Biogen (BIIB) 10-Q 23 Jul 14 2014 Q2 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	6 Months Ended
	Jun. 30, 2014	Jul. 18, 2014
Document and Entity Information [Abstract]	'	'
Entity Registrant Name	'BIOGEN IDEC INC.	'
Entity Central Index Key	'0000875045	'
Current Fiscal Year End Date	'--12-31	'
Entity Filer Category	'Large Accelerated Filer	'
Document Type	'10-Q	'
Document Period End Date	30-Jun-14	'
Document Fiscal Year Focus	'2014	'
Document Fiscal Period Focus	'Q2	'
Amendment Flag	'false	'
Entity Common Stock, Shares Outstanding	'	236,144,705

Condensed_Consolidated_Stateme

Condensed Consolidated Statements of Income (Unaudited) (USD $)	3 Months Ended		6 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Revenues:	'	'	'	'
Product, net	$2,056,292	$1,385,918	$3,799,057	$2,481,697
Unconsolidated joint business	303,296	288,785	600,181	553,391
Other	61,864	48,770	151,965	103,481
Total revenues	2,421,452	1,723,473	4,551,203	3,138,569
Cost and expenses:	'	'	'	'
Cost of sales, excluding amortization of acquired intangible assets	291,887	230,728	571,132	364,477
Research and development	447,273	327,463	976,157	611,803
Selling, general and administrative	576,622	431,012	1,088,296	783,610
Amortization of acquired intangible assets	116,826	82,225	260,084	133,526
Collaboration profit sharing	0	0	0	85,357
(Gain) loss on fair value remeasurement of contingent consideration	4,019	-5,163	3,220	-2,886
Total cost and expenses	1,436,627	1,066,265	2,898,889	1,975,887
Gain on sale of rights	3,900	5,319	7,759	10,370
Income from operations	988,725	662,527	1,660,073	1,173,052
Other income (expense), net	4,861	-10,428	-740	-24,885
Income before income tax expense and equity in loss of investee, net of tax	993,586	652,099	1,659,333	1,148,167
Income tax expense	268,521	159,140	446,935	224,648
Equity in loss of investee, net of tax	1,933	2,289	9,538	6,100
Net income	723,132	490,670	1,202,860	917,419
Net income (loss) attributable to noncontrolling interests, net of tax	8,626	0	8,398	0
Net income attributable to Biogen Idec Inc.	$714,506	$490,670	$1,194,462	$917,419
Net income per share:	'	'	'	'
Basic earnings per share attributable to Biogen Idec Inc.	$3.02	$2.07	$5.05	$3.87
Diluted earnings per share attributable to Biogen Idec Inc.	$3.01	$2.06	$5.03	$3.85
Weighted-average shares used in calculating:	'	'	'	'
Basic earnings per share attributable to Biogen Idec Inc.	236,661	237,484	236,729	237,162
Diluted earnings per share attributable to Biogen Idec Inc.	237,401	238,743	237,634	238,543

Condensed_Consolidated_Stateme1

Condensed Consolidated Statements of Comprehensive Income (Unaudited) (USD $)	3 Months Ended		6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Statement of Comprehensive Income [Abstract]	'	'	'	'
Net income attributable to Biogen Idec Inc.	$714,506	$490,670	$1,194,462	$917,419
Other comprehensive income:	'	'	'	'
Unrealized gains (losses) on securities available for sale, net of tax of $4,029 and $3,569 for the three months ended June 30, 2014 and 2013, respectively; and $3,015 and $2,915 for the six months ended June 30, 2014 and 2013, respectively	-6,865	6,077	-5,140	4,960
Unrealized gains (losses) on foreign currency forward contracts, net of tax of $260 and $59 for the three months ended June 30, 2014 and 2013, respectively; and $5 and $1,480 for the six months ended June 30, 2014 and 2013, respectively	10,760	-2,305	16,551	9,298
Unrealized gains (losses) on pension benefit obligation	-170	1,011	646	2,274
Currency translation adjustment	-8,047	8,056	-10,991	-16,363
Total other comprehensive income (loss), net of tax	-4,322	12,839	1,066	169
Comprehensive income attributable to Biogen Idec Inc.	710,184	503,509	1,195,528	917,588
Comprehensive income attributable to noncontrolling interests, net of tax	8,626	0	8,398	0
Comprehensive income	$718,810	$503,509	$1,203,926	$917,588

Condensed_Consolidated_Stateme2

Condensed Consolidated Statements of Comprehensive Income (Unaudited) (Parenthetical) (USD $)	3 Months Ended		6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Statement of Comprehensive Income [Abstract]	'	'	'	'
Tax effect on net unrealized gains (losses) recognized on securities available for sale	$4,029	$3,569	$3,015	$2,915
Tax effect of net unrealized gains (losses) recognized on foreign currency forward contracts	$260	$59	$5	$1,480

Condensed_Consolidated_Balance

Condensed Consolidated Balance Sheets (Unaudited) (USD $)	Jun. 30, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Current assets:	'	'
Cash and cash equivalents	$863,197	$602,562
Marketable securities	709,869	620,167
Accounts receivable, net	1,002,328	824,406
Due from unconsolidated joint business, net	286,897	252,662
Inventory	715,935	659,003
Other current assets	346,787	226,134
Total current assets	3,925,013	3,184,934
Marketable securities	1,010,837	625,772
Property, plant and equipment, net	1,756,164	1,750,710
Intangible assets, net	4,249,378	4,474,653
Goodwill	1,364,815	1,232,916
Investments and other assets	611,791	594,350
Total assets	12,917,998	11,863,335
Current liabilities:	'	'
Current portion of notes payable and line of credit	3,386	3,494
Taxes payable	192,758	179,685
Accounts payable	224,931	219,913
Accrued expenses and other	1,495,549	1,355,187
Total current liabilities	1,916,624	1,758,279
Notes payable	586,091	592,433
Long-term deferred tax liability	139,092	232,554
Other long-term liabilities	710,965	659,231
Total liabilities	3,352,772	3,242,497
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	128
Additional paid-in capital	4,106,084	4,023,651
Accumulated other comprehensive loss	-26,678	-27,745
Retained earnings	7,543,597	6,349,135
Treasury stock, at cost	-2,061,832	-1,724,927
Total Biogen Idec Inc. shareholders’ equity	9,561,299	8,620,242
Noncontrolling interests	3,927	596
Total equity	9,565,226	8,620,838
Total liabilities and equity	$12,917,998	$11,863,335

Condensed_Consolidated_Balance1

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) (USD $)	Jun. 30, 2014	Dec. 31, 2013
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 $)	6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013
Cash flows from operating activities:	'	'
Net income	$1,202,860	$917,419
Adjustments to reconcile net income to net cash flows from operating activities:	'	'
Depreciation and amortization	355,102	225,880
Share-based compensation	83,856	67,387
Deferred income taxes	-146,506	-118,047
Other	-64,213	-42,495
Changes in operating assets and liabilities, net:	'	'
Accounts receivable	-188,961	-185,688
Inventory	-72,689	-129,171
Accrued expenses and other current liabilities	-30,299	-15,970
Other changes in operating assets and liabilities, net	-43,931	84,836
Net cash flows provided by operating activities	1,095,219	804,151
Cash flows from investing activities:	'	'
Proceeds from sales and maturities of marketable securities	1,317,525	4,404,707
Purchases of marketable securities	-1,787,606	-1,617,974
Acquisition of TYSABRI rights	0	-3,262,719
Purchases of property, plant and equipment	-118,308	-87,440
Acquisitions of business, net of cash acquired	-25,000	0
Other	-10,745	-6,874
Net cash flows used in investing activities	-624,134	-570,300
Cash flows from financing activities:	'	'
Purchase of treasury stock	-336,905	-41,023
Proceeds from issuance of stock for share-based compensation arrangements	33,477	39,002
Repayment of borrowings under senior notes	0	-452,340
Excess tax benefit from stock options	83,940	53,311
Other	12,003	-8,137
Net cash flows used in financing activities	-207,485	-409,187
Net increase (decrease) in cash and cash equivalents	263,600	-175,336
Effect of exchange rate changes on cash and cash equivalents	-2,965	-2,878
Cash and cash equivalents, beginning of the period	602,562	570,721
Cash and cash equivalents, end of the period	$863,197	$392,507

Summary_of_Significant_Account

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

Accounts_Receivable

Inventory	6 Months Ended
	Jun. 30, 2014
Inventory Disclosure [Abstract]	'
Inventory	'
	Inventory
	The components of inventory are summarized as follows:
	(In millions)	As of		As of
		June 30,		December 31,
		2014		2013
	Raw materials	$	124.8	$	115
	Work in process	447.4		435.4
	Finished goods	143.7		108.6
	Total inventory	$	715.9	$	659
	As of June 30, 2014 our inventory includes $45.5 million associated with our PLEGRIDY and Daclizumab High Yield Process programs, which have been capitalized in advance of regulatory approval. As of December 31, 2013, our inventory included $93.7 million associated with our ALPROLIX, ELOCTATE and PLEGRIDY programs, which were capitalized in advance of regulatory approval.

Reserves_for_Discounts_and_All

Inventory	6 Months Ended
	Jun. 30, 2014
Inventory Disclosure [Abstract]	'
Inventory	'
	Inventory
	The components of inventory are summarized as follows:
	(In millions)	As of		As of
		June 30,		December 31,
		2014		2013
	Raw materials	$	124.8	$	115
	Work in process	447.4		435.4
	Finished goods	143.7		108.6
	Total inventory	$	715.9	$	659
	As of June 30, 2014 our inventory includes $45.5 million associated with our PLEGRIDY and Daclizumab High Yield Process programs, which have been capitalized in advance of regulatory approval. As of December 31, 2013, our inventory included $93.7 million associated with our ALPROLIX, ELOCTATE and PLEGRIDY programs, which were capitalized in advance of regulatory approval.

Inventory

Inventory	6 Months Ended
	Jun. 30, 2014
Inventory Disclosure [Abstract]	'
Inventory	'
	Inventory
	The components of inventory are summarized as follows:
	(In millions)	As of		As of
		June 30,		December 31,
		2014		2013
	Raw materials	$	124.8	$	115
	Work in process	447.4		435.4
	Finished goods	143.7		108.6
	Total inventory	$	715.9	$	659
	As of June 30, 2014 our inventory includes $45.5 million associated with our PLEGRIDY and Daclizumab High Yield Process programs, which have been capitalized in advance of regulatory approval. As of December 31, 2013, our inventory included $93.7 million associated with our ALPROLIX, ELOCTATE and PLEGRIDY programs, which were capitalized in advance of regulatory approval.

Intangible_Assets_and_Goodwill

Intangible Assets and Goodwill	6 Months Ended
	Jun. 30, 2014
Goodwill and Intangible Assets Disclosure [Abstract]	'
Intangible Assets and Goodwill	'
	Intangible Assets and Goodwill
	Intangible Assets
	Intangible assets, net of accumulated amortization, impairment charges and adjustments, are summarized as follows:
				As of June 30, 2014									As of December 31, 2013
	(In millions)	Estimated		Cost				Accumulated			Net		Cost		Accumulated			Net
		Life						Amortization							Amortization
	Out-licensed patents	13-23 years		$	543.3			$	(465.9	)	$	77.4	$	578	$	(450.8	)	$	127.2
	Developed	15-23 years		3,005.30				(2,283.7		)	721.6		3,005.30		(2,165.4		)	839.9
	technology
	In-process research and development	Indefinite until commercialization		330.9				—			330.9		327.4		—			327.4
	Trademarks and	Indefinite		64				—			64		64		—			64
	tradenames
	Acquired and in-licensed rights	6-17 years		3,271.50				(216.0		)	3,055.50		3,240.00		(123.8		)	3,116.20
	and patents
	Total intangible assets			$	7,215.00			$	(2,965.6	)	$	4,249.40	$	7,214.70	$	(2,740.0	)	$	4,474.70
	For the three and six months ended June 30, 2014, amortization of acquired intangible assets totaled $116.8 million and $260.1 million, respectively, as compared to $82.2 million and $133.5 million, respectively, in the prior year comparative periods. For the three and six months ended June 30, 2014, compared to the same periods in 2013, the change in amortization of acquired intangible assets was primarily driven by our acquisition of the TYSABRI rights from Elan Pharma International Ltd. (Elan) and an increase in the amount of amortization recorded in relation to our AVONEX intangible asset.
	Out-licensed Patents
	Out-licensed patents to third-parties primarily relate to patents acquired in connection with the merger of Biogen, Inc. and IDEC Pharmaceuticals Corporation in 2003. During the six months ended June 30, 2014, we recorded a charge of $34.7 million related to the impairment of one of our out-licensed patents to reflect a change in its estimated fair value, due to a change in the underlying competitive market for that product, which occurred during the first quarter of 2014. The charge is included in amortization of acquired intangibles. The fair value of the intangible asset was based on discounted cash flow calculated using Level 3 fair value measurements and inputs including estimated revenues.
	Developed Technology
	Developed technology primarily relates to our AVONEX product, which was recorded in connection with the merger of Biogen, Inc. and IDEC Pharmaceuticals Corporation in 2003. The net book value of this asset as of June 30, 2014 was $711.9 million. We amortize this intangible asset using the economic consumption method based on actual and expected revenues generated from the sales of our AVONEX product.
	Acquired and In-licensed Rights and Patents
	Acquired and in-licensed rights and patents primarily relate to our acquisition of the TYSABRI rights from Elan. The net intangible asset capitalized related to this acquisition was $3,178.3 million. 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 revenues generated from the sales of our TYSABRI product. The net book value of this asset as of June 30, 2014 was $2,995.1 million. For a more detailed description of this transaction, please read Note 2, Acquisitions to our consolidated financial statements included within our 2013 Form 10-K.
	The increase in acquired and in-licensed rights and patents during the six months ended June 30, 2014 was primarily related to the $20.0 million contingent payment due to the former owners of Syntonix Pharmaceuticals, Inc., which became payable upon the approval of ALPROLIX in the U.S. by the U.S. Food and Drug Administration (FDA) in the first quarter of 2014. We have recorded an additional $7.8 million of acquired in-licensed rights and patents related to this consideration, along with a corresponding deferred tax liability of the same amount.
	Estimated Future Amortization of Intangible Assets
	Our amortization expense is based on the economic consumption of the intangible assets. Our most significant intangible assets are related to our AVONEX and TYSABRI products. Annually, during our long-range planning cycle, we perform an analysis of anticipated lifetime revenues of AVONEX and TYSABRI. This analysis is updated whenever events or changes in circumstances would significantly affect the anticipated lifetime revenues of either product.
	Our most recent long range planning cycle was updated in the third quarter of 2013, and included the impact of our acquisition of TYSABRI rights from Elan and a decrease in the expected future product revenues of AVONEX, resulting in an increase in amortization expense as compared to prior quarters. The results of our analysis were impacted by changes in the estimated impact of TECFIDERA, as well as other existing and potential oral and alternative MS formulations, including PLEGRIDY, that may compete with AVONEX and TYSABRI. Based upon this recent analysis, the estimated future amortization for acquired intangible assets for the balance of 2014 and the next five years is expected to be as follows:
	(In millions)	As of June 30, 2014
	2014 (remaining six months)	$	217.2
	2015	333.4
	2016	318.8
	2017	322
	2018	323.1
	2019	302.9
	Total	$	1,817.40
	Goodwill
	The following table provides a roll forward of the changes in our goodwill balance:
	(In millions)	As of				As of
		June 30,				December 31,
		2014				2013
	Goodwill, beginning of period	$	1,232.90			$	1,201.30
	Increase to goodwill	131.9				35.7
	Other	—				(4.1		)
	Goodwill, end of period	$	1,364.80			$	1,232.90
	The increase in goodwill during the six months ended June 30, 2014 was related to the accrual of a $150.0 million contingent payment (exclusive of an $18.1 million tax benefit) to be made to former shareholders of Fumapharm AG and holders of their rights. During the quarter ended June 30, 2014, we reached the $2.0 billion cumulative sales level related to the Fumapharm Products. For additional information related to future contingent payments, please read Note 19, Commitments and Contingencies to these condensed consolidated financial statements.
	As of June 30, 2014, we had no accumulated impairment losses related to goodwill.

Fair_Value_Measurements

Fair Value Measurements	6 Months Ended
	Jun. 30, 2014
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
		June 30,			in Active			Observable Inputs			Unobservable
		2014			Markets			(Level 2)			Inputs
					(Level 1)						(Level 3)
	Assets:
	Cash equivalents	$	475.2		$	—		$	475.2		$	—
	Marketable debt securities:
	Corporate debt securities	617.2			—			617.2			—
	Government securities	948			—			948			—
	Mortgage and other asset backed securities	155.4			—			155.4			—
	Marketable equity securities	0.4			0.4			—			—
	Venture capital investments	21.1			—			—			21.1
	Derivative contracts	2.1			—			2.1			—
	Plan assets for deferred compensation	37			—			37			—
	Total	$	2,256.40		$	0.4		$	2,234.90		$	21.1
	Liabilities:
	Derivative contracts	$	9.1		$	—		$	9.1		$	—
	Contingent consideration obligations	279.1			—			—			279.1
	Total	$	288.2		$	—		$	9.1		$	279.1
	(In millions)	As of			Quoted Prices			Significant			Significant
		December 31,			in Active			Other			Unobservable
		2013			Markets			Observable			Inputs
					(Level 1)			Inputs			(Level 3)
								(Level 2)
	Assets:
	Cash equivalents	$	424.7		$	—		$	424.7		$	—
	Marketable debt securities:
	Corporate debt securities	439.8			—			439.8			—
	Government securities	674.7			—			674.7			—
	Mortgage and other asset backed securities	131.4			—			131.4			—
	Marketable equity securities	11.2			11.2			—			—
	Venture capital investments	21.9			—			—			21.9
	Derivative contracts	3.8			—			3.8			—
	Plan assets for deferred compensation	22.7			—			22.7			—
	Total	$	1,730.20		$	11.2		$	1,697.10		$	21.9
	Liabilities:
	Derivative contracts	$	23.5		$	—		$	23.5		$	—
	Contingent consideration obligations	280.9			—			—			280.9
	Total	$	304.4		$	—		$	23.5		$	280.9
	There have been no impairments of our assets measured and carried at fair value during the three and six months ended June 30, 2014. In addition, there were no changes in valuation techniques or inputs utilized or transfers between fair value measurement levels during the three and six months ended June 30, 2014. The fair value of Level 2 instruments classified as cash equivalents and marketable debt securities were determined through third party pricing services. For a description of our validation procedures related to prices provided by third party pricing services, refer to Note 1, Summary of Significant Accounting Policies: Fair Value Measurements, to our consolidated financial statements included within our 2013 Form 10-K.
	Marketable Equity Securities and Venture Capital Investments
	Our marketable equity securities represent investments in publicly traded equity securities. Our venture capital investments, which are all Level 3 measurements, include investments in certain venture capital funds, accounted for at fair value, that primarily invest in small privately-owned, venture-backed biotechnology companies. These venture capital investments represented approximately 0.2% of total assets as of June 30, 2014 and December 31, 2013, respectively.
	The following table provides a roll forward of the fair value of our venture capital investments, which includes Level 3 measurements:
		For the Three Months						For the Six Months
		Ended June 30,						Ended June 30,
	(In millions)	2014			2013			2014			2013
	Fair value, beginning of period	$	23.6		$	18		$	21.9		$	20.3
	Unrealized gains included in earnings	2.1			6.1			5			6.7
	Unrealized losses included in earnings	(0.1		)	(0.6		)	(1.3		)	(2.0		)
	Purchases	—			—			—			—
	Settlements	(4.5		)	—			(4.5		)	(1.5		)
	Fair value, end of period	$	21.1		$	23.5		$	21.1		$	23.5
	Debt Instruments
	The fair and carrying values of our debt instruments, which are Level 2 liabilities, are summarized as follows:
		As of June 30, 2014						As of December 31, 2013
	(In millions)	Fair			Carrying			Fair			Carrying
		Value			Value			Value			Value
	Notes payable to Fumedica	$	13.8		$	12.6		$	17.5		$	15.8
	6.875% Senior Notes due March 1, 2018	645.9			576.9			647.9			580.1
	Total	$	659.7		$	589.5		$	665.4		$	595.9
	The fair value of our notes payable to Fumedica was estimated using market observable inputs, including current interest and foreign currency exchange rates. The fair value of our 6.875% Senior Notes was determined through market, observable, and corroborated sources. For additional information related to our debt instruments, please read Note 12, Indebtedness to our consolidated financial statements included within our 2013 Form 10-K.
Contingent Consideration Obligations
The following table provides a roll forward of the fair values of our contingent consideration obligations which includes Level 3 measurements:
	For the Three Months						For the Six Months
	Ended June 30,						Ended June 30,
(In millions)	2014			2013			2014			2013
Fair value, beginning of period	$	275.1		$	293.7		$	280.9		$	293.9
Additions	—			—			—			—
Changes in fair value	4			(5.2		)	3.2			(2.9		)
Payments	—			(7.5		)	(5.0		)	(10.0		)
Fair value, end of period	$	279.1		$	281		$	279.1		$	281
As of June 30, 2014 and December 31, 2013, approximately $243.4 million and $251.9 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	6 Months Ended
	Jun. 30, 2014
Investments, All Other Investments [Abstract]	'
Financial Instruments	'
	Financial Instruments
	Marketable Securities
	The following tables summarize our marketable debt and equity securities:
	As of June 30, 2014 (In millions)	Fair			Gross		Gross			Amortized
		Value			Unrealized		Unrealized			Cost
					Gains		Losses
	Available-for-sale:
	Corporate debt securities
	Current	$	125		$	0.1	$	—		$	124.9
	Non-current	492.2			0.6		(0.2		)	491.8
	Government securities
	Current	584.9			0.1		(0.1		)	584.9
	Non-current	363.1			0.2		—			362.9
	Mortgage and other asset backed securities
	Current	—			—		—			—
	Non-current	155.4			0.1		(0.1		)	155.4
	Total marketable debt securities	$	1,720.60		$	1.1	$	(0.4	)	$	1,719.90
	Marketable equity securities, non-current	$	0.4		$	—	$	(0.1	)	$	0.5
	As of December 31, 2013 (In millions)	Fair			Gross		Gross			Amortized
		Value			Unrealized		Unrealized			Cost
					Gains		Losses
	Available-for-sale:
	Corporate debt securities
	Current	$	100.7		$	—	$	—		$	100.7
	Non-current	339.1			0.4		(0.1		)	338.8
	Government securities
	Current	519.5			—		—			519.5
	Non-current	155.2			—		(0.1		)	155.3
	Mortgage and other asset backed securities
	Current	—			—		—			—
	Non-current	131.4			—		(0.1		)	131.5
	Total marketable debt securities	$	1,245.90		$	0.4	$	(0.3	)	$	1,245.80
	Marketable equity securities, non-current	$	11.2		$	8.7	$	—		$	2.5
	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
		June 30,			December 31,
		2014			2013
	Commercial paper	$	5.9		$	1.2
	Overnight reverse repurchase agreements	294.8			22.4
	Short-term debt securities	174.5			401.1
	Total	$	475.2		$	424.7
	The carrying values of our commercial paper, including accrued interest, overnight reverse repurchase agreements, and our short-term debt securities approximate fair value due to their short term maturities.
	Summary of Contractual Maturities: Available-for-Sale Securities
	The estimated fair value and amortized cost of our marketable debt securities available-for-sale by contractual maturity are summarized as follows:
		As of June 30, 2014					As of December 31, 2013
	(In millions)	Estimated			Amortized		Estimated			Amortized
		Fair Value			Cost		Fair Value			Cost
	Due in one year or less	$	709.8		$	709.8	$	620.2		$	620.2
	Due after one year through five years	931			930.3		573.1			572.9
	Due after five years	79.8			79.8		52.6			52.7
	Total available-for-sale securities	$	1,720.60		$	1,719.90	$	1,245.90		$	1,245.80
	The average maturity of our marketable debt securities available-for-sale as of June 30, 2014 and December 31, 2013 was 13 months, respectively.
	Proceeds from Marketable Debt Securities
	The proceeds from maturities and sales of marketable debt securities and resulting realized gains and losses are summarized as follows:
		For the Three Months					For the Six Months
		Ended June 30,					Ended June 30,
	(In millions)	2014			2013		2014			2013
	Proceeds from maturities and sales	$	560.1		$	78.3	$	1,317.50		$	4,404.70
	Realized gains	$	0.2		$	—	$	0.4		$	6.3
	Realized losses	$	(0.1	)	$	—	$	(0.2	)	$	(2.0	)
	Strategic Investments
	As of June 30, 2014 and December 31, 2013, our strategic investment portfolio was comprised of investments totaling $51.1 million and $56.9 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 $21.5 million and $33.1 million as of June 30, 2014 and December 31, 2013, 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 $29.6 million and $23.8 million as of June 30, 2014 and December 31, 2013, respectively.
Changes in Fair Value
During the three and six months ended June 30, 2014 and 2013, we realized changes in fair value recorded through income of $2.0 million and $4.9 million, respectively, on our strategic investment portfolio as compared to $4.4 million and $4.1 million, respectively, in the prior year comparative periods.
Impairments
For the three and six months ended June 30, 2014 and 2013, 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 were insignificant.

Derivative_Instruments

Derivative Instruments	6 Months Ended
	Jun. 30, 2014
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 June 30, 2014 and December 31, 2013 had durations of 1 to 18 months, respectively. These contracts have been designated as cash flow hedges and accordingly, to the extent effective, any unrealized gains or losses on these foreign currency forward contracts are reported in accumulated other comprehensive income (loss) (referred to as AOCI in the tables below). Realized gains and losses for the effective portion of such contracts are recognized in revenue when the sale of product in the currency being hedged is recognized. 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
		June 30,				December 31,
		2014				2013
	Euro	$	703.4			$	636.3
	Canadian dollar	19.5				34
	British pound sterling	39.1				72.3
	Total foreign currency forward contracts	$	762			$	742.6
	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 $7.0 million and $23.6 million as of June 30, 2014 and December 31, 2013, respectively. We expect all contracts to be settled over the next 18 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 June 30, 2014 and December 31, 2013, respectively, credit risk did not change the fair value of our foreign currency forward contracts.
	The following table summarizes the effect of derivatives designated as hedging instruments on our condensed consolidated statements of income:
	For the Three Months Ended June 30,
	Net Gains/(Losses)								Net Gains/(Losses)
	Reclassified from AOCI into Net Income								Recognized into Net Income
	(Effective Portion)								(Ineffective Portion)
	Location		2014				2013		Location	2014			2013
	Revenue		$	(5.2	)		$	0.1	Other income (expense)	$	(1.0	)	$	0.1
	For the Six Months Ended June 30,
	Net Gains/(Losses)								Net Gains/(Losses)
	Reclassified from AOCI into Net Income								Recognized into Net Income
	(Effective Portion)								(Ineffective Portion)
	Location		2014				2013		Location	2014			2013
	Revenue		$	(10.0	)		$	1.2	Other income (expense)	$	(1.2	)	$	0.3
	Foreign Currency Forward Contracts - Other Derivatives
	We also enter into other foreign currency forward contracts, usually with one month durations, to mitigate the foreign currency risk related to certain balance sheet positions. We have not elected hedge accounting for these transactions.
	The aggregate notional amount of these outstanding foreign currency contracts was $440.3 million and $273.3 million as of June 30, 2014 and December 31, 2013, respectively. Net losses of $2.4 million and $3.8 million related to these contracts were recognized as a component of other income (expense), net, for three and six months ended June 30, 2014, respectively, as compared to net gains of $0.6 million and $1.5 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 June 30, 2014
	Hedging Instruments:
	Asset derivatives	Other current assets	$	0.4
		Investments and other assets	$	0.6
	Liability derivatives	Accrued expenses and other	$	7.9
		Other long-term liabilities	$	0.5
	Other Derivatives:
	Asset derivatives	Other current assets	$	1.1
	Liability derivatives	Accrued expenses and other	$	0.7
	(In millions)	Balance Sheet Location	Fair Value As of December 31, 2013
	Hedging Instruments:
	Asset derivatives	Other current assets	$	0.6
	Liability derivatives	Accrued expenses and other	$	23.4
	Other Derivatives:
	Asset derivatives	Other current assets	$	3.2
	Liability derivatives	Accrued expenses and other	$	0.1

Indebtedness

Indebtedness	6 Months Ended
	Jun. 30, 2014
Debt Disclosure [Abstract]	'
Indebtedness	'
	Indebtedness
	Credit Facility
	In March 2014, our $750.0 million senior unsecured revolving credit facility expired and was not renewed.

Equity

Equity	6 Months Ended
	Jun. 30, 2014
Equity [Abstract]	'
Equity	'
	Equity
	Total equity as of June 30, 2014 increased $944.4 million compared to December 31, 2013. This increase was primarily driven by net income attributable to Biogen Idec Inc. of $1,194.5 million and an increase in additional paid in capital resulting from our share-based compensation arrangements totaling $82.4 million, partially offset by repurchases of our common stock totaling $336.9 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 six months ended June 30, 2014, we repurchased approximately 1.2 million shares of common stock at a cost of $336.9 million for the purpose of share stabilization. During the six months ended June 30, 2013, we repurchased approximately 0.3 million shares of common stock at a cost of $41.0 million.
	Approximately 3.0 million shares of our common stock remain available for repurchase under the 2011 authorization.
	Noncontrolling Interests
	The following table reconciles equity attributable to noncontrolling interests (NCI):
		For the Three Months					For the Six Months
		Ended June 30,					Ended June 30,
	(In millions)	2014			2013		2014			2013
	Noncontrolling interests, beginning of period	$	4.4		$	0.6	$	0.6		$	2.3
	Net income (loss) attributable to noncontrolling interests, net of tax	8.6			—		8.4			—
	Fair value of net assets and liabilities acquired and assigned to NCI	—			—		4			—
	Distribution to noncontrolling interest	(9.1		)	—		(9.1		)	—
	Deconsolidation of noncontrolling interest	—			—		—			(1.7		)
	Noncontrolling interests, end of period	$	3.9		$	0.6	$	3.9		$	0.6

Accumulated_Other_Comprehensiv

Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss)	6 Months Ended
	Jun. 30, 2014
Equity [Abstract]	'
Accumulated Other Comprehensive Income (Loss)	'
	Accumulated Other Comprehensive Income (Loss)
	The following table summarizes the changes in accumulated other comprehensive income (loss), net of tax by component:
	(In millions)	Unrealized Gains (Losses) on Securities Available for Sale				Unrealized Gains (Losses) on Foreign Currency Forward Contracts				Unfunded Status of Postretirement Benefit Plans				Translation Adjustments				Total
	Balance, as of December 31, 2013	$	5.6			$	(23.7	)		$	(19.6	)		$	10			$	(27.7	)
	Other comprehensive income (loss) before reclassifications	1.4				6.5				0.6				(11.0		)		(2.5		)
	Amounts reclassified from accumulated other comprehensive income (loss)	(6.6		)		10.1				—				—				3.5
	Net current period other comprehensive income (loss)	(5.2		)		16.6				0.6				(11.0		)		1.1
	Balance, as of June 30, 2014	$	0.4			$	(7.1	)		$	(19.0	)		$	(1.0	)		$	(26.7	)
	(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	7.8				10.2				2.3				(16.4		)		3.9
	Amounts reclassified from accumulated other comprehensive income (loss)	(2.8		)		(0.9		)		—				—				(3.7		)
	Net current period other comprehensive income (loss)	5				9.3				2.3				(16.4		)		0.2
	Balance, as of June 30, 2013	$	9.2			$	(1.4	)		$	(19.4	)		$	(43.5	)		$	(55.1	)
	The following table summarizes the amounts reclassified from accumulated other comprehensive income:
	(In millions)	Income Statement Location	Amounts Reclassified from Accumulated Other Comprehensive Income
			For the Three Months								For the Six Months
			Ended June 30,								Ended June 30,
			2014				2013				2014				2013
	Gains (losses) on securities available for sale	Other income (expense)	$	10.1			$	—			$	10.2			$	4.3
		Income tax benefit (expense)	(3.6		)		—				(3.6		)		(1.5		)
	Gains (losses) on foreign currency forward contracts	Revenues	(5.3		)		0.1				(10.0		)		1.2
		Income tax benefit (expense)	(0.5		)		(0.2		)		(0.1		)		(0.3		)
	Total reclassifications, net of tax		$	0.7			$	(0.1	)		$	(3.5	)		$	3.7

Earnings_per_Share

Earnings per Share	6 Months Ended
	Jun. 30, 2014
Earnings Per Share [Abstract]	'
Earnings per Share	'
	Earnings per Share
	Basic and diluted earnings per share are calculated as follows:
		For the Three Months				For the Six Months
		Ended June 30,				Ended June 30,
	(In millions)	2014		2013		2014		2013
	Numerator:
	Net income attributable to Biogen Idec Inc.	$	714.5	$	490.7	$	1,194.50	$	917.4
	Denominator:
	Weighted average number of common shares outstanding	236.7		237.5		236.7		237.2
	Effect of dilutive securities:
	Stock options and employee stock purchase plan	0.1		0.3		0.1		0.4
	Time-vested restricted stock units	0.4		0.6		0.5		0.7
	Market stock units	0.2		0.3		0.3		0.3
	Dilutive potential common shares	0.7		1.2		0.9		1.4
	Shares used in calculating diluted earnings per share	237.4		238.7		237.6		238.5
	Amounts excluded from the calculation of net income per diluted share because their effects were anti-dilutive were insignificant.

Sharebased_Payments

Share-based Payments	6 Months Ended
	Jun. 30, 2014
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 Six Months
		Ended June 30,							Ended June 30,
	(In millions)	2014				2013			2014			2013
	Research and development	$	24.5			$	23.7		$	53.9		$	48.9
	Selling, general and administrative	33.6				45.4			80.8			79.7
	Subtotal	58.1				69.1			134.7			128.6
	Capitalized share-based compensation costs	(2.6		)		(2.7		)	(5.2		)	(5.0		)
	Share-based compensation expense included in total cost and expenses	55.5				66.4			129.5			123.6
	Income tax effect	(16.5		)		(20.3		)	(38.8		)	(36.9		)
	Share-based compensation expense included in net income attributable to Biogen Idec Inc.	$	39			$	46.1		$	90.7		$	86.7
	The following table summarizes share-based compensation expense associated with each of our share-based compensation programs:
		For the Three Months							For the Six Months
		Ended June 30,							Ended June 30,
	(In millions)	2014				2013			2014			2013
	Stock options	$	—			$	0.1		$	—		$	0.4
	Market stock units	8.2				6.6			23.1			15.1
	Time-vested restricted stock units	29.2				24.8			58.4			51.9
	Cash settled performance units	14				35.8			35.4			56.2
	Performance units	4				—			10.3			—
	Employee stock purchase plan	2.7				1.8			7.5			5
	Subtotal	58.1				69.1			134.7			128.6
	Capitalized share-based compensation costs	(2.6		)		(2.7		)	(5.2		)	(5.0		)
	Share-based compensation expense included in total cost and expenses	$	55.5			$	66.4		$	129.5		$	123.6
	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 Six Months
		Ended June 30,
		2014			2013
	Market stock units	222,000			257,000
	Cash settled performance shares	180,000			271,000
	Performance units	50,000			—
	Time-vested restricted stock units	418,000			680,000
	The market stock units (MSUs) granted during the six months ended June 30, 2014 primarily vest in three equal annual increments beginning on the anniversary of the grant date. For these grants, the performance multiplier is derived based on the stock price growth rate between the 30 calendar day average closing stock price on the grant date and the 30 calendar day average closing stock price leading up to and including each of the three vesting dates. These awards may ultimately earn between 0% and 200% of the target number of units granted based on actual stock performance. Any performance multiplier less than 50% results in no shares being earned for that respective tranche.
	During the first quarter of 2014, we began granting awards for performance-vested restricted stock units (PUs), which can be settled in cash or shares of our common stock at the sole discretion of the Compensation and Management Development Committee of the Board of Directors. PUs awarded to employees vest in three equal annual increments beginning on the anniversary of the grant date. The number of PUs granted represents the target number of units that are eligible to be earned based on the attainment of certain performance measures established at the beginning of the performance period, which ends on December 31st of each year. Participants may ultimately earn between 0% and 200% of the target number of units granted based on the degree of actual performance metric achievement, with no units being earned if the performance multiplier is below 50%. Accordingly, additional PUs may be issued or currently outstanding PUs may be cancelled upon final determination of the number of units earned. Settlement of PUs is based on the 30 calendar day average closing stock price through each vesting date once the actual vested and earned number of units is known.
	In addition, for the six months ended June 30, 2014, approximately 112,000 shares were issued under our employee stock purchase plan (ESPP) compared to approximately 160,000 shares issued in the prior year comparative period.

Income_Taxes

Income Taxes	6 Months Ended
	Jun. 30, 2014
Income Tax Disclosure [Abstract]	'
Income Taxes	'
	Income Taxes
	For the three and six months ended June 30, 2014, our effective tax rate was 27.0% and 26.9% respectively, as compared to 24.4% and 19.6%, 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 Six Months
		Ended June 30,				Ended June 30,
		2014		2013		2014		2013
	Statutory rate	35	%	35	%	35	%	35	%
	State taxes	1.1		1.9		1.1		4.4
	Taxes on foreign earnings	(8.7	)	(7.2	)	(8.5	)	(8.4	)
	Credits and net operating loss utilization	(0.7	)	(3.5	)	(0.9	)	(3.4	)
	Purchased intangible assets	1.2		1.8		1.3		1.5
	Manufacturing deduction	(1.7	)	(2.4	)	(1.8	)	(11.1	)
	Other permanent items	0.4		(1.0	)	0.4		1.8
	Other	0.4		(0.2	)	0.3		(0.2	)
	Effective tax rate	27	%	24.4	%	26.9	%	19.6	%
	For the three and six months ended June 30, 2014, compared to the same periods in 2013, the increase in our income tax rate was primarily the result of a 2013 change in our uncertain tax position related to our U.S. federal manufacturing deduction and our unconsolidated joint business described below, the reinstatement for 2013 of the federal research and development tax credit which is now expired and lower current year expenses eligible for the orphan drug credit, partially offset by a higher percentage of our 2014 income being earned outside the U.S.
	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 and state uncertain tax positions in 2013, as discussed below under "Accounting for Uncertainty in Income Taxes".
	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 2013, we received updated technical guidance from the IRS concerning the calculation of our U.S. federal manufacturing deduction and overall tax classification of our unconsolidated joint business for the current and prior year filings. Based on this guidance we reevaluated the level of our unrecognized benefits related to uncertain tax positions, and recorded a $49.8 million income tax benefit. This benefit was for a previously unrecognized position and related to years 2005 through 2012. We recorded an offsetting expense of $10.3 million for non-income based state taxes, which was recorded in other income (expense) within our condensed consolidated statements of income.
	In October 2011, in conjunction with our examination, the IRS proposed a disallowance of approximately $130.0 million in deductions for tax years 2007, 2008 and 2009 related to payments for services provided by our wholly owned Danish subsidiary located in Hillerød, Denmark. We believe that these items represent valid deductible business expenses and are vigorously defending our position. We have initiated a mutual agreement procedure between the IRS and SKAT (the Danish tax authorities) for the years 2001 through 2009, in an attempt to reach agreement on the issue. In addition, we have applied for a bilateral advanced pricing agreement for the years 2010 through 2014 to resolve similar issues for the subsequent years.
	It is reasonably possible that we will adjust the value of our uncertain tax positions related to our unconsolidated joint business and certain transfer pricing issues as we receive additional information from various taxing authorities, including reaching settlements with the authorities. In addition, the IRS and other national tax authorities routinely examine our intercompany transfer pricing with respect to intellectual property related transactions and it is possible that they may disagree with one or more positions we have taken with respect to such valuations.

Other_Consolidated_Financial_S

Other Consolidated Financial Statement Detail	6 Months Ended
	Jun. 30, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]	'
Other Consolidated Financial Statement Detail	'
	Other Consolidated Financial Statement Detail
	Other Income (Expense), Net
	Components of other income (expense), net, are summarized as follows:
		For the Three Months						For the Six Months
		Ended June 30,						Ended June 30,
	(In millions)	2014			2013			2014			2013
	Interest income	$	2.4		$	0.9		$	5.1		$	5.2
	Interest expense	(7.1		)	(7.3		)	(14.7		)	(18.9		)
	Impairments of investments	—			(1.4		)	—			(1.7		)
	Gain (loss) on investments, net	13.6			5.7			16.6			10.1
	Foreign exchange gains (losses), net	(1.4		)	(5.3		)	(4.7		)	(7.9		)
	Other, net	(2.6		)	(3.0		)	(3.0		)	(11.7		)
	Total other income (expense), net	$	4.9		$	(10.4	)	$	(0.7	)	$	(24.9	)
	Accrued Expenses and Other
	Accrued expenses and other consists of the following:
	(In millions)	As of			As of
		June 30,			December 31,
		2014			2013
	Revenue-related rebates	$	321.1		$	264.9
	Employee compensation and benefits	274.1			343.4
	Royalties and licensing fees	190			160.7
	Current portion of contingent consideration obligations	185.7			29
	Deferred revenue	145			172.7
	Clinical development expenses	62.5			55.2
	Collaboration expenses	37.7			18.7
	Construction in progress accrual	15.7			25
	Other	263.7			285.6
	Total accrued expenses and other	$	1,495.50		$	1,355.20
	Other Long-Term Liabilities
	Other long-term liabilities includes long-term taxes payable totaling approximately $198.5 million.

Investments_in_Variable_Intere

Investments in Variable Interest Entities	6 Months Ended
	Jun. 30, 2014
Investments in Variable Interest Entities [Abstract]	'
Investments in Variable Interest Entities	'
	Investments in Variable Interest Entities
	Consolidated Variable Interest Entities
	Our condensed consolidated financial statements include the financial results of variable interest entities in which we are the primary beneficiary.
	Neurimmune SubOne AG
	In 2007, we entered into a collaboration agreement with Neurimmune SubOne AG (Neurimmune), a subsidiary of Neurimmune AG, for the development and commercialization of antibodies for the treatment of Alzheimer’s disease. Neurimmune conducts research to identify potential therapeutic antibodies and we are responsible for the development, manufacturing and commercialization of all products. Our anti-amyloid beta antibody, BIIB037 program, for Alzheimer’s disease resulted from this collaboration. Based upon our current development plans, we may pay Neurimmune up to $335.0 million in remaining milestone payments, as well as royalties on sales of any resulting commercial products.
	Amounts that are incurred by Neurimmune for research and development expenses in support of the collaboration that we reimburse are reflected in research and development expense in our condensed consolidated statements of income. Future milestone payments will be reflected within our condensed consolidated statements of income as a charge to noncontrolling interest, net of tax, when such milestones are achieved.
	For the three and six months ended June 30, 2014, the collaboration incurred development expenses totaling $10.6 million and $21.7 million, respectively, which is reflected as research and development expense within our condensed consolidated statements of income, as compared to $5.7 million and $10.7 million, respectively, in the prior year comparative periods. In addition, for the three months ended June 30, 2014, we recorded a $10.0 million milestone payment in connection with the achievement of certain clinical goals in the Phase 1 trial of our BIIB037 program for Alzheimer’s disease, which is reflected as a charge to noncontrolling interests, net of tax, within our condensed consolidated statements of income.
	The assets and liabilities of Neurimmune are not significant to our financial position or results of operations as it is a research and development organization. We have provided no financing to Neurimmune other than contractually required amounts.
	Ataxion, Inc.
	In February 2014, we paid $1.6 million for preferred stock of Ataxion, Inc. (Ataxion) and entered into an option agreement which gives us the right to purchase all outstanding shares of Ataxion at any time until 30 days after delivery of a Phase 1 clinical trial study report. Ataxion is a discovery-stage biopharmaceutical company developing product candidates focused on a group of orphan genetic disorders referred to as hereditary ataxis. We committed to make additional investments in Ataxion's preferred shares of up to $6.2 million if certain development milestones are achieved. If we exercise our option to purchase the outstanding shares of Ataxion, we could pay additional amounts upon achievement of clinical and commercial milestones.
	In the Ataxion relationship, through our fixed price option to purchase the company, purchases of equity and presence on the program advisory committee, we are deemed to be the primary beneficiary of Ataxion, a variable interest entity. Therefore, we consolidate the results of Ataxion. As part of the initial consolidation of Ataxion, we recorded an in-process research and development intangible asset of $3.5 million and assigned that amount to minority interest within our shareholders' equity.
	The assets and liabilities of Ataxion are not significant to our financial position or results of operations as it is a research and development organization. We have provided no financing to Ataxion other than contractually required amounts.
	Unconsolidated Variable Interest Entities
	We have relationships with other variable interest entities that we do not consolidate as we lack the power to direct the activities that significantly impact the economic success of these entities. These relationships include investments in certain biotechnology companies and research collaboration agreements. For additional information related to our significant collaboration arrangements with unconsolidated variable interest entities, please read Note 19, Investments in Variable Interest Entities to our consolidated financial statements included within our 2013 Form 10-K.
	As of June 30, 2014 and December 31, 2013, 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 $11.4 million and $5.5 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.

Collaborative_and_Other_Relati

Collaborative and Other Relationships	6 Months Ended
	Jun. 30, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]	'
Collaborative and Other Relationships	'
	Collaborative and Other Relationships
	Eisai Co., Ltd.
	On March 4, 2014 we entered into a collaboration with Eisai Co., Ltd. (Eisai) to jointly develop and commercialize two Eisai product candidates for the treatment of Alzheimer’s disease. The agreement also provides Eisai with an option to jointly develop and commercialize two of our candidates for Alzheimer’s disease, the anti-amyloid beta antibody BIIB037 and an anti-tau monoclonal antibody. For additional information related to our candidates for Alzheimer’s disease, please read about our relationship with Neurimmune in Note 16, Investments in Variable Interest Entities to these condensed consolidated financial statements.
	The collaboration initially will be centered on the co-development and co-commercialization of Eisai’s two clinical candidates: E2609, a BACE inhibitor, and BAN2401, an anti-amyloid beta antibody. Eisai will serve as the operational and regulatory lead in the co-development of E2609 and BAN2401 and will pursue marketing authorizations for both compounds worldwide. In major markets, such as the U.S. and the E.U., we and Eisai will co-promote the products following marketing approval. Both companies will share overall cost, including research and development expenses and profits will be split between the companies. The agreement excluded commercialization of these candidates in Japan, but included an option for Eisai to receive an additional one-time payment from us in exchange for expanding joint development and commercialization activities to include Japan.
	We paid $100.0 million upon closing and recorded approximately $17.7 million reflecting the fair value of the options granted under the agreement, both of which were classified as research and development expense within our condensed consolidated statements of income. During the three months ended June 30, 2014, Eisai exercised its option to expand the joint development and commercialization activities to include Japan and we paid Eisai an additional $35.0 million. We recorded $21.6 million in the second quarter of 2014 as research and development expense within our condensed consolidated statements of income, which represents the difference between the payment made upon exercise of the option and the fair value of that option recorded as research and development expense upon closing of the agreement in the first quarter of 2014. We could pay up to an additional $1.0 billion based on the future achievement of certain development, regulatory and commercial milestones.
	Sangamo BioSciences, Inc.
	On February 22, 2014, we completed an exclusive worldwide research, development and commercialization collaboration and license agreement with Sangamo BioSciences, Inc. (Sangamo) under which both companies will develop and commercialize product candidates for the treatment of two inherited blood disorders, sickle cell disease and beta-thalassemia. The collaboration is currently in the research stage of development.
	Under the terms of the agreement, we paid Sangamo an upfront payment of $20.0 million in cash, with additional payments of up to $300.0 million based on the achievement of certain development, regulatory and commercial milestones, plus royalties based on sales. We recorded the $20.0 million upfront payment as research and development expenses. Under this arrangement, Sangamo will be responsible for identifying a product candidate for the treatment of beta-thalassemia and advancing that candidate through a completed Phase 1 human clinical trial, at which point we will assume responsibility for development. We will jointly develop a sickle cell disease candidate through the potential filing of an investigative new drug application, after which we will assume clinical responsibilities. We will lead the global development and commercialization efforts and Sangamo will have the option to assume co-promotion responsibilities in the U.S.
	Isis Pharmaceuticals, Inc.
	In January 2012, we entered into an exclusive, worldwide option and collaboration agreement with Isis Pharmaceuticals, Inc. (Isis) under which both companies will develop and commercialize Isis’ product candidate for the treatment of spinal muscular atrophy (SMA).
	In January 2014, we amended the agreement and agreed to pay the clinical trial costs up to approximately $45.0 million related to the development of ISIS-SMNRx through studies which Isis will be responsible for performing. We will recognize the $45.0 million as research and development expenses as the trial costs are incurred. We are providing input on the clinical trial design and regulatory strategy for the development of ISIS-SMNRx. We have an option to license ISIS-SMNRx following completion and data review of the first successful Phase 2/3 trial or completion of both Phase 2/3 trials.
	Samsung Bioepis
	In February 2012, we entered into a joint venture agreement with Samsung BioLogics Co. Ltd. (Samsung Biologics), establishing an entity, Samsung Bioepis, to develop, manufacture and market biosimilar pharmaceuticals. Under the terms of the joint venture agreement, Samsung Biologics agreed to contribute 280.5 billion South Korean won (approximately $250.0 million) for an 85 percent stake in Samsung Bioepis and we agreed to contribute approximately 49.5 billion South Korean won (approximately $45.0 million) for a 15 percent ownership interest. Our investment is limited to this contribution as we have no obligation to provide any additional funding. As of June 30, 2014, our ownership interest decreased to approximately 12% as Samsung Bioepis secured additional equity financing from Samsung Biologics and we did not participate in such financing. 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 and is based on paying for 49.9 percent of the total investment made to Samsung Bioepis in excess of what we have already contributed, plus interest.
	On December 17, 2013, pursuant to our rights under the joint venture agreement with Samsung Biologics, we entered into an agreement with Samsung Bioepis to commercialize anti-TNF biosimilar product candidates in Europe. Under the terms of this agreement, we paid $36.0 million, which was recorded as a research and development expense within our condensed consolidated statements of income as the programs they relate to had not achieved regulatory approval. Samsung Bioepis is eligible to receive an additional $85.0 million in additional milestones related to clinical development and regulatory approval of the product candidates. Upon commercialization, there will be a 50 percent profit share with Samsung Bioepis.
	As of June 30, 2014 and December 31, 2013, the carrying value of our investment in Samsung Bioepis totaled 15.6 billion and 25.2 billion South Korean won (approximately $14.6 million and $23.9 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 six months ended June 30, 2014, we recognized a loss on our investment of $1.9 million and $9.5 million, respectively, as compared to $2.3 million and $6.1 million, respectively, in the prior year comparative periods.
	Simultaneous with the formation of Samsung Bioepis, we also entered into a license agreement, a technical development services agreement and a manufacturing agreement with Samsung Bioepis. For the three and six months ended June 30, 2014, we recognized $12.3 million and $36.5 million, respectively, in other revenues in relation to these services, as compared to $7.4 million and $14.2 million, respectively, in the prior year comparative periods, which is reflected as a component of other revenues within our condensed consolidated statement of income.
	For additional information related to our other significant collaboration arrangements, please read Note 20, Collaborative and Other Relationships to our consolidated financial statements included within our 2013 Form 10-K.

Litigation

Litigation	6 Months Ended
	Jun. 30, 2014
Loss Contingency, Information about Litigation Matters [Abstract]	'
Litigation	'
	Litigation
	’755 Patent Litigation
	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 that they do not infringe our U.S. Patent No. 7,588,755 ('755 Patent), which claims the use of interferon beta for immunomodulation or treating a viral condition, viral disease, cancers or tumors, and that the patent is invalid and seeking monetary relief in the form of attorneys' fees, costs and expenses. On May 28, 2010, Biogen Idec MA Inc. (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.
	Italian National Medicines Agency
	In the fourth quarter of 2011, Biogen Idec Italia SRL received notice from the Italian National Medicines Agency (Agenzia Italiana del Farmaco or AIFA) that sales of TYSABRI after mid-February 2009 exceeded a reimbursement limit established pursuant to a Price Determination Resolution (Price Resolution) granted by AIFA in December 2006. 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 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 the reimbursement limit in the Price Resolution should apply as written to only "the first 24 months" of TYSABRI sales, which ended in February 2009. The final determination of the appeal is still pending and AIFA and Biogen Idec Italia SRL are in discussions about a resolution of the period from mid-February 2009 through January 2013. On November 21, 2012, the tribunal ruled that the reimbursement limit would not automatically renew.
	On June 10, 2014, AIFA approved a resolution, effective for a 24-month term beginning on June 19, 2014, setting the price for TYSABRI in Italy for this 24-month term. The resolution also eliminates the reimbursement limit from February 2013 going forward.
	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. The court has dismissed certain claims and a trial has been set for May 2015 on the remaining claims. 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 Matters
	We have learned that state and federal governmental authorities are investigating our sales and promotional practices and have received related subpoenas. We have also received a subpoena from the federal government for documents relating to our relationship with certain pharmacy benefit managers. We are cooperating with the government in these matters.
	Qui Tam Litigation
	In August, 2012, we learned that a relator, on behalf of the United States and certain states, filed a suit under seal on February 17, 2011 against us, Elan Corporation, plc, and Elan Pharmaceuticals, Inc. in the United States District Court for the Western District of Virginia. We have neither seen nor been served with the complaint, but understand that it was filed under the Federal False Claims Act.
	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	6 Months Ended
	Jun. 30, 2014
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 are required to make additional contingent payments to former shareholders of Fumapharm AG based on the attainment of certain cumulative sales levels of Fumapharm Products, with the amount of each payment based on 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	$1.0B		$2.0B		$3.0B		Each additional $1.0B up to $20.0B
		Payment Amounts (In Millions)
	< $500 million	$	—	$	—	$	—	$	—
	$500 million - $1.0 billion	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. During the six months ended June 30, 2014, we paid a $25.0 million contingent payment as we reached the $1.0 billion cumulative sales level related to the Fumapharm Products in 2013 and accrued a $150.0 million contingent payment as we reached the $2.0 billion cumulative sales level related to Fumapharm Products in the second quarter of 2014. For additional information, please read the subsection entitled “Contractual Obligations and Off-Balance Sheet Arrangements – Contingent Consideration related to Business Combinations” of the “Management’s Discussion and Analysis of Financial Condition and Results of Operations” included within this report.

Segment_Information

Segment Information	6 Months Ended
	Jun. 30, 2014
Segment Reporting [Abstract]	'
Segment Information	'
	Segment Information
	We operate as one operating segment, which is the business of discovering, developing, manufacturing and marketing therapies for the treatment of neurodegenerative diseases, hematologic conditions and autoimmune disorders and, therefore, our chief operating decision-maker manages the operations of our company as a single operating segment.

Summary_of_Significant_Account1

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

Accounts_Receivable_Tables

Accounts Receivable (Tables)	6 Months Ended
	Jun. 30, 2014
Receivables [Abstract]	'
Net accounts receivable balances from product sales in selected European countries	'
	Our net accounts receivable balances from product sales in selected European countries are summarized as follows:
		As of June 30, 2014
	(In millions)	Current		Non-Current		Total
		Balance Included		Balance Included
		within Accounts		within Investments
		Receivable, net		and Other Assets
	Spain	$	54.7	$	10	$	64.7
	Italy	$	79.3	$	1.3	$	80.6
	Portugal	$	10.7	$	12.7	$	23.4
		As of December 31, 2013
	(In millions)	Current		Non-Current		Total
		Balance Included		Balance Included
		within Accounts		within Investments
		Receivable, net		and Other Assets
	Spain	$	113.3	$	6.8	$	120.1
	Italy	$	76.1	$	2.4	$	78.5
	Portugal	$	10.4	$	8.2	$	18.6

Reserves_for_Discounts_and_All1

Reserves for Discounts and Allowances (Tables)	6 Months Ended
	Jun. 30, 2014
Reserves for Discounts and Allowances [Abstract]	'
Analysis of the changes in reserves	'
	An analysis of the change in reserves for discounts and allowances is summarized as follows:
	(In millions)	Discounts			Contractual			Returns			Total
					Adjustments
	Balance, as of December 31, 2013	$	47		$	335.6		$	33.7		$	416.3
	Current provisions relating to sales in current year	161.2			577.3			14.7			753.2
	Adjustments relating to prior years	(0.4		)	7.6			5.4			12.6
	Payments/returns relating to sales in current year	(111.5		)	(330.3		)	(0.1		)	(441.9		)
	Payments/returns relating to sales in prior years	(44.5		)	(200.8		)	(20.4		)	(265.7		)
	Balance, as of June 30, 2014	$	51.8		$	389.4		$	33.3		$	474.5
Total reserves included in condensed consolidated balance sheets	'
	The total reserves above, included in our condensed consolidated balance sheets, are summarized as follows:
	(In millions)	As of June 30,			As of December 31,
		2014			2013
	Reduction of accounts receivable	$	153.4		$	151.4
	Component of accrued expenses and other	321.1			264.9
	Total reserves	$	474.5		$	416.3

Inventory_Tables

Inventory (Tables)	6 Months Ended
	Jun. 30, 2014
Inventory Disclosure [Abstract]	'
Components of inventory	'
	The components of inventory are summarized as follows:
	(In millions)	As of		As of
		June 30,		December 31,
		2014		2013
	Raw materials	$	124.8	$	115
	Work in process	447.4		435.4
	Finished goods	143.7		108.6
	Total inventory	$	715.9	$	659

Intangible_Assets_and_Goodwill1

Intangible Assets and Goodwill (Tables)	6 Months Ended
	Jun. 30, 2014
Goodwill and Intangible Assets Disclosure [Abstract]	'
Intangible assets	'
	Intangible assets, net of accumulated amortization, impairment charges and adjustments, are summarized as follows:
				As of June 30, 2014									As of December 31, 2013
	(In millions)	Estimated		Cost				Accumulated			Net		Cost		Accumulated			Net
		Life						Amortization							Amortization
	Out-licensed patents	13-23 years		$	543.3			$	(465.9	)	$	77.4	$	578	$	(450.8	)	$	127.2
	Developed	15-23 years		3,005.30				(2,283.7		)	721.6		3,005.30		(2,165.4		)	839.9
	technology
	In-process research and development	Indefinite until commercialization		330.9				—			330.9		327.4		—			327.4
	Trademarks and	Indefinite		64				—			64		64		—			64
	tradenames
	Acquired and in-licensed rights	6-17 years		3,271.50				(216.0		)	3,055.50		3,240.00		(123.8		)	3,116.20
	and patents
	Total intangible assets			$	7,215.00			$	(2,965.6	)	$	4,249.40	$	7,214.70	$	(2,740.0	)	$	4,474.70
Estimated future amortization for acquired intangible assets	'
	Based upon this recent analysis, the estimated future amortization for acquired intangible assets for the balance of 2014 and the next five years is expected to be as follows:
	(In millions)	As of June 30, 2014
	2014 (remaining six months)	$	217.2
	2015	333.4
	2016	318.8
	2017	322
	2018	323.1
	2019	302.9
	Total	$	1,817.40
Summary of roll forward of the changes in goodwill	'
	The following table provides a roll forward of the changes in our goodwill balance:
	(In millions)	As of				As of
		June 30,				December 31,
		2014				2013
	Goodwill, beginning of period	$	1,232.90			$	1,201.30
	Increase to goodwill	131.9				35.7
	Other	—				(4.1		)
	Goodwill, end of period	$	1,364.80			$	1,232.90

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 2014
Fair Value Disclosures [Abstract]	'
Summary of assets and liabilities recorded at fair value	'
	The tables below present information about our assets and liabilities that are regularly measured and carried at fair value and indicate the level within the fair value hierarchy of the valuation techniques we utilized to determine such fair value:
	(In millions)	As of			Quoted Prices			Significant Other			Significant
		June 30,			in Active			Observable Inputs			Unobservable
		2014			Markets			(Level 2)			Inputs
					(Level 1)						(Level 3)
	Assets:
	Cash equivalents	$	475.2		$	—		$	475.2		$	—
	Marketable debt securities:
	Corporate debt securities	617.2			—			617.2			—
	Government securities	948			—			948			—
	Mortgage and other asset backed securities	155.4			—			155.4			—
	Marketable equity securities	0.4			0.4			—			—
	Venture capital investments	21.1			—			—			21.1
	Derivative contracts	2.1			—			2.1			—
	Plan assets for deferred compensation	37			—			37			—
	Total	$	2,256.40		$	0.4		$	2,234.90		$	21.1
	Liabilities:
	Derivative contracts	$	9.1		$	—		$	9.1		$	—
	Contingent consideration obligations	279.1			—			—			279.1
	Total	$	288.2		$	—		$	9.1		$	279.1
	(In millions)	As of			Quoted Prices			Significant			Significant
		December 31,			in Active			Other			Unobservable
		2013			Markets			Observable			Inputs
					(Level 1)			Inputs			(Level 3)
								(Level 2)
	Assets:
	Cash equivalents	$	424.7		$	—		$	424.7		$	—
	Marketable debt securities:
	Corporate debt securities	439.8			—			439.8			—
	Government securities	674.7			—			674.7			—
	Mortgage and other asset backed securities	131.4			—			131.4			—
	Marketable equity securities	11.2			11.2			—			—
	Venture capital investments	21.9			—			—			21.9
	Derivative contracts	3.8			—			3.8			—
	Plan assets for deferred compensation	22.7			—			22.7			—
	Total	$	1,730.20		$	11.2		$	1,697.10		$	21.9
	Liabilities:
	Derivative contracts	$	23.5		$	—		$	23.5		$	—
	Contingent consideration obligations	280.9			—			—			280.9
	Total	$	304.4		$	—		$	23.5		$	280.9
Fair value of venture capital investments	'
	The following table provides a roll forward of the fair value of our venture capital investments, which includes Level 3 measurements:
		For the Three Months						For the Six Months
		Ended June 30,						Ended June 30,
	(In millions)	2014			2013			2014			2013
	Fair value, beginning of period	$	23.6		$	18		$	21.9		$	20.3
	Unrealized gains included in earnings	2.1			6.1			5			6.7
	Unrealized losses included in earnings	(0.1		)	(0.6		)	(1.3		)	(2.0		)
	Purchases	—			—			—			—
	Settlements	(4.5		)	—			(4.5		)	(1.5		)
	Fair value, end of period	$	21.1		$	23.5		$	21.1		$	23.5
Summary of fair and carrying value of debt instruments	'
	The fair and carrying values of our debt instruments, which are Level 2 liabilities, are summarized as follows:
		As of June 30, 2014						As of December 31, 2013
	(In millions)	Fair			Carrying			Fair			Carrying
		Value			Value			Value			Value
	Notes payable to Fumedica	$	13.8		$	12.6		$	17.5		$	15.8
	6.875% Senior Notes due March 1, 2018	645.9			576.9			647.9			580.1
	Total	$	659.7		$	589.5		$	665.4		$	595.9
Fair value of contingent consideration obligations	'
	The following table provides a roll forward of the fair values of our contingent consideration obligations which includes Level 3 measurements:
		For the Three Months						For the Six Months
		Ended June 30,						Ended June 30,
	(In millions)	2014			2013			2014			2013
	Fair value, beginning of period	$	275.1		$	293.7		$	280.9		$	293.9
	Additions	—			—			—			—
	Changes in fair value	4			(5.2		)	3.2			(2.9		)
	Payments	—			(7.5		)	(5.0		)	(10.0		)
	Fair value, end of period	$	279.1		$	281		$	279.1		$	281

Financial_Instruments_Tables

Financial Instruments (Tables)	6 Months Ended
	Jun. 30, 2014
Investments, All Other Investments [Abstract]	'
Marketable debt and equity securities	'
	The following tables summarize our marketable debt and equity securities:
	As of June 30, 2014 (In millions)	Fair			Gross		Gross			Amortized
		Value			Unrealized		Unrealized			Cost
					Gains		Losses
	Available-for-sale:
	Corporate debt securities
	Current	$	125		$	0.1	$	—		$	124.9
	Non-current	492.2			0.6		(0.2		)	491.8
	Government securities
	Current	584.9			0.1		(0.1		)	584.9
	Non-current	363.1			0.2		—			362.9
	Mortgage and other asset backed securities
	Current	—			—		—			—
	Non-current	155.4			0.1		(0.1		)	155.4
	Total marketable debt securities	$	1,720.60		$	1.1	$	(0.4	)	$	1,719.90
	Marketable equity securities, non-current	$	0.4		$	—	$	(0.1	)	$	0.5
	As of December 31, 2013 (In millions)	Fair			Gross		Gross			Amortized
		Value			Unrealized		Unrealized			Cost
					Gains		Losses
	Available-for-sale:
	Corporate debt securities
	Current	$	100.7		$	—	$	—		$	100.7
	Non-current	339.1			0.4		(0.1		)	338.8
	Government securities
	Current	519.5			—		—			519.5
	Non-current	155.2			—		(0.1		)	155.3
	Mortgage and other asset backed securities
	Current	—			—		—			—
	Non-current	131.4			—		(0.1		)	131.5
	Total marketable debt securities	$	1,245.90		$	0.4	$	(0.3	)	$	1,245.80
	Marketable equity securities, non-current	$	11.2		$	8.7	$	—		$	2.5
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents	'
	The following table summarizes our financial assets with maturities of less than 90 days from the date of purchase included within cash and cash equivalents on the accompanying condensed consolidated balance sheet:
	(In millions)	As of			As of
		June 30,			December 31,
		2014			2013
	Commercial paper	$	5.9		$	1.2
	Overnight reverse repurchase agreements	294.8			22.4
	Short-term debt securities	174.5			401.1
	Total	$	475.2		$	424.7
Summary of contractual maturities: available-for-sale securities	'
	The estimated fair value and amortized cost of our marketable debt securities available-for-sale by contractual maturity are summarized as follows:
		As of June 30, 2014					As of December 31, 2013
	(In millions)	Estimated			Amortized		Estimated			Amortized
		Fair Value			Cost		Fair Value			Cost
	Due in one year or less	$	709.8		$	709.8	$	620.2		$	620.2
	Due after one year through five years	931			930.3		573.1			572.9
	Due after five years	79.8			79.8		52.6			52.7
	Total available-for-sale securities	$	1,720.60		$	1,719.90	$	1,245.90		$	1,245.80
Proceeds from marketable debt securities	'
	The proceeds from maturities and sales of marketable debt securities and resulting realized gains and losses are summarized as follows:
		For the Three Months					For the Six Months
		Ended June 30,					Ended June 30,
	(In millions)	2014			2013		2014			2013
	Proceeds from maturities and sales	$	560.1		$	78.3	$	1,317.50		$	4,404.70
	Realized gains	$	0.2		$	—	$	0.4		$	6.3
	Realized losses	$	(0.1	)	$	—	$	(0.2	)	$	(2.0	)

Derivative_Instruments_Tables

Derivative Instruments (Tables)	6 Months Ended
	Jun. 30, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]	'
Foreign currency forward contracts that were entered into to hedge forecasted revenue	'
	The notional value of foreign currency forward contracts that were entered into to hedge forecasted revenues is summarized as follows:
		Notional Amount
	Foreign Currency: (In millions)	As of				As of
		June 30,				December 31,
		2014				2013
	Euro	$	703.4			$	636.3
	Canadian dollar	19.5				34
	British pound sterling	39.1				72.3
	Total foreign currency forward contracts	$	762			$	742.6
Summary of the effect of derivatives designated as hedging instruments on the consolidated statements of income	'
	The following table summarizes the effect of derivatives designated as hedging instruments on our condensed consolidated statements of income:
	For the Three Months Ended June 30,
	Net Gains/(Losses)								Net Gains/(Losses)
	Reclassified from AOCI into Net Income								Recognized into Net Income
	(Effective Portion)								(Ineffective Portion)
	Location		2014				2013		Location	2014			2013
	Revenue		$	(5.2	)		$	0.1	Other income (expense)	$	(1.0	)	$	0.1
	For the Six Months Ended June 30,
	Net Gains/(Losses)								Net Gains/(Losses)
	Reclassified from AOCI into Net Income								Recognized into Net Income
	(Effective Portion)								(Ineffective Portion)
	Location		2014				2013		Location	2014			2013
	Revenue		$	(10.0	)		$	1.2	Other income (expense)	$	(1.2	)	$	0.3
Summary of fair value and presentation of derivatives	'
	The following table summarizes the fair value and presentation in our condensed consolidated balance sheets for our outstanding derivatives including those designated as hedging instruments:
	(In millions)	Balance Sheet Location	Fair Value As of June 30, 2014
	Hedging Instruments:
	Asset derivatives	Other current assets	$	0.4
		Investments and other assets	$	0.6
	Liability derivatives	Accrued expenses and other	$	7.9
		Other long-term liabilities	$	0.5
	Other Derivatives:
	Asset derivatives	Other current assets	$	1.1
	Liability derivatives	Accrued expenses and other	$	0.7
	(In millions)	Balance Sheet Location	Fair Value As of December 31, 2013
	Hedging Instruments:
	Asset derivatives	Other current assets	$	0.6
	Liability derivatives	Accrued expenses and other	$	23.4
	Other Derivatives:
	Asset derivatives	Other current assets	$	3.2
	Liability derivatives	Accrued expenses and other	$	0.1

Equity_Tables

Equity (Tables)	6 Months Ended
	Jun. 30, 2014
Equity [Abstract]	'
Reconciliation of equity attributable to noncontrolling interests	'
	The following table reconciles equity attributable to noncontrolling interests (NCI):
		For the Three Months					For the Six Months
		Ended June 30,					Ended June 30,
	(In millions)	2014			2013		2014			2013
	Noncontrolling interests, beginning of period	$	4.4		$	0.6	$	0.6		$	2.3
	Net income (loss) attributable to noncontrolling interests, net of tax	8.6			—		8.4			—
	Fair value of net assets and liabilities acquired and assigned to NCI	—			—		4			—
	Distribution to noncontrolling interest	(9.1		)	—		(9.1		)	—
	Deconsolidation of noncontrolling interest	—			—		—			(1.7		)
	Noncontrolling interests, end of period	$	3.9		$	0.6	$	3.9		$	0.6

Accumulated_Other_Comprehensiv1

Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Tables)	6 Months Ended
	Jun. 30, 2014
Equity [Abstract]	'
Schedule of accumulated other comprehensive income (loss)	'
	The following table summarizes the changes in accumulated other comprehensive income (loss), net of tax by component:
	(In millions)	Unrealized Gains (Losses) on Securities Available for Sale				Unrealized Gains (Losses) on Foreign Currency Forward Contracts				Unfunded Status of Postretirement Benefit Plans				Translation Adjustments				Total
	Balance, as of December 31, 2013	$	5.6			$	(23.7	)		$	(19.6	)		$	10			$	(27.7	)
	Other comprehensive income (loss) before reclassifications	1.4				6.5				0.6				(11.0		)		(2.5		)
	Amounts reclassified from accumulated other comprehensive income (loss)	(6.6		)		10.1				—				—				3.5
	Net current period other comprehensive income (loss)	(5.2		)		16.6				0.6				(11.0		)		1.1
	Balance, as of June 30, 2014	$	0.4			$	(7.1	)		$	(19.0	)		$	(1.0	)		$	(26.7	)
	(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	7.8				10.2				2.3				(16.4		)		3.9
	Amounts reclassified from accumulated other comprehensive income (loss)	(2.8		)		(0.9		)		—				—				(3.7		)
	Net current period other comprehensive income (loss)	5				9.3				2.3				(16.4		)		0.2
	Balance, as of June 30, 2013	$	9.2			$	(1.4	)		$	(19.4	)		$	(43.5	)		$	(55.1	)
Reclassification out of accumulated other comprehensive income	'
	The following table summarizes the amounts reclassified from accumulated other comprehensive income:
	(In millions)	Income Statement Location	Amounts Reclassified from Accumulated Other Comprehensive Income
			For the Three Months								For the Six Months
			Ended June 30,								Ended June 30,
			2014				2013				2014				2013
	Gains (losses) on securities available for sale	Other income (expense)	$	10.1			$	—			$	10.2			$	4.3
		Income tax benefit (expense)	(3.6		)		—				(3.6		)		(1.5		)
	Gains (losses) on foreign currency forward contracts	Revenues	(5.3		)		0.1				(10.0		)		1.2
		Income tax benefit (expense)	(0.5		)		(0.2		)		(0.1		)		(0.3		)
	Total reclassifications, net of tax		$	0.7			$	(0.1	)		$	(3.5	)		$	3.7

Earnings_per_Share_Tables

Earnings per Share (Tables)	6 Months Ended
	Jun. 30, 2014
Earnings Per Share [Abstract]	'
Basic and diluted earnings per share	'
	Basic and diluted earnings per share are calculated as follows:
		For the Three Months				For the Six Months
		Ended June 30,				Ended June 30,
	(In millions)	2014		2013		2014		2013
	Numerator:
	Net income attributable to Biogen Idec Inc.	$	714.5	$	490.7	$	1,194.50	$	917.4
	Denominator:
	Weighted average number of common shares outstanding	236.7		237.5		236.7		237.2
	Effect of dilutive securities:
	Stock options and employee stock purchase plan	0.1		0.3		0.1		0.4
	Time-vested restricted stock units	0.4		0.6		0.5		0.7
	Market stock units	0.2		0.3		0.3		0.3
	Dilutive potential common shares	0.7		1.2		0.9		1.4
	Shares used in calculating diluted earnings per share	237.4		238.7		237.6		238.5

ShareBased_Payments_Tables

Share-Based Payments (Tables)	6 Months Ended
	Jun. 30, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]	'
Share-based compensation expense included in consolidated statements of income	'
	The following table summarizes share-based compensation expense included within our condensed consolidated statements of income:
		For the Three Months							For the Six Months
		Ended June 30,							Ended June 30,
	(In millions)	2014				2013			2014			2013
	Research and development	$	24.5			$	23.7		$	53.9		$	48.9
	Selling, general and administrative	33.6				45.4			80.8			79.7
	Subtotal	58.1				69.1			134.7			128.6
	Capitalized share-based compensation costs	(2.6		)		(2.7		)	(5.2		)	(5.0		)
	Share-based compensation expense included in total cost and expenses	55.5				66.4			129.5			123.6
	Income tax effect	(16.5		)		(20.3		)	(38.8		)	(36.9		)
	Share-based compensation expense included in net income attributable to Biogen Idec Inc.	$	39			$	46.1		$	90.7		$	86.7
Summary of share-based compensation expense associated with each of our share-based compensating programs	'
	The following table summarizes share-based compensation expense associated with each of our share-based compensation programs:
		For the Three Months							For the Six Months
		Ended June 30,							Ended June 30,
	(In millions)	2014				2013			2014			2013
	Stock options	$	—			$	0.1		$	—		$	0.4
	Market stock units	8.2				6.6			23.1			15.1
	Time-vested restricted stock units	29.2				24.8			58.4			51.9
	Cash settled performance units	14				35.8			35.4			56.2
	Performance units	4				—			10.3			—
	Employee stock purchase plan	2.7				1.8			7.5			5
	Subtotal	58.1				69.1			134.7			128.6
	Capitalized share-based compensation costs	(2.6		)		(2.7		)	(5.2		)	(5.0		)
	Share-based compensation expense included in total cost and expenses	$	55.5			$	66.4		$	129.5		$	123.6
Summary of equity grants to employees, officers and directors under our current stock plans	'
	The following table summarizes our equity grants to employees, officers and directors under our current stock plans:
		For the Six Months
		Ended June 30,
		2014			2013
	Market stock units	222,000			257,000
	Cash settled performance shares	180,000			271,000
	Performance units	50,000			—
	Time-vested restricted stock units	418,000			680,000

Income_Taxes_Tables

Income Taxes (Tables)	6 Months Ended
	Jun. 30, 2014
Income Tax Disclosure [Abstract]	'
Reconciliation between the U.S. federal statutory tax rate and effective tax rate	'
	A reconciliation between the U.S. federal statutory tax rate and our effective tax rate is summarized as follows:
		For the Three Months				For the Six Months
		Ended June 30,				Ended June 30,
		2014		2013		2014		2013
	Statutory rate	35	%	35	%	35	%	35	%
	State taxes	1.1		1.9		1.1		4.4
	Taxes on foreign earnings	(8.7	)	(7.2	)	(8.5	)	(8.4	)
	Credits and net operating loss utilization	(0.7	)	(3.5	)	(0.9	)	(3.4	)
	Purchased intangible assets	1.2		1.8		1.3		1.5
	Manufacturing deduction	(1.7	)	(2.4	)	(1.8	)	(11.1	)
	Other permanent items	0.4		(1.0	)	0.4		1.8
	Other	0.4		(0.2	)	0.3		(0.2	)
	Effective tax rate	27	%	24.4	%	26.9	%	19.6	%

Other_Consolidated_Financial_S1

Other Consolidated Financial Statement Detail (Tables)	6 Months Ended
	Jun. 30, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]	'
Other income (expense), net	'
	Components of other income (expense), net, are summarized as follows:
		For the Three Months						For the Six Months
		Ended June 30,						Ended June 30,
	(In millions)	2014			2013			2014			2013
	Interest income	$	2.4		$	0.9		$	5.1		$	5.2
	Interest expense	(7.1		)	(7.3		)	(14.7		)	(18.9		)
	Impairments of investments	—			(1.4		)	—			(1.7		)
	Gain (loss) on investments, net	13.6			5.7			16.6			10.1
	Foreign exchange gains (losses), net	(1.4		)	(5.3		)	(4.7		)	(7.9		)
	Other, net	(2.6		)	(3.0		)	(3.0		)	(11.7		)
	Total other income (expense), net	$	4.9		$	(10.4	)	$	(0.7	)	$	(24.9	)
Accrued expenses and other	'
	Accrued expenses and other consists of the following:
	(In millions)	As of			As of
		June 30,			December 31,
		2014			2013
	Revenue-related rebates	$	321.1		$	264.9
	Employee compensation and benefits	274.1			343.4
	Royalties and licensing fees	190			160.7
	Current portion of contingent consideration obligations	185.7			29
	Deferred revenue	145			172.7
	Clinical development expenses	62.5			55.2
	Collaboration expenses	37.7			18.7
	Construction in progress accrual	15.7			25
	Other	263.7			285.6
	Total accrued expenses and other	$	1,495.50		$	1,355.20

Commitments_and_Contingencies_

Commitments and Contingencies (Tables)	6 Months Ended
	Jun. 30, 2014
Commitments and Contingencies Disclosure [Abstract]	'
Schedule of milestone payments based on the attainment of certain sales levels	'
	We are required to make additional contingent payments to former shareholders of Fumapharm AG based on the attainment of certain cumulative sales levels of Fumapharm Products, with the amount of each payment based on 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	$1.0B		$2.0B		$3.0B		Each additional $1.0B up to $20.0B
		Payment Amounts (In Millions)
	< $500 million	$	—	$	—	$	—	$	—
	$500 million - $1.0 billion	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

Summary_of_Significant_Account2

Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Details) (USD $)	6 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Business (Textual) [Abstract]	'	'
Interest in subsidiary (less than given percentage)	100.00%	'
Accumulated depreciation	$1,210.60	$1,118.30

Accounts_Receivable_Details

Accounts Receivable (Details) (USD $)	Jun. 30, 2014	Dec. 31, 2013
Accounts receivable balances on product sales from different countries	'	'
Current balance included within accounts receivable, net	$1,002,328,000	$824,406,000
Spain [Member]	'	'
Accounts receivable balances on product sales from different countries	'	'
Current balance included within accounts receivable, net	54,700,000	113,300,000
Non-current balance included within investments and other assets	10,000,000	6,800,000
Total	64,700,000	120,100,000
Italy [Member]	'	'
Accounts receivable balances on product sales from different countries	'	'
Current balance included within accounts receivable, net	79,300,000	76,100,000
Non-current balance included within investments and other assets	1,300,000	2,400,000
Total	80,600,000	78,500,000
Portugal [Member]	'	'
Accounts receivable balances on product sales from different countries	'	'
Current balance included within accounts receivable, net	10,700,000	10,400,000
Non-current balance included within investments and other assets	12,700,000	8,200,000
Total	$23,400,000	$18,600,000

Accounts_Receivable_Details_Te

Accounts Receivable (Details Textual)	3 Months Ended				6 Months Ended		3 Months Ended
	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Dec. 31, 2011	Jul. 31, 2013	Jun. 30, 2013
	USD ($)	USD ($)	USD ($)	USD ($)	USD ($)	USD ($)	Italy [Member]	Italy [Member]	Italy [Member]	Italy [Member]
							USD ($)	EUR (€)	EUR (€)	EUR (€)
Accounts Receivable (Textual) [Abstract]	'	'	'	'	'	'	'	'	'	'
Payment terms of accounts receivable arising from product sales	'	'	'	'	'30 to 90 days	'	'	'	'	'
Minimum term for accounts receivable	'	'	'	'	'30 days	'	'	'	'	'
Maximum term for accounts receivable	'	'	'	'	'90 days	'	'	'	'	'
Net accounts receivable balances overdue more than one year	$15,500,000	'	$45,900,000	'	$15,500,000	'	'	'	'	'
Payments received related to receivables aged greater than 1 and 2 years	'	59,600,000	'	'	'	'	'	'	'	'
Payments received related to receivables aged greater than 2 years	'	'	10,000,000	'	'	'	'	'	'	'
Payment in case of any unfavorable determination received	'	'	'	'	'	'	'	30,700,000	33,300,000	'
Reimbursement limit stated in the resolution	'	'	'	'	'	'	'	'24 months	'	'
Deferred revenue	'	'	'	'	'	'	86,900,000	'	'	'
Loss contingency accrual	'	'	'	'	'	'	'	'	'	15,400,000
Loss Contingency, Adjustment Related To Claim Amount	'	'	'	'	'	'	'	'	'	50.00%
Product, net	$2,056,292,000	'	'	$1,385,918,000	$3,799,057,000	$2,481,697,000	$53,500,000	'	'	'

Reserves_for_Discounts_and_All2

Reserves for Discounts and Allowances (Details) (USD $)	6 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014
Analysis of the amount of, and change in, reserves	'
Balance as of December 31, 2013	$416.30
Current provisions relating to sales in current year	753.2
Adjustments relating to prior years	12.6
Payments/returns relating to sales in current year	-441.9
Payments/returns relating to sales in prior years	-265.7
Balance as of June 30, 2014	474.5
Discounts	'
Analysis of the amount of, and change in, reserves	'
Balance as of December 31, 2013	47
Current provisions relating to sales in current year	161.2
Adjustments relating to prior years	-0.4
Payments/returns relating to sales in current year	-111.5
Payments/returns relating to sales in prior years	-44.5
Balance as of June 30, 2014	51.8
Contractual Adjustments	'
Analysis of the amount of, and change in, reserves	'
Balance as of December 31, 2013	335.6
Current provisions relating to sales in current year	577.3
Adjustments relating to prior years	7.6
Payments/returns relating to sales in current year	-330.3
Payments/returns relating to sales in prior years	-200.8
Balance as of June 30, 2014	389.4
Returns	'
Analysis of the amount of, and change in, reserves	'
Balance as of December 31, 2013	33.7
Current provisions relating to sales in current year	14.7
Adjustments relating to prior years	5.4
Payments/returns relating to sales in current year	-0.1
Payments/returns relating to sales in prior years	-20.4
Balance as of June 30, 2014	$33.30

Reserves_for_Discounts_and_All3

Reserves for Discounts and Allowances (Details 1) (USD $)	Jun. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Summary of total product revenue reserves included in consolidated balance sheets	'	'
Total reserves	$474.50	$416.30
Reduction of accounts receivable	'	'
Summary of total product revenue reserves included in consolidated balance sheets	'	'
Total reserves	153.4	151.4
Component of accrued expenses and other	'	'
Summary of total product revenue reserves included in consolidated balance sheets	'	'
Total reserves	$321.10	$264.90

Inventory_Details

Inventory (Details) (USD $)	Jun. 30, 2014	Dec. 31, 2013
Components of inventories	'	'
Raw materials	$124,800,000	$115,000,000
Work in process	447,400,000	435,400,000
Finished goods	143,700,000	108,600,000
Total inventory	715,935,000	659,003,000
Inventory, Various Programs, Gross	$45,500,000	$93,700,000

Intangible_Assets_and_Goodwill2

Intangible Assets and Goodwill (Details) (USD $)

3 Months Ended

6 Months Ended

12 Months Ended

3 Months Ended

6 Months Ended

6 Months Ended

6 Months Ended

6 Months Ended

6 Months Ended

Jun. 30, 2014

Jun. 30, 2013

Jun. 30, 2014

Jun. 30, 2013

Dec. 31, 2013

Mar. 31, 2014

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Apr. 02, 2013

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Out-licensed patents

Out-licensed patents

Out-licensed patents

Out-licensed patents

Out-licensed patents

Developed technology

Developed technology

Developed technology

Developed technology

Developed technology

Acquired and in-licensed rights and patents

Acquired and in-licensed rights and patents

Acquired and in-licensed rights and patents

Acquired and in-licensed rights and patents

In-process research and development

In-process research and development

Trademarks and trade names

Trademarks and trade names

TYSABRI product

TYSABRI product

ALPROLIX product

ALPROLIX product

Fumapharm AG [Member]

Two billion [Member]

Minimum

Maximum

Minimum

Maximum

AVONEX

Minimum

Maximum

Acquired and in-licensed rights and patents

Acquired and in-licensed rights and patents

TECFIDERA [Member]

Fumapharm AG [Member]

Intangible assets

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Estimated life, (in years)

'

'

'

'

'

'

'

'

'13 years

'23 years

'

'

'15 years

'23 years

'

'

'

'6 years

'17 years

'

'

'

'

'

'

'

'

'

'

Indefinite lived intangible assets useful life

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'Indefinite until commercialization

'

'Indefinite

'

'

'

'

'

'

'

Cost

'

'

'

'

'

'

$543,300,000

$578,000,000

'

'

$3,005,300,000

$3,005,300,000

'

'

'

$3,271,500,000

$3,240,000,000

'

'

'

'

'

'

'

'

'

'

'

'

Cost and Net

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

330,900,000

327,400,000

64,000,000

64,000,000

'

'

'

'

'

'

Total intangible assets, gross

7,215,000,000

'

7,215,000,000

'

7,214,700,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Accumulated Amortization

-2,965,600,000

'

-2,965,600,000

'

-2,740,000,000

'

-465,900,000

-450,800,000

'

'

-2,283,700,000

-2,165,400,000

'

'

'

-216,000,000

-123,800,000

'

'

0

0

0

0

'

'

'

'

'

'

Net

'

'

'

'

'

'

77,400,000

127,200,000

'

'

721,600,000

839,900,000

'

'

711,900,000

3,055,500,000

3,116,200,000

'

'

'

'

'

'

'

2,995,100,000

'

'

'

'

Intangible assets, net

4,249,378,000

'

4,249,378,000

'

4,474,653,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Intangible asset impairment charges

'

'

'

'

'

34,700,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Net intangible asset

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

3,178,300,000

'

'

'

'

'

Contingent payment

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

20,000,000

'

'

'

Increase in in-licensed rights and patents

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

7,800,000

'

'

Income tax benefit (expense)

268,521,000

159,140,000

446,935,000

224,648,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

18,100,000

'

Intangible Assets and Goodwill (Textual) [Abstract]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Amortization of acquired intangible assets

116,826,000

82,225,000

260,084,000

133,526,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Increase to goodwill

'

'

131,900,000

'

35,700,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

150,000,000

'

Cumulative sales level

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

2,000,000,000

Accumulated impairment losses related to goodwill

0

'

0

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Intangible Assets And Goodwill (Additional Textual) [Abstract]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Expected future amortization expense, 2014 (remaining six months)

217,200,000

'

217,200,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Expected future amortization expense, 2015

333,400,000

'

333,400,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Expected future amortization expense, 2016

318,800,000

'

318,800,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Expected future amortization expense, 2017

322,000,000

'

322,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Expected future amortization expense, 2018

323,100,000

'

323,100,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Expected future amortization expense, 2019

302,900,000

'

302,900,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Total

$1,817,400,000

'

$1,817,400,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Intangible_Assets_and_Goodwill3

Intangible Assets and Goodwill (Details 1) (USD $)	6 Months Ended	12 Months Ended
	Jun. 30, 2014	Dec. 31, 2013
Summary of roll forward of the changes in goodwill	'	'
Goodwill, beginning of period	$1,232,916,000	$1,201,300,000
Increase to goodwill	131,900,000	35,700,000
Other	0	-4,100,000
Goodwill, end of period	$1,364,815,000	$1,232,916,000

Fair_Value_Measurements_Detail

Fair Value Measurements (Details) (USD $)	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Assets:	'	'	'	'	'	'
Marketable debt securities	$1,720.60	'	$1,245.90	'	'	'
Fair Value, Measurements, Recurring	'	'	'	'	'	'
Assets:	'	'	'	'	'	'
Cash equivalents	475.2	'	424.7	'	'	'
Derivative contracts	2.1	'	3.8	'	'	'
Plan assets for deferred compensation	37	'	22.7	'	'	'
Total	2,256.40	'	1,730.20	'	'	'
Liabilities:	'	'	'	'	'	'
Derivative contracts	9.1	'	23.5	'	'	'
Contingent consideration obligations	279.1	275.1	280.9	281	293.7	293.9
Total	288.2	'	304.4	'	'	'
Fair Value, Measurements, Recurring | Corporate debt securities	'	'	'	'	'	'
Assets:	'	'	'	'	'	'
Marketable debt securities	617.2	'	439.8	'	'	'
Fair Value, Measurements, Recurring | Government securities	'	'	'	'	'	'
Assets:	'	'	'	'	'	'
Marketable debt securities	948	'	674.7	'	'	'
Fair Value, Measurements, Recurring | Mortgage and other asset backed securities	'	'	'	'	'	'
Assets:	'	'	'	'	'	'
Marketable debt securities	155.4	'	131.4	'	'	'
Fair Value, Measurements, Recurring | Marketable equity securities	'	'	'	'	'	'
Assets:	'	'	'	'	'	'
Investments	0.4	'	11.2	'	'	'
Fair Value, Measurements, Recurring | Venture capital investments	'	'	'	'	'	'
Assets:	'	'	'	'	'	'
Investments	21.1	23.6	21.9	23.5	18	20.3
Quoted Prices in Active Markets, Level 1 | Fair Value, Measurements, Recurring	'	'	'	'	'	'
Assets:	'	'	'	'	'	'
Cash equivalents	0	'	0	'	'	'
Derivative contracts	0	'	0	'	'	'
Plan assets for deferred compensation	0	'	0	'	'	'
Total	0.4	'	11.2	'	'	'
Liabilities:	'	'	'	'	'	'
Derivative contracts	0	'	0	'	'	'
Contingent consideration obligations	0	'	0	'	'	'
Total	0	'	0	'	'	'
Quoted Prices in Active Markets, Level 1 | Fair Value, Measurements, Recurring | Corporate debt securities	'	'	'	'	'	'
Assets:	'	'	'	'	'	'
Marketable debt securities	0	'	0	'	'	'
Quoted Prices in Active Markets, Level 1 | Fair Value, Measurements, Recurring | Government securities	'	'	'	'	'	'
Assets:	'	'	'	'	'	'
Marketable debt securities	0	'	0	'	'	'
Quoted Prices in Active Markets, Level 1 | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities	'	'	'	'	'	'
Assets:	'	'	'	'	'	'
Marketable debt securities	0	'	0	'	'	'
Quoted Prices in Active Markets, Level 1 | Fair Value, Measurements, Recurring | Marketable equity securities	'	'	'	'	'	'
Assets:	'	'	'	'	'	'
Investments	0.4	'	11.2	'	'	'
Quoted Prices in Active Markets, Level 1 | Fair Value, Measurements, Recurring | Venture capital investments	'	'	'	'	'	'
Assets:	'	'	'	'	'	'
Investments	0	'	0	'	'	'
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring	'	'	'	'	'	'
Assets:	'	'	'	'	'	'
Cash equivalents	475.2	'	424.7	'	'	'
Derivative contracts	2.1	'	3.8	'	'	'
Plan assets for deferred compensation	37	'	22.7	'	'	'
Total	2,234.90	'	1,697.10	'	'	'
Liabilities:	'	'	'	'	'	'
Derivative contracts	9.1	'	23.5	'	'	'
Contingent consideration obligations	0	'	0	'	'	'
Total	9.1	'	23.5	'	'	'
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Corporate debt securities	'	'	'	'	'	'
Assets:	'	'	'	'	'	'
Marketable debt securities	617.2	'	439.8	'	'	'
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Government securities	'	'	'	'	'	'
Assets:	'	'	'	'	'	'
Marketable debt securities	948	'	674.7	'	'	'
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities	'	'	'	'	'	'
Assets:	'	'	'	'	'	'
Marketable debt securities	155.4	'	131.4	'	'	'
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Marketable equity securities	'	'	'	'	'	'
Assets:	'	'	'	'	'	'
Investments	0	'	0	'	'	'
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Venture capital investments	'	'	'	'	'	'
Assets:	'	'	'	'	'	'
Investments	0	'	0	'	'	'
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring	'	'	'	'	'	'
Assets:	'	'	'	'	'	'
Cash equivalents	0	'	0	'	'	'
Derivative contracts	0	'	0	'	'	'
Plan assets for deferred compensation	0	'	0	'	'	'
Total	21.1	'	21.9	'	'	'
Liabilities:	'	'	'	'	'	'
Derivative contracts	0	'	0	'	'	'
Contingent consideration obligations	279.1	'	280.9	'	'	'
Total	279.1	'	280.9	'	'	'
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Corporate debt securities	'	'	'	'	'	'
Assets:	'	'	'	'	'	'
Marketable debt securities	0	'	0	'	'	'
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Government securities	'	'	'	'	'	'
Assets:	'	'	'	'	'	'
Marketable debt securities	0	'	0	'	'	'
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities	'	'	'	'	'	'
Assets:	'	'	'	'	'	'
Marketable debt securities	0	'	0	'	'	'
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Marketable equity securities	'	'	'	'	'	'
Assets:	'	'	'	'	'	'
Investments	0	'	0	'	'	'
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Venture capital investments	'	'	'	'	'	'
Assets:	'	'	'	'	'	'
Investments	$21.10	'	$21.90	'	'	'

Fair_Value_Measurements_Detail1

Fair Value Measurements (Details 1) (USD $)	3 Months Ended		6 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012
					Fair Value, Measurements, Recurring	Fair Value, Measurements, Recurring	Fair Value, Measurements, Recurring	Fair Value, Measurements, Recurring	Fair Value, Measurements, Recurring	Fair Value, Measurements, Recurring
					Venture capital investments	Venture capital investments	Venture capital investments	Venture capital investments	Venture capital investments	Venture capital investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]	'	'	'	'	'	'	'	'	'	'
Fair value, beginning of period	'	'	'	'	$21.10	$23.60	$21.90	$23.50	$18	$20.30
Unrealized gains included in earnings	2.1	6.1	5	6.7	'	'	'	'	'	'
Unrealized losses included in earnings	-0.1	-0.6	-1.3	-2	'	'	'	'	'	'
Purchases	0	0	0	0	'	'	'	'	'	'
Settlements	-4.5	0	-4.5	-1.5	'	'	'	'	'	'
Fair value, end of period	'	'	'	'	$21.10	$23.60	$21.90	$23.50	$18	$20.30

Fair_Value_Measurements_Detail2

Fair Value Measurements (Details 2) (USD $)	Jun. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Summary of fair and carrying value of debt instruments	'	'
Notes payable, carrying value	$589.50	$595.90
Debt instruments, fair value	659.7	665.4
Notes Payable to Fumedica	'	'
Summary of fair and carrying value of debt instruments	'	'
Notes payable, fair value	13.8	17.5
Notes payable, carrying value	12.6	15.8
6.875% Senior Notes due 2018	'	'
Summary of fair and carrying value of debt instruments	'	'
Notes payable, fair value	645.9	647.9
Notes payable, carrying value	$576.90	$580.10

Fair_Value_Measurements_Detail3

Fair Value Measurements (Details 3) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012
					Fair Value, Measurements, Recurring	Fair Value, Measurements, Recurring	Fair Value, Measurements, Recurring	Fair Value, Measurements, Recurring	Fair Value, Measurements, Recurring	Fair Value, Measurements, Recurring
Business Acquisition, Contingent Consideration [Line Items]	'	'	'	'	'	'	'	'	'	'
Fair value, beginning of period	'	'	'	'	$279,100,000	$275,100,000	$280,900,000	$281,000,000	$293,700,000	$293,900,000
Additions	0	0	0	0	'	'	'	'	'	'
Changes in fair value	4,019,000	-5,163,000	3,220,000	-2,886,000	'	'	'	'	'	'
Payments	0	-7,500,000	-5,000,000	-10,000,000	'	'	'	'	'	'
Fair value, end of period	'	'	'	'	$279,100,000	$275,100,000	$280,900,000	$281,000,000	$293,700,000	$293,900,000

Fair_Value_Measurements_Detail4

Fair Value Measurements (Details Textual) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Business Acquisition, Contingent Consideration [Line Items]	'	'
Asset impairment charges	$0	'
Changed in valuation techniques	'0	'
Percentage of venture capital investments to assets	0.20%	0.20%
Contingent consideration obligations	$243.40	$251.90
6.875% Senior Notes due 2018	'	'
Business Acquisition, Contingent Consideration [Line Items]	'	'
Stated rate on debt	6.88%	'

Financial_Instruments_Details

Financial Instruments (Details) (USD $)	Jun. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Marketable Debt and Equity Securities	'	'
Fair Value	$1,720.60	$1,245.90
Gross Unrealized Gains	1.1	0.4
Gross Unrealized Losses	-0.4	-0.3
Amortized Cost	1,719.90	1,245.80
Corporate debt securities Current	'	'
Marketable Debt and Equity Securities	'	'
Fair Value	125	100.7
Gross Unrealized Gains	0.1	0
Gross Unrealized Losses	0	0
Amortized Cost	124.9	100.7
Corporate debt securities Non-current	'	'
Marketable Debt and Equity Securities	'	'
Fair Value	492.2	339.1
Gross Unrealized Gains	0.6	0.4
Gross Unrealized Losses	-0.2	-0.1
Amortized Cost	491.8	338.8
Government securities Current	'	'
Marketable Debt and Equity Securities	'	'
Fair Value	584.9	519.5
Gross Unrealized Gains	0.1	0
Gross Unrealized Losses	-0.1	0
Amortized Cost	584.9	519.5
Government securities Non-current	'	'
Marketable Debt and Equity Securities	'	'
Fair Value	363.1	155.2
Gross Unrealized Gains	0.2	0
Gross Unrealized Losses	0	-0.1
Amortized Cost	362.9	155.3
Mortgage and other asset backed securities Current	'	'
Marketable Debt and Equity Securities	'	'
Fair Value	0	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Amortized Cost	0	0
Mortgage and other asset backed securities Non-current	'	'
Marketable Debt and Equity Securities	'	'
Fair Value	155.4	131.4
Gross Unrealized Gains	0.1	0
Gross Unrealized Losses	-0.1	-0.1
Amortized Cost	155.4	131.5
Marketable equity securities	'	'
Marketable Debt and Equity Securities	'	'
Fair Value	0.4	11.2
Gross Unrealized Gains	0	8.7
Gross Unrealized Losses	-0.1	0
Amortized Cost	$0.50	$2.50

Financial_Instruments_Details_

Financial Instruments (Details 1) (USD $)	Jun. 30, 2014	Dec. 31, 2013
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	$475.20	$424.70
Commercial paper	'	'
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents	'	'
Cash equivalents	5.9	1.2
Overnight reverse repurchase agreements	'	'
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents	'	'
Cash equivalents	294.8	22.4
Short-term Debt Securities	'	'
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents	'	'
Cash equivalents	$174.50	$401.10

Financial_Instruments_Details_1

Financial Instruments (Details 2) (USD $)	Jun. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Summary of Contractual Maturities: Available-for-Sale Securities	'	'
Due in one year or less, estimated fair value	$709.80	$620.20
Due in one year or less, amortized cost	709.8	620.2
Due after one year through five years, Estimated Fair Value	931	573.1
Due after one year through five years, Amortized Cost	930.3	572.9
Due after five years, estimated fair value	79.8	52.6
Due after five years, amortized cost	79.8	52.7
Total available-for-sale securities, Fair Value	1,720.60	1,245.90
Total available-for-sale securities, Amortized Cost	$1,719.90	$1,245.80

Financial_Instruments_Details_2

Financial Instruments (Details 3) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Proceeds from Marketable Debt Securities	'	'	'	'
Proceeds from maturities and sales	$560,100,000	$78,300,000	$1,317,525,000	$4,404,707,000
Realized gains	200,000	0	400,000	6,300,000
Realized losses	($100,000)	$0	($200,000)	($2,000,000)

Financial_Instruments_Details_3

Financial Instruments (Details Textual)	6 Months Ended	12 Months Ended
	Jun. 30, 2014	Dec. 31, 2013
Investments, All Other Investments [Abstract]	'	'
Original maturities of commercial paper and short-term debt securities	'less than 90 days	'
Average maturity of marketable securities, months	'13 months	'13 months

Financial_Instruments_Financia

Financial Instruments Financial Instruments (Details Textual 2) (Strategic Investments, USD $)	3 Months Ended		6 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Strategic Investments	'	'	'	'	'
Business Acquisition [Line Items]	'	'	'	'	'
Strategic investment portfolio	$51.10	'	$51.10	'	$56.90
Venture capital investments	21.5	'	21.5	'	33.1
Cost basis of equity securities of certain privately-owned companies and venture capital funds	29.6	'	29.6	'	23.8
Net gains (losses) realized	$2	$4.40	$4.90	$4.10	'

Derivative_Instruments_Details

Derivative Instruments (Details) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
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	'	'	'18 months	'	'18 months
Gain/Loss on fair value of foreign currency forward contracts	$7	'	$7	'	$23.60
Expected settlement time for contracts, in months	'	'	'18 months	'	'
Net gains (losses) of other income (expense) related to foreign currency forward contracts	2.4	0.6	3.8	1.5	'
Designated as hedging instrument	'	'	'	'	'
Derivative Instruments (Textual) [Abstract]	'	'	'	'	'
Aggregate notional amount	762	'	762	'	742.6
Not designated as hedging instrument	'	'	'	'	'
Derivative Instruments (Textual) [Abstract]	'	'	'	'	'
Aggregate notional amount	440.3	'	440.3	'	273.3
Foreign Exchange Contract | Other current assets | Designated as hedging instrument	'	'	'	'	'
Summary of Derivatives designated as Hedging Instruments	'	'	'	'	'
Derivative asset, fair value, net	0.4	'	0.4	'	0.6
Foreign Exchange Contract | Other current assets | Not designated as hedging instrument	'	'	'	'	'
Summary of Derivatives designated as Hedging Instruments	'	'	'	'	'
Derivative asset, fair value, net	1.1	'	1.1	'	3.2
Foreign Exchange Contract | Investments and other assets | Designated as hedging instrument	'	'	'	'	'
Summary of Derivatives designated as Hedging Instruments	'	'	'	'	'
Derivative asset, fair value, net	0.6	'	0.6	'	'
Foreign Exchange Contract | Accrued expenses and other | Designated as hedging instrument	'	'	'	'	'
Summary of Derivatives designated as Hedging Instruments	'	'	'	'	'
Derivative liability, fair value, net	7.9	'	7.9	'	23.4
Foreign Exchange Contract | Accrued expenses and other | Not designated as hedging instrument	'	'	'	'	'
Summary of Derivatives designated as Hedging Instruments	'	'	'	'	'
Derivative liability, fair value, net	0.7	'	0.7	'	0.1
Foreign Exchange Contract | Other long-term liabilities | Designated as hedging instrument	'	'	'	'	'
Summary of Derivatives designated as Hedging Instruments	'	'	'	'	'
Derivative liability, fair value, net	0.5	'	0.5	'	'
Sales | Foreign Exchange Contract	'	'	'	'	'
Derivative Instruments (Textual) [Abstract]	'	'	'	'	'
Net gains (losses) in product revenue for the settlement of certain effective cash flow hedge instruments	-5.2	0.1	-10	1.2	'
Other Income Expense | Foreign Exchange Contract	'	'	'	'	'
Derivative Instruments (Textual) [Abstract]	'	'	'	'	'
Net gains (losses) in earnings of Foreign currency forward contracts due to hedge ineffectiveness	-1	0.1	-1.2	0.3	'
Euro | Designated as hedging instrument	'	'	'	'	'
Derivative Instruments (Textual) [Abstract]	'	'	'	'	'
Aggregate notional amount	703.4	'	703.4	'	636.3
Canadian Dollar | Designated as hedging instrument	'	'	'	'	'
Derivative Instruments (Textual) [Abstract]	'	'	'	'	'
Aggregate notional amount	19.5	'	19.5	'	34
British Pound Sterling | Designated as hedging instrument	'	'	'	'	'
Derivative Instruments (Textual) [Abstract]	'	'	'	'	'
Aggregate notional amount	$39.10	'	$39.10	'	$72.30

Indebtedness_Details

Indebtedness (Details) (USD $)	Mar. 31, 2014
In Millions, unless otherwise specified
Debt Disclosure [Abstract]	'
Senior unsecured revolving credit facility maximum borrowing capacity	$750

Equity_Details

Equity (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Reconciliation of equity attributable to non-controlling interests	'	'	'	'
Noncontrolling interests, beginning of period	$4,400,000	$600,000	$596,000	$2,300,000
Net income (loss) attributable to noncontrolling interests, net of tax	8,626,000	0	8,398,000	0
Fair value of net assets and liabilities acquired and assigned to NCI	0	0	4,000,000	0
Distribution to noncontrolling interest	-9,100,000	0	-9,100,000	0
Deconsolidation of noncontrolling interest	0	0	0	-1,700,000
Noncontrolling interests, end of period	$3,927,000	$600,000	$3,927,000	$600,000

Equity_Details_Textual

Equity (Details Textual) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Feb. 28, 2011
Equity (Textual) [Abstract]	'	'	'	'	'
Equity increase during the period	'	'	$944,400,000	'	'
Net income attributable to Biogen Idec Inc	714,506,000	490,670,000	1,194,462,000	917,419,000	'
Adjustments to additional paid in capital, share-based compensation and exercise of stock options	'	'	82,400,000	'	'
Common stock shares authorized for repurchase	3,000,000	'	3,000,000	'	20,000,000
Repurchase of common stock, shares	'	'	1,200,000	300,000	'
Purchase of treasury stock, value	'	'	$336,905,000	$41,023,000	'

Accumulated_Other_Comprehensiv2

Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Dec. 31, 2012
Accumulated other comprehensive income [Line Items]	'	'	'	'	'	'
Accumulated other comprehensive income (loss), net of tax	($26,678,000)	($55,100,000)	($26,678,000)	($55,100,000)	($27,745,000)	($55,300,000)
Other comprehensive income loss before reclassifications	'	'	-2,500,000	3,900,000	'	'
Amounts reclassified from accumulated other comprehensive income loss	'	'	3,500,000	-3,700,000	'	'
Net current period other comprehensive income loss	-4,322,000	12,839,000	1,066,000	169,000	'	'
Accumulated Net Unrealized Investment Gain (Loss)	'	'	'	'	'	'
Accumulated other comprehensive income [Line Items]	'	'	'	'	'	'
Accumulated other comprehensive income (loss), net of tax	400,000	9,200,000	400,000	9,200,000	5,600,000	4,200,000
Other comprehensive income loss before reclassifications	'	'	1,400,000	7,800,000	'	'
Amounts reclassified from accumulated other comprehensive income loss	'	'	-6,600,000	-2,800,000	'	'
Net current period other comprehensive income loss	'	'	-5,200,000	5,000,000	'	'
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges	'	'	'	'	'	'
Accumulated other comprehensive income [Line Items]	'	'	'	'	'	'
Accumulated other comprehensive income (loss), net of tax	-7,100,000	-1,400,000	-7,100,000	-1,400,000	-23,700,000	-10,700,000
Other comprehensive income loss before reclassifications	'	'	6,500,000	10,200,000	'	'
Amounts reclassified from accumulated other comprehensive income loss	'	'	10,100,000	-900,000	'	'
Net current period other comprehensive income loss	'	'	16,600,000	9,300,000	'	'
Accumulated Defined Benefit Plans Adjustment	'	'	'	'	'	'
Accumulated other comprehensive income [Line Items]	'	'	'	'	'	'
Accumulated other comprehensive income (loss), net of tax	-19,000,000	-19,400,000	-19,000,000	-19,400,000	-19,600,000	-21,700,000
Other comprehensive income loss before reclassifications	'	'	600,000	2,300,000	'	'
Amounts reclassified from accumulated other comprehensive income loss	'	'	0	0	'	'
Net current period other comprehensive income loss	'	'	600,000	2,300,000	'	'
Accumulated Translation Adjustment	'	'	'	'	'	'
Accumulated other comprehensive income [Line Items]	'	'	'	'	'	'
Accumulated other comprehensive income (loss), net of tax	-1,000,000	-43,500,000	-1,000,000	-43,500,000	10,000,000	-27,100,000
Other comprehensive income loss before reclassifications	'	'	-11,000,000	-16,400,000	'	'
Amounts reclassified from accumulated other comprehensive income loss	'	'	0	0	'	'
Net current period other comprehensive income loss	'	'	($11,000,000)	($16,400,000)	'	'

Accumulated_Other_Comprehensiv3

Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Details 1) (USD $)	3 Months Ended		6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]	'	'	'	'
Other income (expense)	$4,861	($10,428)	($740)	($24,885)
Income tax benefit (expense)	268,521	159,140	446,935	224,648
Product, net	2,056,292	1,385,918	3,799,057	2,481,697
Net income attributable to Biogen Idec Inc.	714,506	490,670	1,194,462	917,419
Reclassification out of Accumulated Other Comprehensive Income	'	'	'	'
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]	'	'	'	'
Net income attributable to Biogen Idec Inc.	700	-100	-3,500	3,700
Accumulated Net Unrealized Investment Gain (Loss) | Reclassification out of Accumulated Other Comprehensive Income	'	'	'	'
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]	'	'	'	'
Other income (expense)	10,100	0	10,200	4,300
Income tax benefit (expense)	-3,600	0	-3,600	-1,500
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income	'	'	'	'
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]	'	'	'	'
Income tax benefit (expense)	-500	-200	-100	-300
Product, net	($5,300)	$100	($10,000)	$1,200

Earnings_per_Share_Details

Earnings per Share (Details) (USD $)	3 Months Ended		6 Months Ended
In Thousands, except Share data, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Numerator:	'	'	'	'
Net income attributable to Biogen Idec Inc	$714,506	$490,670	$1,194,462	$917,419
Denominator:	'	'	'	'
Weighted average number of common shares outstanding	236,661,000	237,484,000	236,729,000	237,162,000
Effect of dilutive securities:	'	'	'	'
Dilutive potential common shares	700,000	1,200,000	900,000	1,400,000
Shares used in calculating diluted earnings per share	237,401,000	238,743,000	237,634,000	238,543,000
Stock options and employee stock purchase plan	'	'	'	'
Effect of dilutive securities:	'	'	'	'
Stock options and employee stock purchase plan	100,000	300,000	100,000	400,000
Time-vested restricted stock units	'	'	'	'
Effect of dilutive securities:	'	'	'	'
Stock options and employee stock purchase plan	400,000	600,000	500,000	700,000
Market stock units	'	'	'	'
Effect of dilutive securities:	'	'	'	'
Stock options and employee stock purchase plan	200,000	300,000	300,000	300,000

ShareBased_Payments_Details

Share-Based Payments (Details) (USD $)	3 Months Ended		6 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Share-based Compensation Expense included in consolidated statements of income	'	'	'	'
Subtotal	$58.10	$69.10	$134.70	$128.60
Capitalized share-based compensation costs	-2.6	-2.7	-5.2	-5
Share-based compensation expense included in total costs and expenses	55.5	66.4	129.5	123.6
Income tax effect	-16.5	-20.3	-38.8	-36.9
Research and development	'	'	'	'
Share-based Compensation Expense included in consolidated statements of income	'	'	'	'
Share-based compensation expense	24.5	23.7	53.9	48.9
Selling, general and administrative	'	'	'	'
Share-based Compensation Expense included in consolidated statements of income	'	'	'	'
Share-based compensation expense	33.6	45.4	80.8	79.7
Total share-based compensation expense, net of tax	'	'	'	'
Share-based Compensation Expense included in consolidated statements of income	'	'	'	'
Share-based compensation expense	$39	$46.10	$90.70	$86.70

ShareBased_Payments_Details_1

Share-Based Payments (Details 1) (USD $)	3 Months Ended		6 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Summary of share based compensation expense associated with different programs	'	'	'	'
Subtotal	$58.10	$69.10	$134.70	$128.60
Capitalized share-based compensation costs	-2.6	-2.7	-5.2	-5
Share-based compensation expense included in total costs and expenses	55.5	66.4	129.5	123.6
Stock Options	'	'	'	'
Summary of share based compensation expense associated with different programs	'	'	'	'
Share-based compensation expense	0	0.1	0	0.4
Market stock units	'	'	'	'
Summary of share based compensation expense associated with different programs	'	'	'	'
Share-based compensation expense	8.2	6.6	23.1	15.1
Time-vested restricted stock units	'	'	'	'
Summary of share based compensation expense associated with different programs	'	'	'	'
Share-based compensation expense	29.2	24.8	58.4	51.9
Cash settled performance shares	'	'	'	'
Summary of share based compensation expense associated with different programs	'	'	'	'
Share-based compensation expense	14	35.8	35.4	56.2
Performance units	'	'	'	'
Summary of share based compensation expense associated with different programs	'	'	'	'
Share-based compensation expense	4	0	10.3	0
Employee stock purchase plan	'	'	'	'
Summary of share based compensation expense associated with different programs	'	'	'	'
Share-based compensation expense	$2.70	$1.80	$7.50	$5

ShareBased_Payments_Details_2

Share-Based Payments (Details 2)	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
Market stock units	'	'
Schedule of stock option grants	'	'
Stock units	222,000	257,000
Cash settled performance shares	'	'
Schedule of stock option grants	'	'
Stock units	180,000	271,000
Performance units	'	'
Schedule of stock option grants	'	'
Stock units	50,000	0
Time-vested restricted stock units	'	'
Schedule of stock option grants	'	'
Stock units	418,000	680,000

ShareBased_Payments_Details_Te

Share-Based Payments (Details Textual)	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'
Performance multiplier	0.5	'
Number of days for calculation of average closing stock price	'30 days	'
Shares issued under ESPP	112,000	160,000
Market stock units	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'
Number of equal annual increments	3	'
Minimum target number of market stock units granted based on stock performance in percent	0.00%	'
Maximum target number of market stock units granted based on stock performance in percent	200.00%	'
Performance units	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'
Number of equal annual increments	3	'
Minimum target number of market stock units granted based on stock performance in percent	0.00%	'
Maximum target number of market stock units granted based on stock performance in percent	200.00%	'

Income_Taxes_Details

Income Taxes (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
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	1.10%	1.90%	1.10%	4.40%
Taxes on foreign earnings	-8.70%	-7.20%	-8.50%	-8.40%
Credits and net operating loss utilization	-0.70%	-3.50%	-0.90%	-3.40%
Purchased intangible assets	1.20%	1.80%	1.30%	1.50%
Manufacturing deduction	-1.70%	-2.40%	-1.80%	-11.10%
Permanent items	0.40%	-1.00%	0.40%	1.80%
Other	0.40%	-0.20%	0.30%	-0.20%
Effective tax rate	27.00%	24.40%	26.90%	19.60%

Income_Taxes_Details_Textual

Income Taxes (Details Textual) (USD $)	3 Months Ended		6 Months Ended			1 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Mar. 31, 2013	Oct. 31, 2011	Oct. 31, 2011	Oct. 31, 2011
						Tax Year 2007	Tax Year 2008	Tax Year 2009
Income Tax Contingency [Line Items]	'	'	'	'	'	'	'	'
Effective tax rate	27.00%	24.40%	26.90%	19.60%	'	'	'	'
Net benefit for a previously unrecognized position	'	'	'	'	$49.80	'	'	'
Expense for non-income based state taxes	'	'	'	'	10.3	'	'	'
Proposed deduction disallowance by tax authorities	'	'	'	'	'	$130	$130	$130

Other_Consolidated_Financial_S2

Other Consolidated Financial Statement Detail (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Organization, Consolidation and Presentation of Financial Statements [Abstract]	'	'	'	'	'
Long-term taxes payable	$198,500,000	'	$198,500,000	'	'
Other Income (Expense), Net	'	'	'	'	'
Interest income	2,400,000	900,000	5,100,000	5,200,000	'
Interest expense	-7,100,000	-7,300,000	-14,700,000	-18,900,000	'
Impairment of investments	0	-1,400,000	0	-1,700,000	'
Gain (loss) on sales of investments, net	13,600,000	5,700,000	16,600,000	10,100,000	'
Foreign exchange gains (losses), net	-1,400,000	-5,300,000	-4,700,000	-7,900,000	'
Other, net	-2,600,000	-3,000,000	-3,000,000	-11,700,000	'
Total other income (expense), net	4,861,000	-10,428,000	-740,000	-24,885,000	'
Accrued Expenses and Other	'	'	'	'	'
Revenue-related rebates	474,500,000	'	474,500,000	'	416,300,000
Employee compensation and benefits	274,100,000	'	274,100,000	'	343,400,000
Royalties and licensing fees	190,000,000	'	190,000,000	'	160,700,000
Current portion of contingent consideration obligations	185,700,000	'	185,700,000	'	29,000,000
Deferred revenue	145,000,000	'	145,000,000	'	172,700,000
Clinical development expenses	62,500,000	'	62,500,000	'	55,200,000
Collaboration expenses	37,700,000	'	37,700,000	'	18,700,000
Construction in progress accrual	15,700,000	'	15,700,000	'	25,000,000
Other	263,700,000	'	263,700,000	'	285,600,000
Total accrued expenses and other	1,495,549,000	'	1,495,549,000	'	1,355,187,000
Component of accrued expenses and other	'	'	'	'	'
Accrued Expenses and Other	'	'	'	'	'
Revenue-related rebates	$321,100,000	'	$321,100,000	'	$264,900,000

Investments_in_Variable_Intere1

Investments in Variable Interest Entities (Details) (USD $)

3 Months Ended

6 Months Ended

3 Months Ended

6 Months Ended

1 Months Ended

1 Months Ended

Jun. 30, 2014

Jun. 30, 2013

Jun. 30, 2014

Jun. 30, 2013

Dec. 31, 2013

Jun. 30, 2014

Jun. 30, 2013

Jun. 30, 2014

Jun. 30, 2013

Feb. 28, 2014

Feb. 28, 2014

Feb. 28, 2014

Neurimmune

Neurimmune

Neurimmune

Neurimmune

Ataxion

Commitments to invest

Minority interest

Ataxion

Ataxion

Investment in Variable Interest Entities (Textual) [Abstract]

'

'

'

'

'

'

'

'

'

'

'

'

Remaining potential development milestone payments and royalties on commercial sales under the terms of collaboration agreement

'

'

'

'

'

'

'

$335,000,000

'

'

'

'

Research and development

447,273,000

327,463,000

976,157,000

611,803,000

'

10,600,000

5,700,000

21,700,000

10,700,000

'

'

'

Milestone payments made during period

'

'

'

'

'

'

'

10,000,000

'

'

'

'

Payments to acquire interest in preferred stock of affiliate

'

'

'

'

'

'

'

'

'

1,600,000

'

'

Term for right to purchase outstanding shares

'

'

'

'

'

'

'

'

'

'30 days

'

'

Additional investments committed to if certain milestones are met

'

'

'

'

'

'

'

'

'

'

6,200,000

'

In-process research and development

'

'

'

'

'

'

'

'

'

'

'

3,500,000

Investment in biotechnology companies that are determined to be unconsolidated variable interest entities

$11,400,000

'

$11,400,000

'

$5,500,000

'

'

'

'

'

'

'

Collaborative_and_Other_Relati1

Collaborative and Other Relationships Collaborative and Other Relationships - Collaboration Agreement (Details) (USD $)	3 Months Ended		6 Months Ended			3 Months Ended	1 Months Ended		3 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Mar. 31, 2014	Mar. 31, 2014	Jan. 31, 2014	Mar. 31, 2014	Jun. 30, 2014
					Eisai	Sangamo BioSciences	Isis Pharmaceuticals	Research and development	JAPAN
								Eisai	Eisai
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'	'	'	'	'	'	'	'	'
Total upfront and milestone payments	'	'	'	'	$100,000,000	$20,000,000	'	'	'
Amount recorded to reflect fair value of options granted	'	'	'	'	'	'	'	17,700,000	'
Milestone payments made during period	'	'	'	'	'	'	'	'	35,000,000
Research and development	447,273,000	327,463,000	976,157,000	611,803,000	'	20,000,000	45,000,000	'	21,600,000
Additional milestone payment	'	'	'	'	$1,000,000,000	$300,000,000	'	'	'

Collaborative_and_Other_Relati2

Collaborative and Other Relationships Collaborative and Other Relationships - Equity Method Investments (Details 1)

3 Months Ended

6 Months Ended

1 Months Ended

3 Months Ended

6 Months Ended

Jun. 30, 2014

Jun. 30, 2013

Jun. 30, 2014

Jun. 30, 2013

Feb. 29, 2012

Feb. 29, 2012

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2013

Jun. 30, 2014

Jun. 30, 2013

Jun. 30, 2014

Dec. 31, 2013

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 ($)

USD ($)

USD ($)

USD ($)

USD ($)

KRW

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

14,600,000

23,900,000

'

14,600,000

'

15,600,000,000

25,200,000,000

Percentage of stake in entity minimum

'

'

'

'

15.00%

15.00%

12.00%

'

'

12.00%

'

'

'

Percentage of stake in entity maximum

'

'

'

'

49.90%

49.90%

'

'

'

'

'

'

'

Recognized loss

-1,933,000

-2,289,000

-9,538,000

-6,100,000

'

'

-1,900,000

'

-2,300,000

-9,500,000

-6,100,000

'

'

Revenue related to technical development and technology transfer services

61,864,000

48,770,000

151,965,000

103,481,000

'

'

12,300,000

'

7,400,000

36,500,000

14,200,000

'

'

Research and development

447,273,000

327,463,000

976,157,000

611,803,000

'

'

'

36,000,000

'

'

'

'

'

Additional milestone payment

'

'

'

'

'

'

'

$85,000,000

'

'

'

'

'

Expected profit share with Samsung Bioepis

'

'

'

'

'

'

'

50.00%

'

'

'

'

'

Litigation_Details

Litigation (Details) (ITALY)	3 Months Ended
	Jun. 30, 2014	Dec. 31, 2011
ITALY	'	'
Loss Contingencies [Line Items]	'	'
Limited period for government reimbursement	'	'24 months
Automatic renewal period for the resolution	'	'24 months
Term of agreement to eliminate reimbursement limit	'24 months	'

Commitments_and_Contingencies_1

Commitments and Contingencies (Details) (Fumapharm AG [Member], USD $)	Jun. 30, 2014	Dec. 31, 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	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] | 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] | 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] | 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] | 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] | 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] | 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] | 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 | Five hundred million to one billion [Member]	'	'
Business Acquisition, Contingent Consideration [Line Items]	'	'
Sales level	500,000,000	'
Minimum | One billion to one point five billion [Member]	'	'
Business Acquisition, Contingent Consideration [Line Items]	'	'
Sales level	1,000,000,000	'
Minimum | One point five billion to two billion [Member]	'	'
Business Acquisition, Contingent Consideration [Line Items]	'	'
Sales level	1,500,000,000	'
Minimum | Two billion to two point five billion [Member]	'	'
Business Acquisition, Contingent Consideration [Line Items]	'	'
Sales level	2,000,000,000	'
Minimum | Two point five billion to three billion [Member]	'	'
Business Acquisition, Contingent Consideration [Line Items]	'	'
Sales level	2,500,000,000	'
Maximum | Five hundred million to one billion [Member]	'	'
Business Acquisition, Contingent Consideration [Line Items]	'	'
Sales level	1,000,000,000	'
Maximum | One billion to one point five billion [Member]	'	'
Business Acquisition, Contingent Consideration [Line Items]	'	'
Sales level	1,500,000,000	'
Maximum | One point five billion to two billion [Member]	'	'
Business Acquisition, Contingent Consideration [Line Items]	'	'
Sales level	2,000,000,000	'
Maximum | Two billion to two point five billion [Member]	'	'
Business Acquisition, Contingent Consideration [Line Items]	'	'
Sales level	2,500,000,000	'
Maximum | 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 $)	6 Months Ended		12 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Business Acquisition, Contingent Consideration [Line Items]	'	'	'
Payment of contingent payment for acquisition	$25,000,000	$0	'
Increase to goodwill	131,900,000	'	35,700,000
Fumapharm AG [Member] | TECFIDERA [Member]	'	'	'
Business Acquisition, Contingent Consideration [Line Items]	'	'	'
Payment of contingent payment for acquisition	25,000,000	'	'
Increase to goodwill	150,000,000	'	'
One billion [Member] | Fumapharm AG [Member]	'	'	'
Business Acquisition, Contingent Consideration [Line Items]	'	'	'
Cumulative sales level	1,000,000,000	'	1,000,000,000
Two billion [Member] | Fumapharm AG [Member]	'	'	'
Business Acquisition, Contingent Consideration [Line Items]	'	'	'
Cumulative sales level	$2,000,000,000	'	'

Segment_Information_Details

Segment Information (Details)	6 Months Ended
	Jun. 30, 2014
	segment
Segment Information (Textual) [Abstract]	'
Number of reportable segment	1