets:
Marketable debt securities	60.2		512.9
Fair Value, Measurements, Recurring [Member] | Marketable equity securities [Member]
Assets:
Investments	40.3		9
Fair Value, Measurements, Recurring [Member] | Venture capital investments [Member]
Assets:
Investments	27.6	23.5	20.3	25.2	25.4	23.5
Quoted Prices in Active Markets, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Cash equivalents	0		0
Derivative contracts	0		0
Plan assets for deferred compensation	0		0
Total	40.3		9
Liabilities:
Derivative contracts	0		0
Contingent consideration obligations	0		0
Total	0		0
Quoted Prices in Active Markets, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate debt securities [Member]
Assets:
Marketable debt securities	0		0
Quoted Prices in Active Markets, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Government securities [Member]
Assets:
Marketable debt securities	0		0
Quoted Prices in Active Markets, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Mortgage and other asset backed securities [Member]
Assets:
Marketable debt securities	0		0
Quoted Prices in Active Markets, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Marketable equity securities [Member]
Assets:
Investments	40.3		9
Quoted Prices in Active Markets, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Venture capital investments [Member]
Assets:
Investments	0		0
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Cash equivalents	287.2		439.4
Derivative contracts	1.8		1.8
Plan assets for deferred compensation	21.1		14.3
Total	924.5		3,627.20
Liabilities:
Derivative contracts	17.1		14.4
Contingent consideration obligations	0		0
Total	17.1		14.4
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] | Corporate debt securities [Member]
Assets:
Marketable debt securities	287		1,001
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] | Government securities [Member]
Assets:
Marketable debt securities	267.2		1,657.80
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] | Mortgage and other asset backed securities [Member]
Assets:
Marketable debt securities	60.2		512.9
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] | Marketable equity securities [Member]
Assets:
Investments	0		0
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] | Venture capital investments [Member]
Assets:
Investments	0		0
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Cash equivalents	0		0
Derivative contracts	0		0
Plan assets for deferred compensation	0		0
Total	27.6		20.3
Liabilities:
Derivative contracts	0		0
Contingent consideration obligations	281		293.9
Total	281		293.9
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | Corporate debt securities [Member]
Assets:
Marketable debt securities	0		0
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | Government securities [Member]
Assets:
Marketable debt securities	0		0
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | Mortgage and other asset backed securities [Member]
Assets:
Marketable debt securities	0		0
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | Marketable equity securities [Member]
Assets:
Investments	0		0
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | Venture capital investments [Member]
Assets:
Investments	$27.60		$20.30

Fair_Value_Measurements_Detail1

Fair Value Measurements (Details 1) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Mar. 31, 2012	Dec. 31, 2011
					Fair Value, Measurements, Recurring [Member]	Fair Value, Measurements, Recurring [Member]	Fair Value, Measurements, Recurring [Member]	Fair Value, Measurements, Recurring [Member]	Fair Value, Measurements, Recurring [Member]	Fair Value, Measurements, Recurring [Member]
					Venture Capital Investments [Member]	Venture Capital Investments [Member]	Venture Capital Investments [Member]	Venture Capital Investments [Member]	Venture Capital Investments [Member]	Venture Capital Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, beginning of period					$27.60	$23.50	$20.30	$25.20	$25.40	$23.50
Unrealized gains included in earnings	3.8	1.4	10.5	4.9
Unrealized losses included in earnings	0	-1.6	-2	-3.6
Purchases	0.3	0	0.3	0.4
Settlements	0	0	-1.5	0
Fair value, end of period					$27.60	$23.50	$20.30	$25.20	$25.40	$23.50

Fair_Value_Measurements_Detail2

Fair Value Measurements (Details 2) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Summary of fair and carrying value of debt instruments
Credit facility, amount outstanding	$0	$0
Notes payable, Carrying Value	597	1,054.30
Debt instruments, Fair Value	666.3	1,155.30
Notes Payable to Fumedica [Member]
Summary of fair and carrying value of debt instruments
Notes payable, Fair Value	16.8	20
Notes payable, Carrying Value	15.3	17.9
6.0% Senior Notes due 2013 [Member]
Summary of fair and carrying value of debt instruments
Notes payable, Fair Value	0	453.7
Notes payable, Carrying Value	0	450
6.875% Senior Notes due 2018 [Member]
Summary of fair and carrying value of debt instruments
Notes payable, Fair Value	649.5	681.6
Notes payable, Carrying Value	$581.70	$586.40

Fair_Value_Measurements_Detail3

Fair Value Measurements (Details 3) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Mar. 31, 2012	Dec. 31, 2011
					Fair Value, Measurements, Recurring [Member]	Fair Value, Measurements, Recurring [Member]	Fair Value, Measurements, Recurring [Member]	Fair Value, Measurements, Recurring [Member]	Fair Value, Measurements, Recurring [Member]	Fair Value, Measurements, Recurring [Member]
Business Acquisition, Contingent Consideration [Line Items]
Fair value, beginning of period					$281,000,000	$281,000,000	$293,900,000	$290,300,000	$280,900,000	$151,000,000
Additions	0	0	0	122,200,000
(Gain) loss on fair value remeasurement of contingent consideration	-97,000	9,456,000	-2,983,000	23,573,000
Payments	100,000	0	-9,900,000	-6,500,000
Fair value, end of period					$281,000,000	$281,000,000	$293,900,000	$290,300,000	$280,900,000	$151,000,000

Fair_Value_Measurements_Detail4

Fair Value Measurements (Details Textual 1) (USD $)	3 Months Ended	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2013	Dec. 31, 2012
Business Acquisition, Contingent Consideration [Line Items]
Impairment of Long-Lived Assets Held-for-use	$0	$0
Fair Value Measurements, Changes in Valuation Techniques	0	0
Percentage Of Venture Capital Investments To Assets	0.20%	0.20%	0.20%
Other Liabilities Non Current [Member]
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration obligations	$251.90	$251.90	271.5

Fair_Value_Measurements_Fair_V

Fair Value Measurements Fair Value Measurements (Details Textual 2) (USD $)	3 Months Ended	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2013
Debt Instrument [Line Items]
Impairment of Long-Lived Assets Held-for-use	$0	$0
Fair Value Measurements, Changes in Valuation Techniques	0	0
Senior Notes Six Point Zero Percent Due Twenty Thirteen [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.00%	6.00%
Senior Notes Six Point Eight Seven Five Percent Due Twenty Eighteen [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.88%	6.88%

Financial_Instruments_Details

Financial Instruments (Details) (USD $)	9 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Marketable Debt and Equity Securities
Fair Value	$614.40	$3,171.70
Gross Unrealized Gains	0.3	5.6
Gross Unrealized Losses	0.3	1.9
Amortized Cost	614.4	3,168
Corporate debt securities Current [Member]
Marketable Debt and Equity Securities
Fair Value	76.7	346.9
Gross Unrealized Gains	0	0.3
Gross Unrealized Losses	0	0
Amortized Cost	76.7	346.6
Corporate debt securities Non-current [Member]
Marketable Debt and Equity Securities
Fair Value	210.3	654.1
Gross Unrealized Gains	0.3	2.8
Gross Unrealized Losses	0.1	0.6
Amortized Cost	210.1	651.9
Government securities Current [Member]
Marketable Debt and Equity Securities
Fair Value	164.2	783.4
Gross Unrealized Gains	0	0.3
Gross Unrealized Losses	0	0
Amortized Cost	164.2	783.1
Government securities Non-current [Member]
Marketable Debt and Equity Securities
Fair Value	103	874.4
Gross Unrealized Gains	0	0.8
Gross Unrealized Losses	0.1	0
Amortized Cost	103.1	873.6
Mortgage and other asset backed securities Current [Member]
Marketable Debt and Equity Securities
Fair Value	0	4.8
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Amortized Cost	0	4.8
Mortgage and other asset backed securities Non-current [Member]
Marketable Debt and Equity Securities
Fair Value	60.2	508.1
Gross Unrealized Gains	0	1.4
Gross Unrealized Losses	0.1	1.3
Amortized Cost	60.3	508
Marketable equity securities [Member]
Marketable Debt and Equity Securities
Gross Unrealized Gains	24.3	3
Gross Unrealized Losses	0	0
Amortized Cost	16	6
Fair Value	$40.30	$9

Financial_Instruments_Details_

Financial Instruments (Details 1) (USD $)	Sep. 30, 2013	Dec. 31, 2012
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	$287.20	$439.40
Commercial paper [Member]
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	12	40.7
Overnight Repurchase Agreements [Member]
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	35.5	67.4
Short-term Debt Securities [Member]
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$239.70	$331.30

Financial_Instruments_Details_1

Financial Instruments (Details 2) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Summary of Contractual Maturities: Available-for-Sale Securities
Due in one year or less, Estimated Fair Value	$240.90	$1,135
Due in one year or less, Amortized Cost	240.9	1,134.50
Due after one year through five years, Estimated Fair Value	351.8	1,744.30
Due after one year through five years, Amortized Cost	351.6	1,741.20
Due after five years, Estimated Fair Value	21.7	292.4
Due after five years, Amortized Cost	21.9	292.3
Total available-for-sale securities, Fair Value	614.4	3,171.70
Total available-for-sale securities, Amortized Cost	$614.40	$3,168

Financial_Instruments_Details_2

Financial Instruments (Details 3) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Proceeds from Marketable Debt Securities
Proceeds from maturities and sales	$617,500,000	$491,300,000	$5,025,218,000	$1,913,381,000
Realized gains	200,000	400,000	6,600,000	1,700,000
Realized losses	$0	($800,000)	($2,100,000)	($2,700,000)

Financial_Instruments_Details_3

Financial Instruments (Details Textual) (USD $)	9 Months Ended		12 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013
				Strategic Investments [Member]	Strategic Investments [Member]	Strategic Investments [Member]	Strategic Investments [Member]	TYSABRI product [Member]
Financial Instruments (Textual) [Abstract]
Acquisition of TYSABRI rights	$3,262,719,000	$0						$3,250,000,000
Original maturities of commercial paper and short-term debt securities	less than 90B days
Average maturity of marketable securities, months	14 months		14 months
Impairment charges				$0	$3,500,000	$1,700,000	$4,800,000

Financial_Instruments_Financia

Financial Instruments Financial Instruments (Details Textual 2) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Stromedix [Member]
Business Acquisition [Line Items]
Recognized gain				$9
Strategic Investments [Member]
Business Acquisition [Line Items]
Strategic investment portfolio	93		93		64.2
Venture capital investments	68		68		29.3
Cost basis of equity securities of certain privately-owned companies and venture capital funds	25		25		34.9
Net gains (losses) realized	$3.80	$1.80	$7.90	$11.70

Derivative_Instruments_Details

Derivative Instruments (Details) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
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			15 months		12 months
Gain/Loss on fair value of Foreign currency forward contracts	$16.20		$16.20		$11.80
Expected Settlement time for contracts, in months			15 months
Net gains (losses) of other income (expense) related to foreign currency forward contracts	0.2	5.7	1.3	5.6
Designated as Hedging Instrument [Member]
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	580.6		580.6		524
Not Designated as Hedging Instrument [Member]
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	176.7		176.7		243.2
Foreign Exchange Contract [Member] | Other Current Assets [Member] | Designated as Hedging Instrument [Member]
Summary of Derivatives designated as Hedging Instruments
Derivative Asset, Fair Value, Net	0.4		0.4		0.6
Foreign Exchange Contract [Member] | Other Current Assets [Member] | Not Designated as Hedging Instrument [Member]
Summary of Derivatives designated as Hedging Instruments
Derivative Asset, Fair Value, Net	1.4		1.4		1.2
Foreign Exchange Contract [Member] | Accrued Liabilities [Member] | Designated as Hedging Instrument [Member]
Summary of Derivatives designated as Hedging Instruments
Derivative Liability, Fair Value, Net	15.4		15.4		11.5
Foreign Exchange Contract [Member] | Accrued Liabilities [Member] | Not Designated as Hedging Instrument [Member]
Summary of Derivatives designated as Hedging Instruments
Derivative Liability, Fair Value, Net	1.7		1.7		2.9
Euro [Member] | Designated as Hedging Instrument [Member]
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	570		570		492.2
Canadian Dollar [Member] | Designated as Hedging Instrument [Member]
Derivative Instruments (Textual) [Abstract]
Aggregate notional amount	10.6		10.6		31.8
Sales [Member] | Foreign Exchange Contract [Member]
Derivative Instruments (Textual) [Abstract]
Net gains (losses) in product revenue for the settlement of certain effective cash flow hedge instruments	-7.3	12	-6.1	31
Other Income Expense [Member] | Foreign Exchange Contract [Member]
Derivative Instruments (Textual) [Abstract]
Net gains (losses) in earnings of Foreign currency forward contracts due to hedge ineffectiveness	($0.10)	$0.80	$0.20	$4

Derivative_Instruments_Details1

Derivative Instruments (Details 1) (Foreign Exchange Contract [Member], USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Revenue [Member]
Summary of the effect of derivatives designated as hedging instruments on the consolidated statements of income
Amount Reclassified from Accumulated Other Comprehensive Income into Income Gain/(Loss) (Effective Portion)	($7.30)	$12	($6.10)	$31
Other income (expense) [Member]
Summary of the effect of derivatives designated as hedging instruments on the consolidated statements of income
Net gains (losses) in earnings of Foreign currency forward contracts due to hedge ineffectiveness	($0.10)	$0.80	$0.20	$4

Property_Plant_and_Equipment_D

Property, Plant and Equipment (Details) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Property, Plant and Equipment [Line Items]
Accumulated depreciation	($1,074)		($1,074)		($941.10)
Interest cost capitalization	2.2	6.6	5.7	23.4
Cambridge Leases [Member]
Property, Plant and Equipment [Line Items]
Derecognition of construction in progress assets	47.7		47.7
Construction cost incurred not yet paid	101.3		101.3		86.5
Expected charges to vacate building			25
Hillerod, Denmark Facility [Member]
Property, Plant and Equipment [Line Items]
Transfer from construction in progress		$465.90		$465.90
Average estimated useful life			20 years
Eisai [Member]
Property, Plant and Equipment [Line Items]
Term Of Lease			10 years
Cancellation Period of Lease			5 years

Indebtedness_Details

Indebtedness (Details) (USD $)	9 Months Ended			0 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Mar. 01, 2013
				Senior Notes Six Point Zero Percent Due Twenty Thirteen [Member]
Debt Instrument [Line Items]
Senior unsecured revolving credit facility maximum borrowing capacity	$750,000,000
Credit facility, amount outstanding	0		0
Repayment of senior notes	$452,340,000	$0		$450,000,000
Interest rate on senior notes				6.00%

Equity_Details

Equity (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Mar. 31, 2013	Mar. 31, 2012
Reconciliation of equity attributable to non-controlling interests
Noncontrolling interests, beginning of period			$2,272,000	$1,500,000	$600,000	$2,400,000
Net income attributable to noncontrolling interests, net of tax	0	0	0	0
Currency translation adjustment	0	0	0	100,000
Deconsolidation of noncontrolling interest	0	0	-1,700,000	-500,000
Distributions to noncontrolling interests	0	0	0	1,300,000
Noncontrolling interests, end of period	$596,000	$2,400,000	$596,000	$2,400,000	$600,000	$2,400,000

Equity_Details_Textual

Equity (Details Textual) (USD $)	3 Months Ended		9 Months Ended
Share data in Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Feb. 28, 2011
Equity (Textual) [Abstract]
Equity increase during the period			$1,161,900,000
Net income attributable to Biogen Idec Inc	487,616,000	398,400,000	1,405,035,000	1,087,906,000
Adjustments to Additional Paid in Capital, Share-based Compensation and Exercise of Stock Options			132,900,000
Purchase of treasury stock, value	($359,300,000)		$400,308,000	$963,171,000
Common stock shares authorized for repurchase	4.2		4.2		20
Repurchase of common stock, shares	1.7		2	7.7

Accumulated_Other_Comprehensiv2

Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Accumulated other comprehensive income [Line Items]
Accumulated other comprehensive income (loss), net of tax	($29,315,000)		($29,315,000)		($55,305,000)
Other comprehensive income loss before reclassifications			22,500,000
Amounts reclassified from accumulated other comprehensive income loss			3,500,000
Net current period other comprehensive income loss	25,820,000	-560,000	25,990,000	-24,516,000
Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated other comprehensive income [Line Items]
Accumulated other comprehensive income (loss), net of tax	15,400,000		15,400,000		4,200,000
Other comprehensive income loss before reclassifications			14,100,000
Amounts reclassified from accumulated other comprehensive income loss			-2,900,000
Net current period other comprehensive income loss			11,200,000
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Accumulated other comprehensive income [Line Items]
Accumulated other comprehensive income (loss), net of tax	-16,300,000		-16,300,000		-10,700,000
Other comprehensive income loss before reclassifications			-12,000,000
Amounts reclassified from accumulated other comprehensive income loss			6,400,000
Net current period other comprehensive income loss			-5,600,000
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated other comprehensive income [Line Items]
Accumulated other comprehensive income (loss), net of tax	-18,500,000		-18,500,000		-21,700,000
Other comprehensive income loss before reclassifications			3,200,000
Amounts reclassified from accumulated other comprehensive income loss			0
Net current period other comprehensive income loss			3,200,000
Accumulated Translation Adjustment [Member]
Accumulated other comprehensive income [Line Items]
Accumulated other comprehensive income (loss), net of tax	-9,900,000		-9,900,000		-27,100,000
Other comprehensive income loss before reclassifications			17,200,000
Amounts reclassified from accumulated other comprehensive income loss			0
Net current period other comprehensive income loss			$17,200,000

Accumulated_Other_Comprehensiv3

Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Details 1) (USD $)

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

12 Months Ended

3 Months Ended

9 Months Ended

12 Months Ended

3 Months Ended

9 Months Ended

In Thousands, unless otherwise specified

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2013

Reclassification out of Accumulated Other Comprehensive Income [Member]

Reclassification out of Accumulated Other Comprehensive Income [Member]

Reclassification out of Accumulated Other Comprehensive Income [Member]

Reclassification out of Accumulated Other Comprehensive Income [Member]

Accumulated Net Unrealized Investment Gain (Loss) [Member]

Accumulated Net Unrealized Investment Gain (Loss) [Member]

Accumulated Net Unrealized Investment Gain (Loss) [Member]

Accumulated Net Unrealized Investment Gain (Loss) [Member]

Accumulated Net Unrealized Investment Gain (Loss) [Member]

Accumulated Net Unrealized Investment Gain (Loss) [Member]

Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]

Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]

Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]

Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]

Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]

Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]

Other comprehensive income loss before reclassifications [Member]

Other comprehensive income loss before reclassifications [Member]

Reclassification out of Accumulated Other Comprehensive Income [Member]

Reclassification out of Accumulated Other Comprehensive Income [Member]

Reclassification out of Accumulated Other Comprehensive Income [Member]

Reclassification out of Accumulated Other Comprehensive Income [Member]

Reclassification out of Accumulated Other Comprehensive Income [Member]

Reclassification out of Accumulated Other Comprehensive Income [Member]

Reclassification out of Accumulated Other Comprehensive Income [Member]

Reclassification out of Accumulated Other Comprehensive Income [Member]

Accumulated Net Unrealized Investment Gain (Loss) [Member]

Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]

Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]

Other income (expense)

($4,640)

($4,548)

($29,525)

$13,546

$200

($300)

$4,500

($1,100)

Income tax expense

186,105

131,044

410,753

334,213

9,000

2,500

-100

100

-1,600

400

100

1,100

0

0

-300

-1,900

6,600

1,200

Revenues

1,827,780

1,385,554

4,966,349

4,098,520

-7,300

12,000

-6,100

31,000

Net income

$487,616

$398,400

$1,405,035

$1,087,906

($7,200)

$11,800

($3,500)

$28,400

Earnings_per_Share_Details

Earnings per Share (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, except Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Numerator:
Net income attributable to Biogen Idec Inc	$487,616	$398,400	$1,405,035	$1,087,906
Denominator:
Weighted average number of common shares outstanding	237,070,000	236,474,000	237,131,000	238,331,000
Effect of dilutive securities:
Dilutive potential common shares	1,200,000	1,600,000	1,400,000	1,800,000
Shares used in calculating diluted earnings per share	238,349,000	238,125,000	238,508,000	240,137,000
Stock options and employee stock purchase plan [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	200,000	400,000	400,000	500,000
Time-vested restricted stock units [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	700,000	900,000	700,000	1,000,000
Market stock units [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	300,000	300,000	300,000	300,000

ShareBased_Payments_Details

Share-Based Payments (Details) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Share-based Compensation Expense included in consolidated statements of income
Subtotal	$60.60	$45.70	$189.10	$137.30
Capitalized share-based compensation costs	-2.4	-1.5	-7.4	-4
Share-based compensation expense included in total costs and expenses	58.2	44.2	181.7	133.3
Income tax effect	-17.4	-13	-54.3	-40.1
Research and development
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	20.7	18	69.6	55.8
Selling, general and administrative [Member]
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	39.9	27.7	119.5	81.5
Total share-based compensation expense, net of tax [Member]
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense	$40.80	$31.20	$127.40	$93.20

ShareBased_Payments_Details_1

Share-Based Payments (Details 1) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Summary of share based compensation expense associated with different programs
Subtotal	$60.60	$45.70	$189.10	$137.30
Capitalized share-based compensation costs	-2.4	-1.5	-7.4	-4
Share-based compensation expense included in total costs and expenses	58.2	44.2	181.7	133.3
Stock Options [Member]
Summary of share based compensation expense associated with different programs
Share-based compensation expense	0.1	0.7	0.5	1.6
Market stock units [Member]
Summary of share based compensation expense associated with different programs
Share-based compensation expense	12.2	5.6	27.2	17.2
Time-vested restricted stock units [Member]
Summary of share based compensation expense associated with different programs
Share-based compensation expense	25.3	21.4	77.2	69.5
Performance-vested restricted stock units settled in shares [Member]
Summary of share based compensation expense associated with different programs
Share-based compensation expense	0	0	0	0.1
Cash settled performance shares [Member]
Summary of share based compensation expense associated with different programs
Share-based compensation expense	20.6	16.4	76.8	45.1
Employee stock purchase plan [Member]
Summary of share based compensation expense associated with different programs
Share-based compensation expense	$2.40	$1.60	$7.40	$3.80

ShareBased_Payments_Details_2

Share-Based Payments (Details 2)	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Market stock units [Member]
Schedule of stock option grants
Stock units	268,000	312,000
Cash settled performance shares [Member]
Schedule of stock option grants
Stock units	273,000	327,000
Time-vested restricted stock units [Member]
Schedule of stock option grants
Stock units	708,000	902,000

ShareBased_Payments_Details_Te

Share-Based Payments (Details Textual)	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Share-Based Payments (Textual) [Abstract]
Shares issued under ESPP	208,000	225,000
Number of stock option	0	0
Performance Vested Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock units	0	0

Income_Taxes_Details

Income Taxes (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Reconciliation between the U.S. federal statutory tax rate and effective tax rate
Statutory rate	35.00%	35.00%	35.00%	35.00%
State taxes	2.50%	0.80%	3.70%	0.80%
Taxes on foreign earnings	-6.60%	-7.10%	-7.70%	-7.60%
Credits and net operating loss utilization	-2.20%	-3.40%	-3.00%	-3.70%
Purchased intangible assets	1.90%	1.30%	1.60%	1.20%
Manufacturing deduction	-2.50%	-2.50%	-8.00%	-2.30%
Permanent items	-0.30%	0.40%	1.00%	-0.40%
Other	-0.40%	0.20%	-0.10%	0.50%
Effective tax rate	27.40%	24.70%	22.50%	23.50%

Income_Taxes_Details_Textual

Income Taxes (Details Textual) (USD $)	0 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Oct. 15, 2013	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2007	Jun. 08, 2010
Income Taxes (Textual) [Abstract]
Effective tax rate		27.40%	24.70%	22.50%	23.50%
Deferred tax liability recognized				$203.70
Net benefit for a previously unrecognized position		33		33
Expense for non-income based state taxes		10		10
Net benefit from remeasurement of uncertain tax position		7		7
Proposed deduction disallowance by tax authorities						130	130	130
Notice of Assessment of corporate excise tax including penalties and interest									103.5
Settlement with the Massachusetts DOR	$5

Other_Consolidated_Financial_S2

Other Consolidated Financial Statement Detail (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Other Income (Expense), Net
Interest income	$1,500,000	$5,900,000	$6,700,000	$22,600,000
Interest expense	-6,600,000	-8,700,000	-25,500,000	-23,100,000
Impairment of investments	0	-3,500,000	-1,700,000	-4,800,000
Gain (loss) on sales of investments, net	3,900,000	1,300,000	14,000,000	15,600,000
Foreign exchange gains (losses), net	-3,100,000	100,000	-11,000,000	200,000
Other, net	-300,000	400,000	-12,000,000	3,100,000
Total other income (expense), net	-4,640,000	-4,548,000	-29,525,000	13,546,000
Accrued Expenses and Other
Employee-related Liabilities	261,500,000		261,500,000		248,500,000
Revenue-related rebates	247,400,000		247,400,000		191,000,000
Deferred revenue	240,700,000		240,700,000		148,000,000
Royalties and licensing fees	117,300,000		117,300,000		45,200,000
Clinical development expenses	57,100,000		57,100,000		51,600,000
Production expenses accrual	43,600,000		43,600,000		45,600,000
Current portion of contingent consideration obligations	29,100,000		29,100,000		22,400,000
Construction in progress accrual	21,900,000		21,900,000		12,300,000
Other	185,800,000		185,800,000		215,300,000
Total accrued expenses and other	$1,204,437,000		$1,204,437,000		$979,945,000

Investments_in_Variable_Intere1

Investments in Variable Interest Entities (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Investment in Variable Interest Entities (Textual) [Abstract]
Research and development	$410,017,000	$304,217,000	$1,021,820,000	$989,738,000
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities	6,800,000		6,800,000		9,400,000
Knopp [Member]
Investment in Variable Interest Entities (Textual) [Abstract]
Percentage of funding for R&D cost required in support of the collaboration agreement			100.00%
Research and development		15,700,000		58,900,000
Neurimmune [Member]
Investment in Variable Interest Entities (Textual) [Abstract]
Percentage of funding for R&D cost required in support of the collaboration agreement			100.00%
Research and development	6,100,000	3,400,000	16,800,000	8,500,000
Remaining potential development milestone payments and royalties on commercial sales under the terms of collaboration agreement			$345,000,000
Common Class B [Member] | Knopp [Member]
Investment in Variable Interest Entities (Textual) [Abstract]
Purchase of common shares in variable interest entities			30.00%

Collaborative_and_Other_Relati1

Collaborative and Other Relationships Collaborative and Other Relationships - Equity Method Investments (Details)

3 Months Ended

9 Months Ended

1 Months Ended

3 Months Ended

9 Months Ended

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Feb. 29, 2012

Feb. 29, 2012

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Dec. 31, 2012

Dec. 31, 2012

USD ($)

USD ($)

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]

Samsung Biosimilar Agreement [Member]

USD ($)

KRW

USD ($)

KRW

USD ($)

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 Idec to develop, manufacture and market biosimilar pharmaceuticals

45,000,000

49,500,000,000

26,500,000

28,500,000,000

26,500,000

27,800,000

29,700,000,000

Percentage of stake in entity minimum

15.00%

15.00%

Percentage of stake in entity maximum

49.90%

49.90%

Cash contributions to Samsung Bioepis

5,800,000

6,500,000,000

Recognized loss

-6,170,000

-1,258,000

-12,270,000

-1,769,000

-6,200,000

-1,300,000

-12,300,000

-1,800,000

Revenue related to technical development and technology transfer services

$71,016,000

$58,652,000

$174,497,000

$150,147,000

$4,300,000

$4,300,000

$18,500,000

$9,900,000

Collaborative_and_Other_Relati2

Collaborative and Other Relationships Collaborative and Other Relationships - Collaboration Agreement (Details 1) (USD $)	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013
			Isis Pharmaceuticals	Isis Pharmaceuticals	Isis Pharmaceuticals	Other research and discovery [Member]
				Research and development expense	Prepaid research and development
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Term of research collaboration			6 years
Upfront payment made to collaborative partner			$100,000,000	$75,000,000	$25,000,000
Expected license fee			70,000,000
Additional milestone payments upon achievement of certain regulatory milestones			150,000,000
Additional milestone payments for product candidate using a different modality			90,000,000
Investments and other assets	636,028,000	274,054,000				35,000,000
Research and development expense						$2,000,000

Litigation_Details

Litigation (Details)	3 Months Ended			12 Months Ended	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2013	Mar. 31, 2013	Jun. 30, 2011	Dec. 31, 2008	Dec. 31, 2011	Sep. 30, 2013
	Hoechst [Member]	Hoechst [Member]	Hoechst [Member]	Canada Lease Dispute [Member]	ITALY	ITALY
	USD ($)	EUR (€)	USD ($)	USD ($)	EUR (€)	EUR (€)
Loss Contingencies [Line Items]
Compensatory damages and interest		€ 108		$7
Prejudgment interest		54
Decrease in share of co-promotion revenue due to estimated damages and interest	41.5		50
Payment in case of any unfavorable determination received					30.7
Limited period for government reimbursement					24 months
Automatic renewal period for the resolution					24 months
Reimbursement limit stated in the resolution					24 months
Loss contingency, expected settlement						33.3
Plaintiffs estimated expert report of damages				2.5
Cost seeks for damages				$0.40

Commitments_and_Contingencies_1

Commitments and Contingencies (Details) (Fumapharm AG [Member], USD $)	Sep. 30, 2013
Five hundred million [Member]
Business Acquisition, Contingent Consideration [Line Items]
Cumulative sales level	$500,000,000
One billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Cumulative sales level	1,000,000,000
Two billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Cumulative sales level	2,000,000,000
Three billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Cumulative sales level	3,000,000,000
Each additional one billion up to twenty billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Cumulative sales level	20,000,000,000
Each additional sales level	1,000,000,000
Less than five hundred million [Member]
Business Acquisition, Contingent Consideration [Line Items]
Sales level	500,000,000
Less than five hundred million [Member] | Five hundred million [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	0
Less than five hundred million [Member] | One billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	0
Less than five hundred million [Member] | Two billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	0
Less than five hundred million [Member] | Three billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	0
Less than five hundred million [Member] | Each additional one billion up to twenty billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	0
Five hundred million to one billion [Member] | Five hundred million [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	22,000,000
Five hundred million to one billion [Member] | One billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	25,000,000
Five hundred million to one billion [Member] | Two billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	50,000,000
Five hundred million to one billion [Member] | Three billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	50,000,000
Five hundred million to one billion [Member] | Each additional one billion up to twenty billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	50,000,000
One billion to one point five billion [Member] | Five hundred million [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	0
One billion to one point five billion [Member] | One billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	50,000,000
One billion to one point five billion [Member] | Two billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	100,000,000
One billion to one point five billion [Member] | Three billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	100,000,000
One billion to one point five billion [Member] | Each additional one billion up to twenty billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	100,000,000
One point five billion to two billion [Member] | Five hundred million [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	0
One point five billion to two billion [Member] | One billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	0
One point five billion to two billion [Member] | Two billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	150,000,000
One point five billion to two billion [Member] | Three billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	150,000,000
One point five billion to two billion [Member] | Each additional one billion up to twenty billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	150,000,000
Two billion to two point five billion [Member] | Five hundred million [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	0
Two billion to two point five billion [Member] | One billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	0
Two billion to two point five billion [Member] | Two billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	200,000,000
Two billion to two point five billion [Member] | Three billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	200,000,000
Two billion to two point five billion [Member] | Each additional one billion up to twenty billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	200,000,000
Two point five billion to three billion [Member] | Five hundred million [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	0
Two point five billion to three billion [Member] | One billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	0
Two point five billion to three billion [Member] | Two billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	0
Two point five billion to three billion [Member] | Three billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	250,000,000
Two point five billion to three billion [Member] | Each additional one billion up to twenty billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	250,000,000
Greater than three billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Sales level	3,000,000,000
Greater than three billion [Member] | Five hundred million [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	0
Greater than three billion [Member] | One billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	0
Greater than three billion [Member] | Two billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	0
Greater than three billion [Member] | Three billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	0
Greater than three billion [Member] | Each additional one billion up to twenty billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Milestone payment due on the prior 12 month sales	300,000,000
Minimum [Member] | Five hundred million to one billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Sales level	500,000,000
Minimum [Member] | One billion to one point five billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Sales level	1,000,000,000
Minimum [Member] | One point five billion to two billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Sales level	1,500,000,000
Minimum [Member] | Two billion to two point five billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Sales level	2,000,000,000
Minimum [Member] | Two point five billion to three billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Sales level	2,500,000,000
Maximum [Member] | Five hundred million to one billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Sales level	1,000,000,000
Maximum [Member] | One billion to one point five billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Sales level	1,500,000,000
Maximum [Member] | One point five billion to two billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Sales level	2,000,000,000
Maximum [Member] | Two billion to two point five billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Sales level	2,500,000,000
Maximum [Member] | Two point five billion to three billion [Member]
Business Acquisition, Contingent Consideration [Line Items]
Sales level	$3,000,000,000

Commitments_and_Contingences_D

Commitments and Contingences (Details Textual) (USD $)	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013	Dec. 31, 2006	Sep. 30, 2013
			TECFIDERA [Member]	Fumapharm AG [Member]	Fumapharm AG [Member]
					TECFIDERA [Member]
Business Acquisition, Contingent Consideration [Line Items]
Amount paid in cash				$220
Additional payment if a Fumapharm Product is approved				15
Goodwill acquired during the period	$13.50	$48.20	$15		$15

Segment_Information_Details

Segment Information (Details)	9 Months Ended
	Sep. 30, 2013
	segment
Segment Information (Textual) [Abstract]
Number of reportable segment	1