Alexion Pharmaceuticals (ALXN) 10-Q 6 May 20 2020 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information Document - shares	3 Months Ended
	Mar. 31, 2020	May 04, 2020
Document and Entity Information [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2020
Document Transition Report	false
Entity File Number	0-27756
Entity Registrant Name	ALEXION PHARMACEUTICALS, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	13-3648318
Entity Address, Address Line One	121 Seaport Boulevard
Entity Address, City or Town	Boston
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02210
City Area Code	475
Local Phone Number	230-2596
Title of 12(b) Security	Common Stock $0.0001 par value
Trading Symbol	ALXN
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Central Index Key	0000899866
Current Fiscal Year End Date	--12-31
Entity Common Stock, Shares Outstanding		220,827,431
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Assets
Cash and cash equivalents	$ 2,315	$ 2,685.5
Marketable securities	47.8	64
Trade accounts receivable, net	1,345.2	1,243.2
Inventories	586.8	627.6
Prepaid expenses and other current assets	575.4	456.1
Total current assets	4,870.2	5,076.4
Property, plant and equipment, net	1,159.9	1,163.3
Intangible assets, net	4,187.6	3,344.3
Goodwill	5,072.1	5,037.4
Right of use operating assets	203.2	204
Deferred tax assets	2,223.5	2,290.2
Other assets	432	429
Total assets	18,148.5	17,544.6
Liabilities and Stockholders' Equity
Accounts payable and accrued expenses	862.8	966.7
Current portion of long-term debt	126.7	126.7
Other current liabilities	130.4	100.9
Total current liabilities	1,119.9	1,194.3
Long-term debt, less current portion	2,343.3	2,375
Contingent consideration	358.9	192.4
Deferred tax liabilities	2,113.3	2,081.4
Noncurrent operating lease liabilities	162.8	164.1
Other liabilities	302.5	265.6
Total liabilities	6,400.7	6,272.8
Commitments and contingencies (Note 16)
Stockholders' Equity:
Common stock, $0.0001 par value; 290.0 shares authorized; 238.9 and 237.8 shares issued at March 31, 2020 and December 31, 2019, respectively	0	0
Additional paid-in capital	8,864.9	8,804.7
Treasury stock, at cost, 17.8 and 16.5 shares at March 31, 2020 and December 31, 2019, respectively	(2,213)	(2,105.9)
Accumulated other comprehensive loss	(101.5)	(66.8)
Retained earnings	5,197.4	4,639.8
Total stockholders' equity	11,747.8	11,271.8
Total liabilities and stockholders' equity	$ 18,148.5	$ 17,544.6

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares shares in Millions	Mar. 31, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	290	290
Common stock, shares issued	238.9	237.8
Treasury Stock, Shares	17.8	16.5

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Total revenues	$ 1,444.8	$ 1,140.4
Costs and expenses:
Cost of sales (exclusive of amortization of purchased intangible assets)	111.7	85.8
Research and development	200.9	195.9
Selling, general and administrative	319.9	281.5
Amortization of purchased intangible assets	73.7	80
Change in fair value of contingent consideration	5.8	(28.7)
Acquisition-related costs	38.1	0
Restructuring expenses	(0.8)	9.1
Total costs and expenses	749.3	623.6
Operating income	695.5	516.8
Other income and expense:
Investment (expense) income	(5.2)	42.5
Interest expense	(25.8)	(19.9)
Other income and (expense)	(0.9)	2.4
Income before income taxes	663.6	541.8
Income tax expense (benefit)	106	(46.1)
Net income	$ 557.6	$ 587.9
Earnings per common share
Basic	$ 2.52	$ 2.63
Diluted	$ 2.50	$ 2.61
Shares used in computing earnings per common share
Basic	221.6	223.8
Diluted	222.6	225.5
Net Product Sales
Total revenues	$ 1,444.6	$ 1,140.2
Other Revenue
Total revenues	$ 0.2	$ 0.2

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Comprehensive Income Statement - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Net income	$ 557.6	$ 587.9
Other comprehensive income (loss), net of tax:
Foreign currency translation	(8)	(2)
Unrealized (losses) gains on debt securities	(0.2)	0.2
Unrealized losses on hedging activities, net of tax of $(8.2) and $(2.2), respectively	(26.5)	(9)
Other comprehensive income (loss), net of tax	(34.7)	(10.8)
Comprehensive income	$ 522.9	$ 577.1

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Unrealized losses on hedging activities - tax effect	$ (8.2)	$ (2.2)

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Changes in Stockholder's Equity Statement - USD ($) shares in Millions, $ in Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Treasury Stock [Member]	AOCI Attributable to Parent [Member]	Retained Earnings [Member]
Balances, common shares at Dec. 31, 2018		236.2
Balances, value at Dec. 31, 2018	$ 9,165.3	$ 0	$ 8,539.1	$ (1,689.9)	$ (9.7)	$ 2,325.8
Balances, treasury shares at Dec. 31, 2018				12.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Repurchase of common stock, shares	0.1			0.1
Repurchase of common stock	$ (11.3)			$ (11.3)
Issuance of common stock under stock option and stock purchase plans, shares		0.1
Issuance of common stock under stock option and stock purchase plans	10.1		10.1
Issuance of restricted common stock, shares		0.7
Issuance of restricted common stock	0
Share-based compensation expense	55.7		55.7
Net income	587.9					587.9
Other comprehensive loss	(10.8)				(10.8)
Balances, treasury shares at Mar. 31, 2019				12.8
Balances, value at Mar. 31, 2019	9,706.6	$ 0	8,604.9	$ (1,701.2)	(20.5)	2,823.4
Balances, common shares at Mar. 31, 2019		237
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adoption of new accounting standards	(90.3)					(90.3)
Balances, common shares at Dec. 31, 2019		237.8
Balances, value at Dec. 31, 2019	$ 11,271.8	$ 0	8,804.7	$ (2,105.9)	(66.8)	4,639.8
Balances, treasury shares at Dec. 31, 2019	16.5			16.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Repurchase of common stock, shares	1.3			1.3
Repurchase of common stock	$ (107.1)			$ (107.1)
Issuance of common stock under stock option and stock purchase plans, shares		0.1
Issuance of common stock under stock option and stock purchase plans	2.8		2.8
Issuance of restricted common stock, shares		1
Issuance of restricted common stock	0
Share-based compensation expense	57.4		57.4
Net income	557.6					557.6
Other comprehensive loss	$ (34.7)				(34.7)
Balances, treasury shares at Mar. 31, 2020	17.8			17.8
Balances, value at Mar. 31, 2020	$ 11,747.8	$ 0	$ 8,864.9	$ (2,213)	$ (101.5)	$ 5,197.4
Balances, common shares at Mar. 31, 2020		238.9

Condensed Consolidated Statem_5

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Cash flows from operating activities:
Net income	$ 557.6		$ 587.9
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization	89.3		97.2
Change in fair value of contingent consideration	5.8		(28.7)
Share-based compensation expense	57.6		56.7
Deferred taxes (benefit)	49		(81.1)
Unrealized foreign currency loss	7.1		1.9
Unrealized (gain) loss on forward contracts	(15)		5.5
Unrealized loss (gain) on strategic equity investments	9.2		(33.8)
Other	13.7		(2.4)
Changes in operating assets and liabilities, excluding the effect of acquisitions:
Accounts receivable	(120.9)		(95.3)
Inventories	37.3		(11.2)
Prepaid expenses, right of use operating assets and other assets	(72.9)		(58.6)
Accounts payable, accrued expenses, lease liabilities and other liabilities	(68.2)		(8.2)
Net cash provided by operating activities	549.6		429.9
Cash flows from investing activities:
Purchases of available-for-sale debt securities	(19.4)		0
Proceeds from maturity or sale of available-for-sale debt securities	141.4		92.6
Purchases of mutual funds related to nonqualified deferred compensation plan	(6.9)		(5.8)
Proceeds from sale of mutual funds related to nonqualified deferred compensation plan	3.3		3.7
Purchases of property, plant and equipment	(12.2)		(36)
Payment for acquisition of business, net of cash acquired	(837.7)		0
Purchases of strategic equity investments and options	(34.5)		(43.8)
Purchase of intangible assets	0		(8)
Other	0		0.2
Net cash (used in) provided by investing activities	(766)		2.9
Cash flows from financing activities:
Payments on term loan	(32.6)		0
Payments on revolving credit facility	0		(250)
Repurchases of common stock	(107.1)		(11.3)
Net proceeds from issuance of common stock under share-based compensation arrangements	2.8		10.2
Other	(1.3)		(1.3)
Net cash used in financing activities	(138.2)		(252.4)
Effect of exchange rate changes on cash and cash equivalents and restricted cash	(13.2)		(1.4)
Net change in cash and cash equivalents and restricted cash	(367.8)		179
Cash and cash equivalents and restricted cash at beginning of period	2,723.6		1,367.3
Cash and cash equivalents and restricted cash at end of period	2,355.8	$ 2,723.6	1,546.3
Supplemental cash flow disclosures from investing and financing activities:
Contingent consideration issued in acquisitions	155		0
Fair value of strategic investment and purchase option	0		27.1
Operating ROU lease assets obtained in exchange for operating lease liabilities	3.7		0
Accounts payable and accrued expenses for purchases of property, plant and equipment and intangible assets	13		10.8
Cash and cash equivalents	2,315	$ 2,685.5	1,544.8
Restricted cash included in other current assets	40.7		0.1
Restricted cash included in other noncurrent assets	$ 0.1		$ 1.4

Business

Business	3 Months Ended
	Mar. 31, 2020
Business [Abstract]
Business	Business Alexion Pharmaceuticals, Inc. (Alexion, the Company, we, our or us) is a global biopharmaceutical company focused on serving patients and families affected by rare and devastating diseases through the discovery, development and commercialization of life-changing therapies. As the global leader in complement biology and inhibition for more than 20 years, Alexion has developed and commercializes two approved complement inhibitors to treat patients with paroxysmal nocturnal hemoglobinuria (PNH) and atypical hemolytic uremic syndrome (aHUS), as well as the first and only approved complement inhibitor to treat anti-acetylcholine receptor (AChR) antibody-positive generalized myasthenia gravis (gMG) and neuromyelitis optica spectrum disorder (NMOSD) in patients who are anti-aquaporin-4 (AQP4) antibody positive. Alexion also has two highly innovative enzyme replacement therapies and the first and only approved therapies for patients with life-threatening and ultra-rare metabolic disorders, hypophosphatasia (HPP) and lysosomal acid lipase deficiency (LAL-D). In addition to our marketed therapies, we have a diverse pipeline resulting from internal innovation and business development. Alexion focuses its research efforts on novel molecules and targets in the complement cascade and its development efforts on the core therapeutic areas of hematology, nephrology, neurology, metabolic disorders and cardiology. We were incorporated in 1992 under the laws of the State of Delaware.

Basis of Presentation and Princ

Basis of Presentation and Principles Of Consolidation	3 Months Ended
	Mar. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation and Principles of Consolidation	Basis of Presentation and Principles of Consolidation The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by accounting principles generally accepted in the United States for complete financial statements. These accounting principles were applied on a basis consistent with those of the consolidated financial statements contained in the Company's Annual Report on Form 10-K for the year ended December 31, 2019 . In our opinion, the accompanying unaudited condensed consolidated financial statements include all adjustments, consisting of only normal recurring adjustments, necessary for a fair statement of our financial statements for interim periods presented in accordance with accounting principles generally accepted in the United States. The condensed consolidated balance sheet as of December 31, 2019 was derived from audited annual financial statements but does not include all disclosures required by accounting principles generally accepted in the United States. These interim financial statements should be read in conjunction with the audited financial statements for the year ended December 31, 2019 included in our Annual Report on Form 10-K for the year ended December 31, 2019 . The results of operations for the three months ended March 31, 2020 are not necessarily indicative of the results to be expected for the full year or any other future periods. The financial statements of our subsidiaries with functional currencies other than the U.S. dollar are translated into U.S. dollars using period-end exchange rates for assets and liabilities, historical exchange rates for stockholders' equity and weighted average exchange rates for operating results. Translation gains and losses are included in accumulated other comprehensive income (loss), net of tax, in stockholders' equity. Foreign currency transaction gains and losses are included in the results of operations in other income and expense. The accompanying unaudited condensed consolidated financial statements include the accounts of Alexion Pharmaceuticals, Inc. and its subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Our significant accounting policies are described in Note 1 of the notes to the consolidated financial statements included in our Annual Report on Form 10-K for the year ended December 31, 2019 . Updates to our accounting policies, including impacts from the adoption of new accounting standards, are discussed below in this Note 2 . Reclassifications Certain items in the prior period’s condensed consolidated financial statements have been reclassified to conform to the current presentation. Use of Estimates Preparation of the condensed consolidated financial statements in conformity with U.S. GAAP requires us to make estimates, judgments and assumptions that may affect the reported amounts of assets, liabilities, revenues, expenses and disclosure of contingent liabilities in our condensed consolidated financial statements. Due to the COVID-19 pandemic, there has been uncertainty and disruption in the global economy and financial markets. The full extent to which the COVID-19 pandemic will directly or indirectly impact our business, results of operations and financial condition, including sales, expenses, reserves and allowances, manufacturing, clinical trials, research and development costs and employee-related amounts, will depend on future developments that are highly uncertain. We are not aware of any specific event or circumstance that would require an update to our estimates, judgments and assumptions or a revision of the carrying value of our assets or liabilities as of May 6, 2020 , the date of issuance of this Quarterly Report on Form 10-Q. These estimates may change, as new events occur and additional information is obtained. Actual results may differ from these estimates under different assumptions or conditions and such differences may be material. New Accounting Pronouncements Accounting Standards Update (ASU) 2019-12, “Income Taxes: Simplifying the Accounting for Income Taxes”: In December 2019, the Financial Accounting Standards Board (FASB) issued a new standard intended to simplify the accounting for income taxes by eliminating certain exceptions related to the approach for intraperiod tax allocation, the methodology for calculating income taxes in an interim period and the recognition of deferred tax liabilities for outside basis differences. The new standard also simplifies aspects of the accounting for franchise taxes and enacted changes in tax laws or rates and clarifies the accounting for transactions that result in a step-up in the tax basis of goodwill. The standard is effective for annual periods beginning after December 15, 2020 and interim periods within, with early adoption permitted. Adoption of the standard requires certain changes to be made prospectively, with some changes to be made retrospectively. We are currently assessing the impact of this standard on our financial condition and results of operations. ASU 2020-01, “Investments - Equity Securities, Investments - Equity Method and Joint Ventures, and Derivatives and Hedging - Clarifying the Interactions Between Topic 321, Topic 323, and Topic 815” : In January 2020, the FASB issued a new standard intended to clarify the interactions between ASC 321, ASC 323 and ASC 815. The new standard addresses accounting for the transition into and out of the equity method and measurement of certain purchased options and forward contracts to acquire investments. The standard is effective for annual and interim periods beginning after December 15, 2020, with early adoption permitted. Adoption of the standard requires changes to be made prospectively. We are currently assessing the impact of this standard on our financial condition and results of operations. ASU 2020-04, “Reference Rate Reform, Facilitation of the Effects of Reference Rate Reform on Financial Reporting" : In response to concerns about structural risks of interbank offered rates , and, particularly, the risk of cessation of the London Interbank Offered Rate (LIBOR), regulators around the world have undertaken reference rate reform initiatives to identify alternative reference rates that are more observable or transaction-based and less susceptible to manipulation. In March 2020, the FASB issued a new standard that provides optional guidance for a limited time to ease the potential burden in accounting for the effects of reference rate reform, including optional expedients and exceptions for the accounting implications of contracts, hedging relationships, and other transactions affected by reference rate reform if certain criteria are met. The amendments in this new standard only apply to contracts and hedging relationships that reference LIBOR or another reference rate expected to be discontinued due to reference rate reform. The expedients and exceptions provided by the standard do not apply to contract modifications made and hedging relationships entered into or evaluated after December 31, 2022. We are currently reviewing our contracts impacted by reference rate reform and are assessing the impact of this standard on our financial condition and results of operations. Recently Adopted Accounting Pronouncements ASU 2018-15, "Customer's Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract" : In August 2018, the FASB issued a new standard on a customer's accounting for implementation, set-up, and other upfront costs incurred in a cloud computing arrangement (CCA) that aligns the requirements for capitalizing implementation costs in a CCA service contract with existing internal-use software guidance. The standard also provides classification guidance on these implementation costs as well as additional quantitative and qualitative disclosures. The standard is effective for interim and annual periods beginning after December 15, 2019, with early adoption permitted, and can be adopted prospectively or retrospectively. We adopted the new standard on January 1, 2020 on a prospective basis. The adoption of this standard had no impact on our financial statements at the date of adoption; however, we anticipate the adoption of this standard will result in an increase in capitalized assets related to qualifying CCA implementation costs in future periods. Qualifying CCA implementation, set-up and other upfront costs incurred after January 1, 2020 are capitalized as other assets in our condensed consolidated balance sheets. These assets will be expensed over the term of the hosting arrangement and such expense will be presented within the same line item in our condensed consolidated statements of operations as the expense for fees for the associated hosting arrangement. These capitalized costs will be evaluated for impairment when events or changes in circumstances indicate that the carrying value of the capitalized implementation costs is not recoverable. For the period ended March 31, 2020, capitalized CCA implementation costs were not material. ASU 2016-13, "Measurement of Credit Losses on Financial Instruments" : In June 2016, the FASB issued a new standard intended to improve reporting requirements specific to loans, receivables and other financial instruments. The new standard requires that credit losses on financial assets measured at amortized cost be determined using an expected loss model, instead of the current incurred loss model, and requires that credit losses related to available-for-sale debt securities be recorded through an allowance for credit losses and limited to the amount by which carrying value exceeds fair value. The new standard also requires enhanced disclosure of credit risk associated with financial assets. The standard is effective for interim and annual periods beginning after December 15, 2019 with early adoption permitted. We adopted the new standard on January 1, 2020 and have completed our assessment of the standard based on the composition of our portfolio of financial instruments and current and forecasted economic conditions at that date. Our significant financial assets that are within the scope of the new standard consist of trade accounts receivable and available for sale debt securities. We have not historically experienced any material credit losses associated with our trade accounts receivable or available for debt securities. We monitor economic conditions, including volatility associated with international economies and the associated impacts on the financial markets and our business. We disaggregate our trade accounts receivable population into pools of similar risk characteristics based on underlying customer type and geographical location. Current expected credit loss allowances are estimated for each risk pool based on available information, including i) historical credit loss experience, ii) current economic conditions and, iii) reasonable and supportable forecasts of future economic conditions that may affect the collectibility of the recorded amounts. Based on the relevant facts and economic conditions as of the date of adoption, we concluded that the expected credit losses on our trade accounts receivable were immaterial. Additionally, unrealized losses on our available for sale investment portfolio were immaterial.

Acquisitions

Acquisitions	3 Months Ended
	Mar. 31, 2020
Business Combinations [Abstract]
Acquisitions	Acquisitions Business Combinations Achillion Pharmaceuticals, Inc. In October 2019, Alexion entered into a definitive agreement to acquire Achillion Pharmaceuticals, Inc. (Achillion), a clinical-stage biopharmaceutical company focused on the development of oral Factor D inhibitors. Achillion was developing oral small molecule Factor D inhibitors to treat people with complement alternative pathway-mediated rare diseases, such as PNH and C3 glomerulopathy (C3G). Achillion had two clinical stage medicines in development, including danicopan (ACH-4471/ALXN2040) and ACH-5228 (ALXN2050). The acquisition of Achillion closed on January 28, 2020. Under the terms of the agreement, we acquired all outstanding common stock of Achillion for $6.30 per share, or an aggregate of $926.2 , inclusive of the settlement of Achillion's outstanding equity awards. The acquisition was funded with cash on hand. The transaction includes the potential for additional consideration in the form of non-tradeable contingent value rights (CVRs), which will be paid to Achillion shareholders if certain clinical and regulatory milestones are achieved within specified periods. These include $1.00 per share for the U.S. Food and Drug Administration (FDA) approval of danicopan and $1.00 per share for the initiation of a Phase III clinical trial in ACH-5228. The transaction was accounted for as a business combination. The following table summarizes the total consideration transferred to acquire Achillion and the estimated fair value of the identified assets acquired and liabilities assumed at the acquisition date: Consideration Upfront payment to shareholders and option holders $ 926.2 Upfront payment, fair value of equity compensation attributable to the post-combination service period (20.0 ) Upfront cash paid, net 906.2 Contingent consideration 160.7 Contingent consideration, fair value of equity compensation attributable to the post-combination service period (5.7 ) Total consideration $ 1,061.2 Assets Acquired and Liabilities Assumed Cash and cash equivalents $ 68.5 Marketable securities 106.1 In-process research & development assets (IPR&D) 918.0 Goodwill 34.7 Deferred tax liabilities, net (59.8 ) Other assets and liabilities, net (6.3 ) Total net assets acquired $ 1,061.2 Our accounting for this acquisition is preliminary. The fair value estimates for the assets acquired and liabilities assumed were based on preliminary calculations, and our estimates and assumptions are subject to change as we obtain additional information for our estimates during the measurement period. The areas that rely on preliminary estimates that are not yet finalized relate to deferred tax liabilities, net, as a result of ongoing studies to determine the tax deductibility of certain acquisition-related costs and the valuation of historical net operating loss and income tax credit carryforwards. The initial fair value estimate of the contingent consideration in the form of non-tradeable CVRs was $160.7 , which was recorded as a noncurrent liability in our condensed consolidated balance sheet, including $5.7 related to compensation attributable to the post-combination service period. We determined the fair value of these milestone-related payment obligations using various estimates, including probabilities of success prior to expiration of the specified period, discount rates and the amount of time until the conditions of the milestone payments are expected to be met. This fair value measurement is based on significant inputs not observable in the market, representing Level 3 measurements within the fair value hierarchy. The resulting probability-weighted cash flows were discounted using a cost of debt rate ranging from 2.1% to 2.3% .The range of estimated milestone payments is from zero , if no milestones are achieved for any product, to $306.3 if certain development and regulatory milestones are achieved. Subsequent to the acquisition date, we have adjusted the contingent consideration to fair value with changes in fair value recognized in operating earnings. Changes in fair values reflect new information about the probability and timing of meeting the conditions of the milestone payments. In the absence of new information, changes in fair value will only reflect the interest component of contingent consideration related to the passage of time as development work progresses towards the potential achievement of the milestones. At March 31, 2020 , the fair value of the contingent consideration for the Achillion acquisition was $162.4 based on the probability-weighted cash flows, discounted using a cost of debt ranging from 1.6% to 1.8% . Changes in fair value of the contingent consideration associated with the Achillion acquisition for the three months ended March 31, 2020 was $1.7 . The aggregate fair value of equity compensation attributable to the post-combination service period was $25.7 . This amount was excluded from the total consideration transferred and was recognized as a charge to acquisition-related costs in our condensed consolidated statements of operations. These amounts were associated with the accelerated vesting of stock options previously granted to Achillion employees. Excluding the $5.7 of contingent consideration related to equity compensation attributable to the post-combination service period, such amounts were paid during the three months ended March 31, 2020 . Intangible assets associated with IPR&D relate to two development-stage programs, ACH-4471 (ALXN2040) and ACH-5228 (ALXN2050). The estimated fair value of $918.0 was determined using the excess earnings method, a variation of the income approach. The excess earnings method estimates the value of an intangible asset equal to the present value of the incremental after-tax cash flows attributable to that intangible asset. Some of the more significant assumptions utilized in our asset valuations included the estimated net cash flows for each asset, including net revenues, cost of sales, research and development and other operating expenses, the potential regulatory and commercial success rates, competitive trends impacting the assets, and tax rates. The fair value using the excess earnings method was determined using an estimated weighted average cost of capital for Achillion of 11.5% , which represents a rate of return that a market participant would expect for these assets. These fair value measurements were based on significant inputs not observable in the market and thus represent Level 3 fair value measurements. The excess of purchase price over the fair value of the assets acquired and liabilities assumed represents the goodwill resulting from the acquisition. The goodwill, which is not tax-deductible, has been recorded as a noncurrent asset and is not amortized, but is subject to an annual review for impairment. The factors that contributed to the recognition of goodwill include the value of the acquired workforce, synergies that are specific to our business and not available to market participants, and early research in preclinical Factor D inhibitors, as well as the effects of the establishment of a deferred tax liability for the acquired IPR&D intangible assets, which have no tax basis. We recorded a net deferred tax liability of $59.8 . This amount was primarily comprised of $205.3 of deferred tax liabilities relating to the IPR&D acquired, offset by $145.5 of deferred tax assets related to net operating loss carryforwards (NOLs), income tax credits, and other temporary differences. Achillion's results of operations are included in the condensed consolidated financial statements from the date of acquisition. For the three months ended March 31, 2020 , we recorded $13.9 of pre-tax operating losses associated with the operations of Achillion in our condensed consolidated statements of operations. We also recorded acquisition-related costs in connection with the acquisition during the three months ended March 31, 2020 as presented below. Pro forma financial information (unaudited) The following unaudited pro forma information presents the combined results of Alexion and Achillion as if the acquisition of Achillion had been completed on January 1, 2019, with adjustments to give effect to pro forma events that are directly attributable to the acquisition. The unaudited pro forma results do not reflect operating efficiencies or potential cost savings that may have resulted from the consolidation of operations. Accordingly, the unaudited pro forma financial information is not necessarily indicative of the results of operations had we completed the transaction on January 1, 2019. Three months ended March 31, 2020 2019 Pro forma revenue $ 1,444.8 $ 1,140.4 Pro forma net income 574.9 515.7 The unaudited pro forma consolidated results include pro forma adjustments related to non-recurring activity. Alexion and Achillion acquisition-related costs of $53.3 , net of tax, were excluded from income for the three months ended March 31, 2020 . These expenses were included in net income for the three months ended March 31, 2019 . Acquisition-Related Costs Acquisition-related costs recorded within the condensed consolidated statement of operations associated with our business combinations for the three months ended March 31, 2020 and 2019 include the following: Three months ended March 31, 2020 2019 Transaction costs (1) $ 1.4 $ — Integration costs 0.1 — Fair value of equity compensation attributable to the post-combination service period 25.7 — Restructuring-related costs (2) 10.9 — $ 38.1 $ — (1) Transaction costs include legal fees and costs incurred to effectuate the settlement of the outstanding options (2) Restructuring-related costs include severance payments and one-time short-term retention awards agreed to in connection with the acquisition

Inventories

Inventories	3 Months Ended
	Mar. 31, 2020
Inventory, Net [Abstract]
Inventories	Inventories The components of inventory are as follows: March 31, December 31, 2020 2019 Raw materials $ 49.6 $ 41.2 Work-in-process 108.8 180.8 Finished goods 428.4 405.6 $ 586.8 $ 627.6 As of March 31, 2020 and December 31, 2019 , the carrying value of capitalized inventory manufactured at production facilities that have not yet received regulatory approval was $67.3 and $60.5 , respectively.

Intangible Assets and Goodwill

Intangible Assets and Goodwill	3 Months Ended
	Mar. 31, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible Assets and Goodwill	Intangible Assets and Goodwill The following table summarizes the carrying amount of our intangible assets and goodwill, net of accumulated amortization: March 31, 2020 December 31, 2019 Estimated Cost Accumulated Net Cost Accumulated Net Licensing rights 3-8 $ 57.0 $ (35.6 ) $ 21.4 $ 57.0 $ (34.7 ) $ 22.3 Patents 7 10.5 (10.5 ) — 10.5 (10.5 ) — Purchased technology 6-16 4,710.5 (1,462.4 ) 3,248.1 4,710.5 (1,388.7 ) 3,321.8 Other intangibles 5 0.4 (0.3 ) 0.1 0.4 (0.2 ) 0.2 Acquired IPR&D Indefinite 918.0 — 918.0 — — — Total $ 5,696.4 $ (1,508.8 ) $ 4,187.6 $ 4,778.4 $ (1,434.1 ) $ 3,344.3 Goodwill Indefinite $ 5,075.0 $ (2.9 ) $ 5,072.1 $ 5,040.3 $ (2.9 ) $ 5,037.4 In connection with our acquisition of Achillion during the first quarter of 2020, we acquired IPR&D programs with a fair value of $918.0 and recorded goodwill of $34.7 . For additional information on our acquisition of Achillion, please see Note 3, Acquisitions . Amortization expense for the three months ended March 31, 2020 and 2019 was $74.7 and $80.5 , respectively. As of March 31, 2020 , assuming no changes in the gross cost basis of intangible assets, the total estimated amortization expense for finite-lived intangible assets is $224.0 for the nine months ending December 31, 2020 , and approximately $298.0 for each of the years ending December 31, 2021 through December 31, 2025 . As of March 31, 2020 , the net book value of our purchased technology includes $2,927.4 associated with the KANUMA intangible asset, which we acquired in the acquisition of Synageva BioPharma Corp. As part of our standard quarterly procedures, we reviewed the KANUMA asset as of March 31, 2020 and determined that there were no indicators of impairment. Cash flow models used in our assessments are based on our commercial experience to date with KANUMA and require the use of significant estimates, which include, but are not limited to, long-range pricing expectations and patient-related assumptions, including patient identification, conversion and retention rates. We will continue to review the related valuation and accounting of this asset as new information becomes available to us. The following summarizes the changes in the carrying amount of goodwill: March 31, 2020 Balance at December 31, 2019 $ 5,037.4 Goodwill resulting from the acquisition of Achillion 34.7 Balance at March 31, 2020 $ 5,072.1

Debt

Debt	3 Months Ended
	Mar. 31, 2020
Debt [Abstract]
Debt	Debt On June 7, 2018, we entered into an Amended and Restated Credit Agreement (the Credit Agreement), with Bank of America, N.A. as Administrative Agent. The Credit Agreement amended and restated our credit agreement dated as of June 22, 2015 (the Prior Credit Agreement). The Credit Agreement provides for a $1,000.0 revolving credit facility and a $2,612.5 term loan facility. The revolving credit facility and the term loan facility mature on June 7, 2023. Beginning with the quarter ending June 30, 2019, we are required to make payments of 5.00% of the original principal amount of the term loan facility annually, payable in equal quarterly installments. In connection with entering into the Credit Agreement and the Prior Credit Agreement, we paid an aggregate of $53.1 in financing costs in 2018. Financing costs are amortized as interest expense over the life of the debt. Amortization expense associated with deferred financing costs for each of the three months ended March 31, 2020 and 2019 was $1.2 . Remaining unamortized deferred financing costs as of March 31, 2020 and December 31, 2019 were $14.6 and $15.8 , respectively. We made principal payments of $32.6 on the term loan during the three months ended March 31, 2020 , and as of March 31, 2020 , we had $2,481.9 outstanding on the term loan. We had no outstanding borrowings under the revolving credit facility as of March 31, 2020 . As of March 31, 2020 , we had open letters of credit of $1.0 that offset our availability in the revolving credit facility. The amount outstanding under the term loan of $2,481.9 as of March 31, 2020 is subject to variable interest rates, which are based on current market rates, and as such, the Company believes the carrying amount of the obligation approximates fair value. We were in compliance with all applicable covenants under the Credit Agreement as of March 31, 2020 .

Earnings Per Common Share

Earnings Per Common Share	3 Months Ended
	Mar. 31, 2020
Earnings Per Common Share [Abstract]
Earnings Per Common Share	Earnings Per Common Share Basic earnings per common share (EPS) is computed by dividing net income by the weighted-average number of shares of common stock outstanding. For purposes of calculating diluted EPS, the denominator reflects the potential dilution that could occur if stock options, unvested restricted stock units or other contracts to issue common stock were exercised or converted into common stock, using the treasury stock method. The following table summarizes the calculation of basic and diluted EPS for the three months ended March 31, 2020 and 2019 : Three months ended March 31, 2020 2019 Net income used for basic and diluted calculation $ 557.6 $ 587.9 Shares used in computing earnings per common share—basic 221.6 223.8 Weighted-average effect of dilutive securities: Stock awards 1.0 1.7 Shares used in computing earnings per common share—diluted 222.6 225.5 Earnings per common share: Basic $ 2.52 $ 2.63 Diluted $ 2.50 $ 2.61 We exclude from diluted EPS the weighted-average number of securities whose effect is anti-dilutive. Excluded from the calculation of diluted EPS for the three months ended March 31, 2020 and 2019 were 3.2 and 2.6 shares of common stock, respectively, because their effect is anti-dilutive.

Marketable Securities

Marketable Securities	3 Months Ended
	Mar. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Marketable securities	Marketable Securities The amortized cost, gross unrealized holding gains, gross unrealized holding losses and fair value of available-for-sale debt securities by type of security as of March 31, 2020 and December 31, 2019 were as follows: March 31, 2020 Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Fair Value Commercial paper $ 134.2 $ — $ — $ 134.2 Corporate bonds 3.6 — (0.1 ) 3.5 Other government-related obligations: U.S. 60.0 — — 60.0 Bank certificates of deposit 6.5 — — 6.5 Total available-for-sale debt securities $ 204.3 $ — $ (0.1 ) $ 204.2 December 31, 2019 Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Fair Value Commercial paper $ 246.9 $ — $ — $ 246.9 Corporate bonds 24.3 — — 24.3 Other government-related obligations: U.S. 70.4 — — 70.4 Bank certificates of deposit 27.4 — — 27.4 Total available-for-sale debt securities $ 369.0 $ — $ — $ 369.0 The aggregate fair value of available-for-sale debt securities in an unrealized loss position as of March 31, 2020 and December 31, 2019 was $20.2 and $21.5 , respectively. We did not have any investments in a continuous unrealized loss position for more than twelve months as of March 31, 2020 and December 31, 2019 . As of March 31, 2020 , we believe that the cost basis of our available-for-sale debt securities is recoverable. No allowance for credit losses was recorded as of March 31, 2020 . The fair values of available-for-sale debt securities by classification in the condensed consolidated balance sheets were as follows: March 31, 2020 December 31, 2019 Cash and cash equivalents $ 179.1 $ 328.1 Marketable securities 25.1 40.9 $ 204.2 $ 369.0 The fair values of available-for-sale debt securities as of March 31, 2020 , by contractual maturity, are summarized as follows: March 31, 2020 Due in one year or less $ 204.2 Due after one year through three years — $ 204.2 We sponsor a nonqualified deferred compensation plan which allows certain highly-compensated employees to elect to defer income to future periods. Participants in the plan earn a return on their deferrals based on several investment options, which mirror returns on underlying mutual fund investments. We choose to invest in the underlying mutual fund investments to offset the liability associated with our nonqualified deferred compensation plan. These mutual fund investments are valued at net asset value per share and are carried at fair value with gains and losses included in investment income. The changes in the underlying liability to the employee are recorded in operating expenses. As of March 31, 2020 and December 31, 2019 , the fair value of these investments was $22.7 and $23.1 , respectively. We utilize the specific identification method in computing realized gains and losses. Realized gains and losses on our marketable securities were not material for the three months ended March 31, 2020 and 2019 .

Derivative Instruments and Hedg

Derivative Instruments and Hedging Activities	3 Months Ended
	Mar. 31, 2020
Derivative Instruments and Hedging Activities [Abstract]
Derivative Instruments and Hedging Activities	Derivative Instruments and Hedging Activities We operate internationally and, in the normal course of business, are exposed to fluctuations in foreign currency exchange rates. The exposures result from portions of our revenues, as well as the related receivables, and expenses that are denominated in currencies other than the U.S. dollar, primarily the Euro and Japanese Yen. We are also exposed to fluctuations in interest rates on outstanding borrowings under our revolving credit facility, if any, and term loan facility. We manage these exposures within specified guidelines through the use of derivatives. All of our derivative instruments are utilized for risk management purposes, and we do not use derivatives for speculative trading purposes. We enter into foreign exchange forward contracts, with durations of up to 60 months, to hedge exposures resulting from portions of our forecasted revenues, including intercompany revenues, and certain forecasted expenses that are denominated in currencies other than the U.S. dollar. The purpose of these hedges is to reduce the volatility of exchange rate fluctuations on our operating results. These hedges are designated as cash flow hedges upon contract inception. As of March 31, 2020 , we had open revenue related foreign exchange forward contracts with notional amounts totaling $1,096.6 that qualified for hedge accounting with current contract maturities through June 2021. As of March 31, 2020 , we had open expense related foreign exchange forward contracts with notional amounts totaling $12.7 that qualified for hedge accounting with contract maturities through September 2022. To achieve a desired mix of floating and fixed interest rates on our term loan, we enter into interest rate swap agreements that qualify for and are designated as cash flow hedges. These contracts convert the floating interest rate on a portion of our debt to a fixed rate, plus a borrowing spread. The following table summarizes the total interest rate swap contracts executed as of March 31, 2020 : Type of Interest Rate Swap Contract Notional Amount Effective Date Termination Date Fixed Interest Rate or Rate Range Floating to Fixed 450.0 December 2018 December 2022 2.60% - 2.79% Floating to Fixed 1,300.0 December 2019 December 2022 2.37% - 2.83% The amount of gains and (losses) recognized in the condensed consolidated statements of operations for the three months ended March 31, 2020 and 2019 from foreign exchange and interest rate swap contracts that qualified as cash flow hedges were as follows: Three months ended Three months ended March 31, 2020 March 31, 2019 Financial Statement Line Item in which the Effects of Cash Flow Hedges are Recorded Net Product Sales Interest Expense Net Product Sales Interest Expense Total amount presented in the Condensed Consolidated Statements of Operations $ 1,444.6 $ (25.8 ) $ 1,140.2 $ (19.9 ) Impact of cash flow hedging relationships: Foreign exchange forward contracts $ 11.4 $ — $ 7.0 $ — Interest rate swap contracts $ — $ (4.6 ) $ — $ 4.6 The impact on accumulated other comprehensive income (AOCI) and earnings from foreign exchange and interest rate swap contracts that qualified as cash flow hedges, for the three months ended March 31, 2020 and 2019 were as follows: Three months ended March 31, 2020 2019 Foreign Exchange Forward Contracts: Gain (loss) recognized in AOCI, net of tax $ 26.0 $ 14.2 Gain (loss) reclassified from AOCI to net product sales, net of tax $ 8.8 $ 5.4 Interest Rate Swap Contracts: Gain (loss) recognized in AOCI, net of tax $ (47.3 ) $ (14.2 ) Gain (loss) reclassified from AOCI to interest expense, net of tax $ (3.6 ) $ 3.6 Assuming no change in foreign exchange rates from market rates at March 31, 2020 , $27.8 of gains recognized in AOCI will be reclassified to revenue over the next 12 months. Assuming no change in LIBOR-based interest rates from market rates at March 31, 2020 , $42.1 of losses recognized in AOCI will be reclassified to interest expense over the next 12 months. Amounts recognized in AOCI for expense related foreign exchange forward contracts were immaterial as of March 31, 2020 . We enter into foreign exchange forward contracts, with durations of up to 7 months , designed to limit the balance sheet exposure of monetary assets and liabilities. We enter into these hedges to reduce the impact of fluctuating exchange rates on our operating results. Hedge accounting is not applied to these derivative instruments as gains and losses on these hedge transactions are designed to offset gains and losses on underlying balance sheet exposures. As of March 31, 2020 , the notional amount of foreign exchange contracts where hedge accounting is not applied was $1,066.7 . We recognized a gain (loss) of $16.2 and $3.2 , in other income and (expense) for the three months ended March 31, 2020 and 2019 , respectively, associated with the foreign exchange contracts not designated as hedging instruments. These amounts were partially offset by gains or losses on monetary assets and liabilities. The following tables summarize the fair value of outstanding derivatives as of March 31, 2020 and December 31, 2019 : March 31, 2020 Derivative Assets Derivative Liabilities Balance Sheet Fair Balance Sheet Fair Derivatives designated as hedging instruments: Foreign exchange forward contracts Prepaid expenses and other current assets $ 29.1 Other current liabilities $ 1.7 Foreign exchange forward contracts Other assets 1.1 Other liabilities 0.8 Interest rate swap contracts Prepaid expenses and other current assets — Other current liabilities 42.1 Interest rate swap contracts Other assets — Other liabilities 75.8 Derivatives not designated as hedging instruments: Foreign exchange forward contracts Prepaid expenses and other current assets 24.1 Other current liabilities 12.4 Total fair value of derivative instruments $ 54.3 $ 132.8 December 31, 2019 Derivative Assets Derivative Liabilities Balance Sheet Fair Balance Sheet Fair Derivatives designated as hedging instruments: Foreign exchange forward contracts Prepaid expenses and other current assets $ 12.7 Other current liabilities $ 6.2 Foreign exchange forward contracts Other assets 0.6 Other liabilities 1.1 Interest rate swap contracts Prepaid expenses and other current assets — Other current liabilities 19.5 Interest rate swap contracts Other assets — Other liabilities 41.9 Derivatives not designated as hedging instruments: Foreign exchange forward contracts Prepaid expenses and other current assets 17.2 Other current liabilities 20.4 Total fair value of derivative instruments $ 30.5 $ 89.1 Although we do not offset derivative assets and liabilities within our condensed consolidated balance sheets, our International Swap and Derivatives Association agreements provide for net settlement of transactions that are due to or from the same counterparty upon early termination of the agreement due to an event of default or other termination event. The following tables summarize the potential effect on our condensed consolidated balance sheets of offsetting our foreign exchange forward contracts and interest rate contracts subject to such provisions: March 31, 2020 Gross Amounts Not Offset in the Condensed Consolidated Balance Sheet Description Gross Amounts of Recognized Assets/Liabilities Gross Amounts Offset in the Condensed Consolidated Balance Sheet Net Amounts of Assets/Liabilities Presented in the Condensed Consolidated Balance Sheet Derivative Financial Instruments Cash Collateral Received (Pledged) Net Amount Derivative assets $ 54.3 $ — $ 54.3 $ (14.9 ) $ — $ 39.4 Derivative liabilities (132.8 ) — (132.8 ) 14.9 — (117.9 ) December 31, 2019 Gross Amounts Not Offset in the Condensed Consolidated Balance Sheet Description Gross Amounts of Recognized Assets/Liabilities Gross Amounts Offset in the Condensed Consolidated Balance Sheet Net Amounts of Assets/Liabilities Presented in the Condensed Consolidated Balance Sheet Derivative Financial Instruments Cash Collateral Received (Pledged) Net Amount Derivative assets $ 30.5 $ — $ 30.5 $ (21.4 ) $ — $ 9.1 Derivative liabilities (89.1 ) — (89.1 ) 21.4 — (67.7 )

Other Investments

Other Investments	3 Months Ended
	Mar. 31, 2020
Investments, All Other Investments [Abstract]
Other Investments	Other Investments Other investments include strategic investments in equity securities of certain biotechnology companies which we acquired in connection with strategic business development transactions, including license and option agreements. These investments are included in other assets in our condensed consolidated balance sheets. Moderna During 2014, we purchased $37.5 of preferred stock of Moderna Therapeutics, Inc. (Moderna), a privately held biotechnology company, which was initially recorded at cost. We began recording the investment at fair value, with the effects of a holding period restriction estimated using an option pricing valuation model, upon Moderna's completion of its initial public offering (IPO) in 2018. During the first quarter 2019, we recognized an unrealized gain of $29.8 to adjust our investment in Moderna to fair value as of March 31, 2019 . On December 9, 2019, we sold our investment in Moderna for $114.7 in net proceeds, resulting in a realized gain of $77.2 on our initial investment. Dicerna In October 2018, we purchased $10.3 of Dicerna Pharmaceuticals Inc. (Dicerna) common stock in connection with an agreement that we entered into with Dicerna, a publicly-traded biopharmaceutical company. As our equity investment in Dicerna common stock has a readily determinable fair value, we are recording the investment at fair value. During the three months ended March 31, 2020 and 2019 , we recognized an unrealized loss of $3.1 and an unrealized gain of $3.3 , respectively, in investment income to adjust our equity investment in Dicerna to fair value. The fair value of this investment was $15.3 and $18.4 as of March 31, 2020 and December 31, 2019 , respectively. Caelum In January 2019, we purchased $41.0 of preferred stock of Caelum Biosciences (Caelum), a privately-held biotechnology company, and a $16.1 option to acquire the remaining equity in Caelum, based on Phase II data, in connection with an agreement that we entered into with Caelum. Following discussions with the FDA, Caelum changed the design of its clinical development program and initiated expanded Phase II/III trials during the first quarter of 2020. In December 2019, we amended the terms of the agreement with respect to the option to acquire the remaining equity in Caelum based on data from the expanded Phase II/III trials. We accounted for the amendment as an exchange transaction as the terms of the modified option were determined to be substantially different than the terms of the original option. In conjunction with this amendment, we recognized a gain of $32.0 during the fourth quarter 2019 in other income and (expense), which reflects an increase in the fair value of the option, less $20.0 in incremental upfront funding which we accrued as of December 31, 2019 and paid during the first quarter 2020, and $4.1 associated with the change in the fair value of contingent payments which we also modified as part of the amendment. See Note 16, Commitments and Contingencies, for additional information on the agreement. As our equity investment in Caelum and the option to acquire the remaining equity in Caelum do not have a readily determinable fair value, we only adjust the carrying value of the assets for impairment and any subsequent changes resulting from an observable price change in an orderly transaction for identical or similar equity securities of the same issuer. There were no observable price changes associated with these assets during the three months ended March 31, 2020 and 2019 . As of March 31, 2020 and December 31, 2019 , the carrying value of the investment and option, respectively, was $41.0 and $64.0 . The investment and option were not impaired as of March 31, 2020 . Zealand In March 2019, we purchased $13.8 of Zealand Pharma A/S (Zealand) common stock in connection with an agreement that we entered into with Zealand, a publicly-traded biopharmaceutical company based in Copenhagen, Denmark. See Note 16, Commitments and Contingencies, for additional information on the agreement. As our equity investment in Zealand common stock has a readily determinable fair value, we are recording the investment at fair value. During the three months ended March 31, 2020 and 2019 , we recognized an unrealized loss of $0.2 and unrealized gain of $0.7 , respectively, in investment income to adjust our equity investment in Zealand to fair value. The fair value of this investment was $27.7 and $28.5 as of March 31, 2020 and December 31, 2019 , respectively. Eidos In September 2019, we purchased $19.9 of Eidos Therapeutics, Inc. (Eidos) common stock, in connection with an agreement that we entered into with Eidos, a publicly-traded biopharmaceutical company and subsidiary of BridgeBio Pharma, Inc. See Note 16, Commitments and Contingencies, for additional information on the agreement. As our equity investment in Eidos common stock has a readily determinable fair value, we are recording the investment at fair value, with the effects of a one year holding period restriction estimated using an option pricing valuation model. During the three months ended March 31, 2020 , we recognized an unrealized loss of $3.7 in investment income to adjust our equity investment in Eidos to fair value. The fair value of this investment was $24.1 and $27.8 as of March 31, 2020 and December 31, 2019 , respectively. Stealth In October 2019, we purchased $9.6 of Stealth BioTherapeutics Corp. (Stealth) common stock, in connection with an agreement that we entered into with Stealth, a publicly traded clinical-stage biotechnology company. As our equity investment in Stealth common stock has a readily determinable fair value, we are recording the investment at fair value. During the three months ended March 31, 2020 , we recognized an unrealized loss of $2.8 in investment income to adjust our equity investment in Stealth to fair value. The fair value of this investment was $1.6 and $4.4 as of March 31, 2020 and December 31, 2019 , respectively. Portola In March 2020, we purchased of $14.5 of common stock of Portola Pharmaceuticals, Inc., a publicly traded commercial-stage biotechnology company. As our equity investment in the common stock has a readily determinable fair value, we are recording the investment at fair value. During the three months ended March 31, 2020 , we recognized an unrealized gain of $0.6 in investment income to adjust our equity investment to fair value. The fair value of this investment was $15.1 as of March 31, 2020 .

Stockholders' Equity

Stockholders' Equity	3 Months Ended
	Mar. 31, 2020
Equity [Abstract]
Stockholders' Equity	Stockholders' Equity Share Repurchases In November 2012, our Board of Directors authorized a share repurchase program. In February 2017, our Board of Directors increased the amount that we are authorized to expend on future repurchases to $1,000.0 under our repurchase program, which superseded all prior repurchase programs. On October 22, 2019, the Board of Directors approved an additional share repurchase authorization of up to $1,000.0 . The repurchase program does not have an expiration date and we are not obligated to acquire a particular number of shares. The repurchase program may be discontinued at any time at our discretion. Under the program, we repurchased 1.3 and 0.1 shares of our common stock at a cost of $107.1 and $11.3 during the three months ended March 31, 2020 and 2019 , respectively. Subsequent to March 31, 2020 , we repurchased 0.4 shares of common stock under our repurchase program at a cost of $35.0 . As of May 4, 2020 , there is a total of $893.3

Other Comprehensive Income and

Other Comprehensive Income and Accumulated Other Comprehensive Income	3 Months Ended
	Mar. 31, 2020
Accumulated Other Comprehensive Income [Abstract]
Other Comprehensive Income and Accumulated Other Comprehensive Income	Other Comprehensive Income and Accumulated Other Comprehensive Income The following tables summarize the changes in AOCI, by component, for the three months ended March 31, 2020 and 2019 : Defined Benefit Pension Plans Unrealized Gains (Losses) from Debt Securities Unrealized Gains (Losses) from Hedging Activities Foreign Currency Translation Adjustment Total Accumulated Other Comprehensive Income (Loss) Balances, December 31, 2019 $ (9.2 ) $ (0.1 ) $ (40.1 ) $ (17.4 ) $ (66.8 ) Other comprehensive income (loss) before reclassifications — (0.2 ) (21.3 ) (8.0 ) (29.5 ) Amounts reclassified from other comprehensive income — — (5.2 ) — (5.2 ) Net other comprehensive income (loss) — (0.2 ) (26.5 ) (8.0 ) (34.7 ) Balances, March 31, 2020 $ (9.2 ) $ (0.3 ) $ (66.6 ) $ (25.4 ) $ (101.5 ) Defined Benefit Pension Plans Unrealized Gains (Losses) from Debt Securities Unrealized Gains (Losses) from Hedging Activities Foreign Currency Translation Adjustment Total Accumulated Other Comprehensive Income (Loss) Balances, December 31, 2018 $ (2.6 ) $ (0.3 ) $ 9.6 $ (16.4 ) $ (9.7 ) Other comprehensive income (loss) before reclassifications — 0.2 — (2.0 ) (1.8 ) Amounts reclassified from other comprehensive income — — (9.0 ) — (9.0 ) Net other comprehensive income (loss) — 0.2 (9.0 ) (2.0 ) (10.8 ) Balances, March 31, 2019 $ (2.6 ) $ (0.1 ) $ 0.6 $ (18.4 ) $ (20.5 ) The table below provides details regarding significant reclassifications from AOCI during the three months ended March 31, 2020 and 2019 : Details about Accumulated Other Comprehensive Income Components Amount Reclassified From Accumulated Other Comprehensive Income during the three months ended March 31, Affected Line Item in the Condensed Consolidated Statements of Operations 2020 2019 Unrealized Gains (Losses) on Hedging Activity Foreign exchange forward contracts $ 11.4 $ 7.0 Net product sales Interest rate swap contracts (4.6 ) 4.6 Interest expense 6.8 11.6 (1.6 ) (2.6 ) Income tax (benefit) expense $ 5.2 $ 9.0

Fair Value Measurement

Fair Value Measurement	3 Months Ended
	Mar. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurement	Fair Value Measurement Authoritative guidance establishes a valuation hierarchy for disclosure of the inputs to the valuation used to measure fair value. This hierarchy prioritizes the inputs into three broad levels as follows. Level 1 inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities. Level 2 inputs are quoted prices for similar assets and liabilities in active markets or inputs that are observable for the asset or liability, either directly or indirectly through market corroboration, for substantially the full term of the financial instrument. Level 3 inputs are unobservable inputs based on our own assumptions used to measure assets and liabilities at fair value. The following tables present information about our assets and liabilities that are measured at fair value on a recurring basis as of March 31, 2020 and December 31, 2019 , and indicate the fair value hierarchy of the valuation techniques we utilized to determine such fair value. Fair Value Measurement at Balance Sheet Type of Instrument Total Level 1 Level 2 Level 3 Cash equivalents Money market funds $ 679.4 $ — $ 679.4 $ — Cash equivalents Commercial paper $ 119.1 $ — $ 119.1 $ — Cash equivalents Other government-related obligations $ 60.0 $ — $ 60.0 $ — Marketable securities Mutual funds $ 22.7 $ 22.7 $ — $ — Marketable securities Commercial paper $ 15.1 $ — $ 15.1 $ — Marketable securities Corporate bonds $ 3.5 $ — $ 3.5 $ — Marketable securities Bank certificates of deposit $ 6.5 $ — $ 6.5 $ — Other assets Equity securities $ 83.8 $ 59.7 $ 24.1 $ — Prepaid expenses and other current assets Foreign exchange forward contracts $ 53.2 $ — $ 53.2 $ — Other assets Foreign exchange forward contracts $ 1.1 $ — $ 1.1 $ — Other current liabilities Foreign exchange forward contracts $ 14.1 $ — $ 14.1 $ — Other liabilities Foreign exchange forward contracts $ 0.8 $ — $ 0.8 $ — Other current liabilities Interest rate contracts $ 42.1 $ — $ 42.1 $ — Other liabilities Interest rate contracts $ 75.8 $ — $ 75.8 $ — Contingent consideration Acquisition-related contingent consideration $ 358.9 $ — $ — $ 358.9 Other current liabilities Other contingent payments $ 26.3 $ — $ — $ 26.3 Fair Value Measurement at Balance Sheet Type of Instrument Total Level 1 Level 2 Level 3 Cash equivalents Money market funds $ 635.9 $ — $ 635.9 $ — Cash equivalents Commercial paper $ 227.9 $ — $ 227.9 $ — Cash equivalents Corporate bonds $ 20.6 $ — $ 20.6 $ — Cash equivalents Bank certificates of deposit $ 19.2 $ — $ 19.2 $ — Cash equivalents Other government-related obligations $ 60.4 $ — $ 60.4 $ — Marketable securities Mutual funds $ 23.1 $ 23.1 $ — $ — Marketable securities Commercial paper $ 19.0 $ — $ 19.0 $ — Marketable securities Corporate bonds $ 3.7 $ — $ 3.7 $ — Marketable securities Other government-related obligations $ 10.0 $ — $ 10.0 $ — Marketable securities Bank certificates of deposit $ 8.2 $ — $ 8.2 $ — Other assets Equity securities $ 79.0 $ 51.2 $ 27.8 $ — Prepaid expenses and other current assets Foreign exchange forward contracts $ 29.9 $ — $ 29.9 $ — Other assets Foreign exchange forward contracts $ 0.6 $ — $ 0.6 $ — Other current liabilities Foreign exchange forward contracts $ 26.6 $ — $ 26.6 $ — Other liabilities Foreign exchange forward contracts $ 1.1 $ — $ 1.1 $ — Other current liabilities Interest rate contracts $ 19.5 $ — $ 19.5 $ — Other liabilities Interest rate contracts $ 41.9 $ — $ 41.9 $ — Contingent consideration Acquisition-related contingent consideration $ 192.4 $ — $ — $ 192.4 Other current liabilities Other contingent payments $ 24.0 $ — $ — $ 24.0 There were no securities transferred between Level 1, 2 and 3 during the three months ended March 31, 2020 . Valuation Techniques We classify mutual fund investments and equity securities, which are valued based on quoted market prices in active markets with no valuation adjustment, as Level 1 assets within the fair value hierarchy. Cash equivalents and marketable securities classified as Level 2 within the valuation hierarchy include money market funds, commercial paper, U.S. and foreign government-related debt, corporate debt securities and certificates of deposit. We estimate the fair values of these marketable securities by taking into consideration valuations obtained from third-party pricing sources. These pricing sources utilize industry standard valuation models, including both income and market-based approaches, for which all significant inputs are observable, either directly or indirectly, to estimate fair value. These inputs include market pricing based on real-time trade data for similar securities, issuer credit spreads, benchmark yields, and other observable inputs. We validate the prices provided by our third-party pricing sources by understanding the models used, obtaining market values from other pricing sources and analyzing pricing data in certain instances. Other investments in equity securities of publicly traded companies which are subject to holding period restrictions are carried at fair value using an option pricing valuation model and classified as Level 2 equity securities within the fair value hierarchy. The most significant assumptions within the option pricing valuation model are the term of the restrictions and the stock price volatility, which is based upon the historical volatility of the applicable company or similar companies. We also use a constant maturity risk-free interest rate to match the remaining term of the restrictions on such investments. Our derivative assets and liabilities include foreign exchange and interest rate derivatives that are measured at fair value using observable market inputs such as forward rates, interest rates, our own credit risk as well as an evaluation of our counterparties’ credit risks. Based on these inputs, the derivative assets and liabilities are classified within Level 2 of the valuation hierarchy. Contingent consideration liabilities related to business acquisitions and derivative liabilities associated with other contingent payments are classified as Level 3 within the valuation hierarchy and are valued based on various estimates, including probability of success, discount rates and amount of time until the conditions of the milestone payments are anticipated to be met. As of March 31, 2020 , there has not been any impact to the fair value of our derivative liabilities due to our own credit risk. Similarly, there has not been any significant adverse impact to our derivative assets based on our evaluation of our counterparties’ credit risks. Acquisition-Related Contingent Consideration In connection with our business combinations, we may be required to pay future consideration that is contingent upon the achievement of specified development, regulatory approvals or sales-based milestone events. We determine the fair value of these obligations using various estimates that are not observable in the market and represent a Level 3 measurement within the fair value hierarchy. As of March 31, 2020 , the resulting probability-weighted cash flows were discounted using a cost of debt ranging from 1.6% to 1.8% for developmental and regulatory milestones and a weighted average cost of capital of 9.0% for sales-based milestones. As of December 31, 2019 , the resulting probability-weighted cash flows were discounted using a weighted average cost of capital of 9.0% for sales-based milestones. Each reporting period, we adjust the contingent consideration to fair value with changes in fair value recognized in operating earnings. Changes in fair values reflect new information about the probability and anticipated timing of meeting the conditions of the milestone payments. In the absence of new information, changes in fair value will only reflect the interest component of contingent consideration related to the passage of time. As of March 31, 2020 , estimated future contingent milestone payments related to our business combinations range from zero if no milestone events are achieved, to a maximum of $908.3 if all development, regulatory and sales-based milestones are reached. As of March 31, 2020 , the fair value of acquisition-related contingent consideration was $358.9 . Amounts issued during the period represent the fair value of the non-tradeable CVRs recorded in connection with the acquisition of Achillion. See Note 3, Acquisitions . The following table represents a roll-forward of our acquisition-related contingent consideration: March 31, 2020 Balance at beginning of period $ 192.4 Amounts issued 160.7 Changes in fair value 5.8 Balance at end of period $ 358.9 Other Contingent Payments In January 2019, we entered into an agreement with Caelum, a biotechnology company that is developing CAEL-101 for light chain (AL) amyloidosis. Under the terms of the agreement, we acquired a minority equity interest in preferred stock of Caelum and an exclusive option to acquire the remaining equity in Caelum based on Phase II data, for pre-negotiated economics. We paid $30.0 during the first quarter 2019 and agreed to pay up to an additional $30.0 in contingent development milestones prior to our exercise of the option to acquire the remaining equity in Caelum. These contingent payments meet the definition of a derivative liability and were initially recorded at fair value of $27.1 , based on the probability-weighted cash flows, discounted using a cost of debt ranging from 3.3% to 3.5% . In December 2019, following FDA feedback which resulted in the redesign and expansion of Caelum's planned clinical development program for CAEL-101, we amended the terms of our existing option agreement with Caelum. The amendment modified the terms of the option to acquire the remaining equity in Caelum based on data from the expanded Phase II/III trials. The amendment also modified the development-related milestone events associated with the initial $30.0 in contingent payments, provided for an additional $20.0 in upfront funding, which we accrued as of December 31, 2019 and paid during the first quarter 2020, as well as funding of $60.0 in exchange for an additional equity interest at fair value upon achievement of a specific development-related milestone event. Following the amendment, the $30.0 in contingent payments continues to meet the definition of a derivative liability. Each reporting period, we adjust the derivative liability associated with the contingent payments to fair value with changes in fair value recognized in other income and expense. Changes in fair values reflect new information about the probability and anticipated timing of meeting the conditions of the milestone payments. In the absence of new information, changes in fair value will only reflect the interest component of the liability related to the passage of time. As of March 31, 2020 , the fair value of our contingent payments was $26.3 , based on the probability-weighted cash flows, discounted using a cost of debt of 2.1% . We recorded $2.3 in other income and (expense) during the three months ended March 31, 2020 related to the change in the fair value of the liability. The change in fair value was immaterial for the three months ended March 31, 2019. In April 2020, we paid an aggregate of $15.0

Revenue Recognition

Revenue Recognition	3 Months Ended
	Mar. 31, 2020
Revenue from Contract with Customer [Abstract]
Revenue Recognition	Revenue Recognition Disaggregation of Revenue The Company disaggregates revenue from contracts with customers into product and geographical regions as summarized below. Three months ended March 31, 2020 2019 SOLIRIS United States $ 556.2 $ 463.7 Europe 263.5 264.5 Asia Pacific 87.1 100.9 Rest of World 116.1 132.9 Total $ 1,022.9 $ 962.0 ULTOMIRIS United States $ 131.5 $ 24.6 Europe 33.8 — Asia Pacific 57.1 — Rest of World 0.4 — Total $ 222.8 $ 24.6 STRENSIQ United States $ 128.1 $ 99.5 Europe 24.0 17.5 Asia Pacific 13.6 9.9 Rest of World 6.5 3.2 Total $ 172.2 $ 130.1 KANUMA United States $ 16.4 $ 13.8 Europe 7.5 6.3 Asia Pacific 0.9 0.8 Rest of World 1.9 2.6 Total $ 26.7 $ 23.5 Total Net Product Sales $ 1,444.6 $ 1,140.2 Contract Balances and Receivables Contract liabilities relate to consideration received and/or billed for goods that have not been delivered to the customer and for which the performance obligation has not yet been completed. These amounts are included within other current liabilities in the condensed consolidated balance sheets. The following table provides information about receivables and contract liabilities from our contracts with customers. March 31, 2020 December 31, 2019 Receivables, which are included in "Trade accounts receivable, net" $ 1,345.2 $ 1,243.2 Contract liabilities, which are included in "Other current liabilities" $ 22.9 $ 6.8

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes Coronavirus Aid, Relief and Economic Security Act In response to the market volatility and instability resulting from the COVID-19 pandemic, the Coronavirus Aid, Relief, and Economic Security Act (CARES Act) was signed into law on March 27, 2020. The CARES Act lifts certain deduction limitations originally imposed by the Tax Cuts and Jobs Act of 2017 (2017 Tax Act). Under the 2017 Tax Act, federal net operating losses (NOLs) generated after 2017 could not be carried back and utilization was limited to 80% of taxable income. The CARES Act allows for a five-year carryback of federal NOLs generated in 2018 through 2020 and eliminates the 80% taxable income limitation by allowing corporate entities to fully utilize NOL carryforwards to offset taxable income in 2018 through 2020. In addition, the CARES Act generally allows taxpayers to deduct interest up to 50% of adjusted taxable income (30% limit under the 2017 Tax Act) for tax years 2019 and 2020. The CARES Act also allows taxpayers with prior year alternative minimum tax (repealed by the 2017 Tax Act) (AMT) credits to accelerate refund claims to tax years beginning in 2018 and 2019 instead of recovering the credits over a period of years, as originally enacted by the 2017 Tax Act. Additionally, the CARES Act raises the corporate charitable deduction limit to 25% of taxable income and provides a technical correction to the 2017 Tax Act to generally provide qualified improvement property a 15-year cost-recovery period and allow 100% bonus depreciation. The enactment of the CARES Act did not result in any material adjustments to our income tax provision for the three months ended March 31, 2020, or to our U.S. federal and state net deferred tax liabilities as of March 31, 2020. Tax Rate The following table provides a comparative summary of our income tax expense and effective income tax rate for the three months ended March 31, 2020 and 2019 : Three months ended March 31, 2020 2019 Income tax expense (benefit) $ 106.0 $ (46.1 ) Effective income tax rate 16.0 % (8.5 )% Income tax expense (benefit) is attributable to the U.S. federal, state and foreign income taxes on our profitable operations. The increase in the effective tax rate for the three months ended March 31, 2020 as compared to the same period in the prior year is primarily attributable to the one-time tax benefits recorded during the three months ended March 31, 2019 , in connection with the future integration of intellectual property of Wilson Therapeutics into the Alexion corporate structure. The deferred tax benefits included $95.7 and $30.3 associated with a tax election made with respect to intellectual property of Wilson Therapeutics and a valuation allowance release and corresponding recognition of net operating losses, respectively. These deferred tax benefits decreased the effective tax rate for the three months ended March 31, 2019 by approximately 23.3% . In April 2020 we became aware of a European withholding tax regulation that could be interpreted to apply to certain of our previous intra-group transactions. We are evaluating whether the interpretation of this regulation could apply to our facts and circumstances, and, upon conclusion of our analysis, we may establish a reserve related to this matter during the second quarter of 2020. An estimate of the impact on the financial statements cannot be made at this time. In 2017, the Internal Revenue Service (IRS) commenced an examination of our U.S. income tax returns for 2015. We anticipate this audit will conclude within the next twelve months. We have not been notified of any significant adjustments proposed by the IRS. It is reasonably possible that previously unrecognized tax benefits could be recognized upon the conclusion of the IRS examination. At this time, an estimate of the change in unrecognized tax benefits cannot be made. We have recorded tax on the undistributed earnings of our controlled foreign corporation (CFC) subsidiaries. To the extent CFC earnings may not be repatriated to the U.S. as a dividend distribution due to limitations imposed by law, we have not recorded the related potential withholding, foreign, local, and U.S. state income taxes. We continue to maintain a valuation allowance against certain deferred tax assets where realization is not certain.

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Asset Acquisition and In-License Agreements We have entered into asset purchase agreements, license agreements, and option arrangements in order to advance and obtain technologies and services related to our business. These agreements generally require us to pay an initial fee and certain agreements call for future payments upon the attainment of agreed upon development, regulatory and/or commercial milestones. These agreements may also require minimum royalty payments based on sales of products developed from the applicable technologies, if any. In January 2019, we entered into an agreement with Caelum, a biotechnology company that is developing CAEL101 for light chain (AL) amyloidosis. Under the terms of the agreement, we acquired a minority equity interest in preferred stock of Caelum and an exclusive option to acquire the remaining equity in Caelum based on Phase II data, for pre-negotiated economics. We paid $30.0 in the first quarter 2019 and agreed to pay up to an additional $30.0 in contingent development milestones prior to the exercise the option to acquire the remaining equity in Caelum. These contingent payments meet the definition of a derivative liability and were initially recorded at fair value of $27.1 . We allocated the total consideration of $57.1 , inclusive of the fair value of the contingent payments, to the equity investment in Caelum and the option to acquire the remaining equity in Caelum based on the relative fair values of the assets. Following discussions with the FDA, Caelum changed the design of its clinical development program and initiated expanded Phase II/III trials during the first quarter 2020. In December 2019, we amended the terms of the agreement with Caelum to modify the option to acquire the remaining equity in Caelum based on data from the expanded Phase II/III trials. The amendment also modified the development-related milestone events associated with the initial $30.0 in contingent payments, provided for an additional $20.0 in upfront funding, which we accrued as of December 31, 2019 and paid during the first quarter 2020 , as well as funding of $60.0 in exchange for an additional equity interest at fair value upon achievement of a specific development-related milestone event. The agreement with Caelum also provides for additional payments, in the event Alexion exercises the purchase option, for up to $500.0 , which includes an upfront option exercise payment and potential regulatory and commercial milestone payments. In April 2020, we paid an aggregate of $15.0 of contingent payments to Caelum related to two development-based milestones. In March 2019, we entered into an agreement with Zealand which provides us with exclusive worldwide licenses, as well as development and commercial rights, for subcutaneously delivered preclinical peptide therapies directed at up to four complement pathway targets. Pursuant to the agreement, Zealand will lead joint discovery and research efforts through the preclinical stage, and Alexion will lead development efforts beginning with the investigational new drug filing and Phase I studies. In addition to the agreement, we made an equity investment in Zealand (see Note 10, Other Investments ). Under the terms of the agreement, we made an upfront payment of $40.0 for an exclusive license to the lead target and the equity investment, as well as for preclinical research services to be performed by Zealand in relation to the lead target. The market value of the equity investment was $13.8 as of the date of acquisition, which we recorded in other assets in our condensed consolidated balance sheets. We also recognized prepaid research and development expense of $5.0 within the condensed consolidated balance sheets associated with the research activities to be performed by Zealand. Due to the early stage of the asset we are licensing, we recorded the upfront license payment of $21.2 as research and development expense during the first quarter 2019. As of March 31, 2020 , we could be required to pay up to $610.0 , for the lead target, upon the achievement of specified development, regulatory and commercial milestones, as well as royalties on commercial sales. In addition, we could be required to pay up to an additional $115.0 in development and regulatory milestones if both a long-acting and short-acting product are developed with respect to the lead target. Each of the three subsequent targets can be selected for an option fee of $15.0 and has the potential for additional development, regulatory and commercial milestones, as well as royalty payments, at a reduced price to the lead target. In April 2019, we entered into an agreement with Affibody AB (Affibody), through which Alexion obtained an exclusive worldwide license, as well as development and commercial rights, to ABY-039, a bivalent antibody-mimetic that targets the FcRn. Under the terms of the agreement, we made an upfront payment of $25.0 for the exclusive license to ABY-039. Due to the early stage of the asset we licensed, we recorded the upfront license payment as research and development expense during the second quarter 2019. In February 2020, based on data from our Phase I study, we terminated the agreement to co-develop ABY-039 with Affibody. In September 2019, we entered into an agreement with Eidos through which Alexion obtained an exclusive license to develop and commercialize AG10 in Japan. AG10 is a small molecule designed to treat the root cause of transthyretin amyloidosis (ATTR) and is currently in a Phase III study in the U.S. and Europe for ATTR cardiomyopathy (ATTR-CM). In addition, we made an equity investment in Eidos (see Note 10, Other Investments ). Under the terms of the agreement, we made an upfront payment of $50.0 for the exclusive license to AG10 in Japan and the equity investment. The market value of the equity investment was $19.9 as of the date of acquisition, which we recorded in other assets in our condensed consolidated balance sheets. Due to the early stage of the asset we are licensing, we recorded the upfront license payment of $30.1 as research and development expense during the third quarter 2019. As of March 31, 2020 , we could also be required to pay $30.0 upon achievement of a Japanese-based regulatory milestone as well as royalties on commercial sales. In October 2018, we entered into a collaboration agreement with Dicerna that provides us with exclusive worldwide licenses and development and commercial rights for two preclinical RNA interference (RNAi) subcutaneously delivered molecules for complement-mediated diseases, as well as an exclusive option for other preclinical RNAi molecules for two additional targets within the complement pathway. In addition to the collaboration agreement, we made an equity investment in Dicerna. Under the terms of the agreements, we made an upfront payment of $37.0 for the exclusive licenses and the equity investment. The market value of the equity investment was $10.3 as of the date of acquisition, which we recorded in other assets in our condensed consolidated balance sheets. Due to the early stage of the assets we are licensing, we recorded the upfront license payment of $26.7 as research and development expense during the fourth quarter 2018. In December 2019, we exercised our option for exclusive rights to two additional targets within the complement pathway under an existing agreement with Dicerna, which expands our existing research collaboration and license agreement with Dicerna to include a total of four targets within the complement pathway. In connection with the option exercise, we paid Dicerna $20.0 , which we recorded as research and development expense in the fourth quarter 2019. As of March 31, 2020 , we could be required to pay up to $614.1 for amounts due upon the achievement of specified research, development, regulatory and commercial milestones on the four licensed targets, as well as royalties on commercial sales. In December 2017, we entered into a collaboration and license agreement with Halozyme Therapeutics, Inc. that allows us to use drug-delivery technology in the development of subcutaneous formulations for our portfolio of products for up to four targets. Due to the early stage of the assets we are licensing, we recorded expense for the upfront payment of $40.0 during the fourth quarter 2017. In addition, as of March 31, 2020 , we could be required to pay up to $160.0 for each target developed, subject to achievement of specified development, regulatory and sales-based milestones, as well as royalties on commercial sales. In connection with our prior acquisition of Syntimmune, Inc., a clinical-stage biotechnology company developing an antibody therapy targeting the neonatal Fc receptor (FcRn), we could be required to pay up to $800.0 upon the achievement of specified development, regulatory and commercial milestones. In addition, excluding accrued milestones, as of March 31, 2020 , we have other license agreements under which we may be required to pay up to an additional $42.5 for currently licensed targets, if certain development, regulatory and commercial milestones are met. Additional amounts may be payable if we elect to acquire licenses to additional targets, as applicable, under the terms of these agreements. Asset Sale and Out-License Arrangements In connection with prior asset sale and out-license arrangements, Alexion is entitled to receive contingent payments upon the achievement of various regulatory and commercial milestones and other events, as well as royalties on commercial sales. The amount of contingent consideration related to these agreements is fully constrained and therefore has not been recognized as of March 31, 2020 . Manufacturing Agreements We have various manufacturing development and license agreements to support our clinical and commercial product needs. We rely on Lonza, a third party manufacturer, to produce a portion of commercial and clinical quantities of our commercial products and product candidates. We have various manufacturing and license agreements with Lonza, with remaining total non-cancellable future commitments of approximately $1,053.2 . This amount includes $97.7 of undiscounted, fixed payments applicable to our CMO embedded lease arrangement with Lonza. If we terminate certain supply agreements with Lonza without cause, we will be required to pay for product scheduled for manufacture under our arrangement. Under an existing arrangement with Lonza, we also pay Lonza a royalty on sales of SOLIRIS that was manufactured at the Alexion Rhode Island Manufacturing Facility (ARIMF facility) prior to the sale of the facility and a payment with respect to sales of SOLIRIS manufactured at Lonza facilities. We also pay Lonza a royalty on the sales of ULTOMIRIS. In addition to our commitments with Lonza, as of March 31, 2020 we have non-cancellable commitments of approximately $64.6 through 2020 with other third party manufacturers. Contingent Liabilities We are currently involved in various claims, disputes, lawsuits, investigations, administrative proceedings and legal proceedings. On a quarterly basis, we review the status of each significant matter and assess its potential financial exposure. In accordance with generally accepted accounting principles, if the potential loss from any claim, asserted or unasserted, or legal proceeding is considered probable and the amount can be reasonably estimated, we accrue a liability for the estimated loss. Because of uncertainties related to claims, proceedings and litigation, accruals are based on our best estimates based on information available at the time of the assessment. On a periodic basis, as additional information becomes available, or based on specific events such as the outcome of litigation, court decisions or settlement of claims (and offers of settlement), we may reassess the potential liability related to these matters and may revise these estimates, which could result in a material adverse adjustment to our operating results. Costs associated with our involvement in legal proceedings are expensed as incurred. The outcome of any such proceedings, regardless of the merits, is inherently uncertain. If we were unable to prevail in any such proceedings, our consolidated financial position, results of operations, and future cash flows may be materially impacted. We have received, and may in the future receive, notices from third parties claiming that their patents may be infringed by the use, development, manufacture, importation or sale of our products or product candidates. Under the guidance of ASC 450, Contingencies , we record a royalty accrual based on our best estimate of the fair value percent of net sales of our products that we could be required to pay the owners of patents for technology used in the manufacture and sale of our products. A costly license, or inability to obtain a necessary license, could have a material adverse effect on our financial results. In May 2015, we received a subpoena in connection with an investigation by the Enforcement Division of the Securities and Exchange Commission (SEC) requesting information related to our grant-making activities and compliance with the Foreign Corrupt Practices Act (FCPA) in various countries. In addition, in October 2015, we received a request from the Department of Justice (DOJ) for the voluntary production of documents and other information pertaining to Alexion’s compliance with the FCPA. The SEC and DOJ also seek information related to Alexion’s recalls of specific lots of SOLIRIS and related securities disclosures. Alexion has been cooperating with these investigations. The investigations have focused on operations in various countries, including Brazil, Colombia, Japan, Russia and Turkey, and Alexion's compliance with the FCPA and other applicable laws. DOJ recently informed us that it has closed its inquiry into these matters. As previously disclosed, we have been engaged in discussions with the SEC to resolve this investigation. Following March 31, 2020, we reached an agreement in principle with the SEC Enforcement Division staff to settle the claims relating to the SEC investigation, pursuant to which we would pay an aggregate amount less than $25 million to resolve the investigation. As a result, we have accrued a liability in such amount in our financial statements for the quarter ended March 31, 2020. While we have reached an agreement in principle with the SEC, there can be no assurance that a definitive agreement reflecting the terms of the agreement in principle will be agreed to or finalized. If no such definitive agreement is reached, or if the terms of any definitive agreement are different than the agreement in principle, it is possible that a loss related to these matters may be incurred and such amount may be greater than the amounts contemplated by the agreement in principle. In addition, if we fail to reach a definitive agreement and the SEC continues its investigation, the SEC may pursue litigation to determine whether our operations or activities are not or were not in compliance with existing laws or regulations and such action could result in the imposition of fines, civil penalties, equitable remedies, including disgorgement, injunctive relief, and/or other sanctions against us, and remediation of any such findings could have an adverse effect on our business operations. Alexion is committed to strengthening its compliance program and is currently enhancing and continuing to implement a comprehensive company-wide transformation plan that is designed to enhance our business processes, structures, controls, training, talent, and systems across Alexion’s global operations. As previously reported, on December 29, 2016, a shareholder filed a putative class action against the Company and certain former employees in the U.S. District Court for the District of Connecticut, alleging that defendants made misrepresentations and omissions about SOLIRIS. On April 12, 2017, the court appointed a lead plaintiff. On July 14, 2017, the lead plaintiff filed an amended putative class action complaint against the Company and seven current or former employees. Defendants moved to dismiss the amended complaint on September 12, 2017. Plaintiffs filed an opposition to defendants’ motion to dismiss on November 13, 2017, and defendants filed a reply brief in further support of their motion on December 28, 2017. On March 26, 2019, the court held a telephonic status conference. During that conference, the court informed counsel that it was preparing a ruling granting the defendants’ pending motion to dismiss. The court inquired of plaintiffs’ counsel whether they intended to seek leave to amend their complaint, and indicated that if they wished to file a second amended complaint, they would be allowed to do so. On April 2, 2019, the court granted plaintiffs until May 31, 2019 to file a second amended complaint, thereby rendering moot defendants’ pending motion to dismiss. On May 31, 2019, plaintiffs filed a second amended complaint against the same defendants. The complaint alleges that defendants engaged in securities fraud, including by making misrepresentations and omissions in its public disclosures concerning the Company’s SOLIRIS sales practices, management changes, and related investigations, between January 30, 2014 and May 26, 2017, and that the Company's stock price dropped upon the purported disclosure of the alleged fraud. The plaintiffs seek to recover unspecified monetary relief, unspecified equitable and injunctive relief, interest, and attorneys’ fees and costs. Defendants’ filed a motion to dismiss the amended complaint on August 2, 2019; plaintiffs’ filed their opposition to that motion on October 2, 2019; and defendants’ filed their reply in further support of their motion on November 16, 2019. Given the early stage of these proceedings, we cannot presently predict the likelihood of obtaining dismissal of the case (or the ultimate outcome of the case if the motion to dismiss is denied by the court), nor can we estimate the possible loss or range of loss at this time. In December 2016, we received a subpoena from the U.S. Attorney's Office for the District of Massachusetts requesting documents relating generally to our support of Patient Services, Inc. (PSI) and National Organization for Rare Disorders (NORD), 501(c)(3) organizations that provide financial assistance to Medicare patients taking drugs sold by Alexion; Alexion’s provision of free drug to Medicare patients; and Alexion compliance policies and training materials concerning the anti-kickback statute and information on donations to PSI and NORD from 2010 through 2016. In April 2019, we entered into a civil settlement agreement with the DOJ and the Office of Inspector General (OIG) of the U.S. Department of Health and Human Services to resolve this matter. As part of the settlement agreement, Alexion paid $13.1 to the DOJ and OIG. OIG did not require a Corporate Integrity Agreement with Alexion because it made fundamental organizational changes, including hiring a new executive leadership team, replacing half of the members of its Board of Directors, and effecting a significant change in the workforce. In May 2017, Brazilian authorities seized records and data from our Sao Paulo, Brazil offices as part of an investigation being conducted into Alexion’s Brazilian operations. We are cooperating with this inquiry. In June 2017, we received a demand to inspect certain of our books and records pursuant to Section 220 of the General Corporation Law of the State of Delaware on behalf of a purported stockholder. Among other things, the demand sought to determine whether to institute a derivative lawsuit against certain of the Company’s directors and officers in relation to the investigation by our Audit and Finance Committee announced in November 2016 and the investigations instituted by the SEC, DOJ, U.S. Attorney’s Office for the District of Massachusetts, and Brazilian law enforcement officials that are described above. We have responded to the demand. Given the early stages of this matter, an estimate of the possible loss or range of loss cannot be made at this time. On September 27, 2017, a hearing panel of the Canadian Patented Medicine Prices Review Board (PMPRB) issued a decision in a previously pending administrative pricing matter that we had excessively priced SOLIRIS in a manner inconsistent with the Canadian pricing rules and guidelines. In its decision, the PMPRB ordered Alexion to decrease the price of SOLIRIS to an upper limit based upon pricing in certain other countries, and to forfeit excess revenues for the period between 2009 and 2017. The amount of excess revenues for the period between 2009 and 2017 was not determined to be a material amount and was paid in 2018. In October 2017, Alexion filed an application for judicial review of the PMPRB’s decision in the Federal Court of Canada. On May 23, 2019, the Federal Court of Canada dismissed Alexion's application for judicial review and, as a consequence, affirmed the decision of the PMPRB that we had excessively priced SOLIRIS. On June 21, 2019, Alexion filed a notice of appeal of the Federal Court of Canada's ruling and on October 17, 2019, Alexion filed a memorandum of fact and law in support of the appeal. On December 3, 2019, the Attorney General of Canada filed its memorandum of fact and law in support of the Federal Court of Canada's dismissal of Alexion's appeal of the PMPRB's decision. On December 19, 2019, intervenor, the Minister of Health for the Province of British Columbia, filed a separate memorandum of fact and law in support of the Federal Court of Canada's decision. Pursuant to an order made by the Federal Court of Canada, as of May 6, 2020 , we have placed approximately $45.4 in escrow to secure our obligations pending the final resolution of all appeals in this matter. This amount reflects the difference between the list price for SOLIRIS and the price determined by the PMPRB to be non-excessive for sales of SOLIRIS in Canada for the period beginning September 2017 through March 31, 2020. In addition, on a quarterly basis until the appeals process has concluded, Alexion will be required to place amounts into escrow for each vial of SOLIRIS sold in the applicable quarter equal to the list price for SOLIRIS and the price determined by the PMPRB to be non-excessive. Our revenues in Canada have been reduced by $31.4 cumulatively to date, which is our current best estimate of our liability through March 31, 2020 if we lose the appeal of this matter (the amount of our ultimate liability, however, may be greater than this estimate when the appeal process for this matter is concluded). Chugai Pharmaceutical Co., Ltd. has filed three lawsuits against Alexion. The first was filed in November 2018 in the United States District Court for the District of Delaware against Alexion Pharmaceuticals, Inc. alleging that ULTOMIRIS infringes one U.S. patent held by Chugai Pharmaceutical Co., Ltd. Upon issuance of a new U.S. patent on November 12, 2019, Chugai filed a second lawsuit in the United States alleging that ULTOMIRIS infringes the new patent. The parties have agreed to consolidate the November 2018 and November 2019 lawsuits. Chugai filed a third lawsuit in December 2018 in the Tokyo District Court against Alexion Pharma GK (a wholly-owned subsidiary of Alexion) in Japan and alleges that ULTOMIRIS infringes two Japanese patents held by Chugai Pharmaceutical Co., Ltd. Chugai’s complaints seek unspecified damages and certain injunctive relief. On March 5, 2020, the Supreme Court of Japan conclusively affirmed an earlier IP High Court of Japan decision which held that one of the Chugai patents-in-suit is invalid. Subsequently Chugai filed a correction to the claims of this patents-in-suit and Alexion has countered that the corrected claims are still invalid and not infringed. In all cases, Alexion has denied the charges and countered that the patents are neither valid nor infringed. A trial date for the U.S. case has been set for July 2021. The case is still at the briefing stage in Japan. Given the early stages of these litigations, an estimate of the possible loss or range of loss cannot be made at this time. On February 28, 2019, Amgen Inc. (Amgen) petitioned the U.S. Patent and Trademark Office (PTO) to institute Inter Partes Review (IPR) of three patents owned by Alexion that relate to SOLIRIS: U.S. Patent Nos. 9,725,504; 9,718,880; and 9,732,149. In each case, Amgen alleges the patented subject matter was anticipated and/or obvious in view of prior art, and that the patent claims are therefore invalid. On August 30, 2019, the PTO instituted IPRs of each of the three patents. We expect the PTO to review written submissions by both parties, hold an oral argument, and issue decisions on the three IPRs by August 30, 2020. At this time we cannot determine what decision the PTO will make. In connection with an ongoing matter, in August 2019, the Brazilian Federal Revenue Service provided a Notice of Tax and Description of the Facts (the “Tax Assessment”) to two Alexion subsidiaries (the "Brazil Subsidiaries"), as well as to two additional entities, a logistics provider utilized by Alexion and a distributor. The Tax Assessment focuses on the importation of SOLIRIS vials pursuant to Alexion’s free drug supply to patients program (referred to as Global Access to Medicines, or GATM) in Brazil. In September 2019, the Brazil Subsidiaries filed defenses to the Tax Assessment disputing the basis for liability under the Tax Assessment based on, among others, the following: in connection with the operation of GATM, during the period from September 2014 to June 2019: (i) the importers responsible for the importation of the GATM SOLIRIS vials into Brazil were correctly identified and (ii) the correct customs value was utilized for the purpose of importing the GATM SOLIRIS vials provided to the patients free of charge. The defenses filed by Alexion are pending judgment at the first level of administrative appeals within the Brazilian federal administrative proceeding system. There are three separate levels of administrative appeals within the Brazilian federal administrative proceeding system and, if the outcome of these administrative appeals is unfavorable, the final decision of the federal administrative proceeding system can be disputed to the federal court systems in Brazil (at this time, Alexion intends to appeal the Tax Assessment if it is not overturned in the course of administrative appeals). Given the early stage of these proceedings, Alexion is unable to predict the duration, scope or outcome of this matter, but we expect that a final resolution will take three years or more. While it is possible that a loss related to the Tax Assessment may be incurred, given its ongoing nature, we cannot reasonably estimate the potential magnitude of any such possible loss or range of loss, or the cost of the ongoing administrative appeals (and potential appeals to the federal court system) of the Tax Assessment. Any determination that any aspects of the importation of free of charge medications into Brazil as set forth in the Tax Assessment are not or were not in compliance with existing laws or regulations could result in the imposition of fines, civil penalties and, potentially criminal penalties, and/or other sanctions against us and could have an adverse impact on our Brazilian operations.

Subsequent Events

Subsequent Events	3 Months Ended
	Mar. 31, 2020
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On May 5, 2020, Alexion entered into a definitive merger agreement to acquire Portola Pharmaceuticals, Inc. (Portola), a commercial-stage biopharmaceutical company focused on life-threatening blood-related disorders. Portola’s commercialized medicine, Andexxa®, marketed as Ondexxya® in Europe, is the first and only approved Factor Xa inhibitor reversal agent, and has demonstrated transformative clinical value by rapidly reversing the anticoagulant effects of Factor Xa inhibitors rivaroxaban and apixaban in severe and uncontrolled bleeding. Under the terms of the merger agreement, a subsidiary of Alexion will commence a tender offer to acquire all of the outstanding shares of Portola’s common stock at a price of $18.00 per share in cash, or approximately $1,380.0 , excluding shares of Portola stock currently held by Alexion, with the final aggregate purchase price to be determined upon closing. The tender offer is subject to customary conditions, including the tender of a majority of the outstanding shares of Portola common stock, the expiration or termination of the waiting period under the Hart-Scott Rodino Antitrust Improvements Act and receipt of certain other regulatory approvals. Following successful completion of the tender offer, Alexion will acquire all remaining shares not tendered in the offer at the same price of $18.00

Basis of Presentation and Pri_2

Basis of Presentation and Principles Of Consolidation (Policies)	3 Months Ended
	Mar. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Use of Estimates	Use of Estimates Preparation of the condensed consolidated financial statements in conformity with U.S. GAAP requires us to make estimates, judgments and assumptions that may affect the reported amounts of assets, liabilities, revenues, expenses and disclosure of contingent liabilities in our condensed consolidated financial statements. Due to the COVID-19 pandemic, there has been uncertainty and disruption in the global economy and financial markets. The full extent to which the COVID-19 pandemic will directly or indirectly impact our business, results of operations and financial condition, including sales, expenses, reserves and allowances, manufacturing, clinical trials, research and development costs and employee-related amounts, will depend on future developments that are highly uncertain. We are not aware of any specific event or circumstance that would require an update to our estimates, judgments and assumptions or a revision of the carrying value of our assets or liabilities as of May 6, 2020 , the date of issuance of this Quarterly Report on Form 10-Q. These estimates may change, as new events occur and additional information is obtained. Actual results may differ from these estimates under different assumptions or conditions and such differences may be material.
New Accounting Pronouncements	New Accounting Pronouncements Accounting Standards Update (ASU) 2019-12, “Income Taxes: Simplifying the Accounting for Income Taxes”: In December 2019, the Financial Accounting Standards Board (FASB) issued a new standard intended to simplify the accounting for income taxes by eliminating certain exceptions related to the approach for intraperiod tax allocation, the methodology for calculating income taxes in an interim period and the recognition of deferred tax liabilities for outside basis differences. The new standard also simplifies aspects of the accounting for franchise taxes and enacted changes in tax laws or rates and clarifies the accounting for transactions that result in a step-up in the tax basis of goodwill. The standard is effective for annual periods beginning after December 15, 2020 and interim periods within, with early adoption permitted. Adoption of the standard requires certain changes to be made prospectively, with some changes to be made retrospectively. We are currently assessing the impact of this standard on our financial condition and results of operations. ASU 2020-01, “Investments - Equity Securities, Investments - Equity Method and Joint Ventures, and Derivatives and Hedging - Clarifying the Interactions Between Topic 321, Topic 323, and Topic 815” : In January 2020, the FASB issued a new standard intended to clarify the interactions between ASC 321, ASC 323 and ASC 815. The new standard addresses accounting for the transition into and out of the equity method and measurement of certain purchased options and forward contracts to acquire investments. The standard is effective for annual and interim periods beginning after December 15, 2020, with early adoption permitted. Adoption of the standard requires changes to be made prospectively. We are currently assessing the impact of this standard on our financial condition and results of operations. ASU 2020-04, “Reference Rate Reform, Facilitation of the Effects of Reference Rate Reform on Financial Reporting" : In response to concerns about structural risks of interbank offered rates , and, particularly, the risk of cessation of the London Interbank Offered Rate (LIBOR), regulators around the world have undertaken reference rate reform initiatives to identify alternative reference rates that are more observable or transaction-based and less susceptible to manipulation. In March 2020, the FASB issued a new standard that provides optional guidance for a limited time to ease the potential burden in accounting for the effects of reference rate reform, including optional expedients and exceptions for the accounting implications of contracts, hedging relationships, and other transactions affected by reference rate reform if certain criteria are met. The amendments in this new standard only apply to contracts and hedging relationships that reference LIBOR or another reference rate expected to be discontinued due to reference rate reform. The expedients and exceptions provided by the standard do not apply to contract modifications made and hedging relationships entered into or evaluated after December 31, 2022. We are currently reviewing our contracts impacted by reference rate reform and are assessing the impact of this standard on our financial condition and results of operations. Recently Adopted Accounting Pronouncements ASU 2018-15, "Customer's Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract" : In August 2018, the FASB issued a new standard on a customer's accounting for implementation, set-up, and other upfront costs incurred in a cloud computing arrangement (CCA) that aligns the requirements for capitalizing implementation costs in a CCA service contract with existing internal-use software guidance. The standard also provides classification guidance on these implementation costs as well as additional quantitative and qualitative disclosures. The standard is effective for interim and annual periods beginning after December 15, 2019, with early adoption permitted, and can be adopted prospectively or retrospectively. We adopted the new standard on January 1, 2020 on a prospective basis. The adoption of this standard had no impact on our financial statements at the date of adoption; however, we anticipate the adoption of this standard will result in an increase in capitalized assets related to qualifying CCA implementation costs in future periods. Qualifying CCA implementation, set-up and other upfront costs incurred after January 1, 2020 are capitalized as other assets in our condensed consolidated balance sheets. These assets will be expensed over the term of the hosting arrangement and such expense will be presented within the same line item in our condensed consolidated statements of operations as the expense for fees for the associated hosting arrangement. These capitalized costs will be evaluated for impairment when events or changes in circumstances indicate that the carrying value of the capitalized implementation costs is not recoverable. For the period ended March 31, 2020, capitalized CCA implementation costs were not material. ASU 2016-13, "Measurement of Credit Losses on Financial Instruments" : In June 2016, the FASB issued a new standard intended to improve reporting requirements specific to loans, receivables and other financial instruments. The new standard requires that credit losses on financial assets measured at amortized cost be determined using an expected loss model, instead of the current incurred loss model, and requires that credit losses related to available-for-sale debt securities be recorded through an allowance for credit losses and limited to the amount by which carrying value exceeds fair value. The new standard also requires enhanced disclosure of credit risk associated with financial assets. The standard is effective for interim and annual periods beginning after December 15, 2019 with early adoption permitted. We adopted the new standard on January 1, 2020 and have completed our assessment of the standard based on the composition of our portfolio of financial instruments and current and forecasted economic conditions at that date. Our significant financial assets that are within the scope of the new standard consist of trade accounts receivable and available for sale debt securities. We have not historically experienced any material credit losses associated with our trade accounts receivable or available for debt securities. We monitor economic conditions, including volatility associated with international economies and the associated impacts on the financial markets and our business. We disaggregate our trade accounts receivable population into pools of similar risk characteristics based on underlying customer type and geographical location. Current expected credit loss allowances are estimated for each risk pool based on available information, including i) historical credit loss experience, ii) current economic conditions and, iii) reasonable and supportable forecasts of future economic conditions that may affect the collectibility of the recorded amounts. Based on the relevant facts and economic conditions as of the date of adoption, we concluded that the expected credit losses on our trade accounts receivable were immaterial. Additionally, unrealized losses on our available for sale investment portfolio were immaterial.

Acquisitions (Tables)

Acquisitions (Tables)	3 Months Ended
	Mar. 31, 2020
Business Combinations [Abstract]
Summary of Total Consideration Transferred	The following table summarizes the total consideration transferred to acquire Achillion and the estimated fair value of the identified assets acquired and liabilities assumed at the acquisition date: Consideration Upfront payment to shareholders and option holders $ 926.2 Upfront payment, fair value of equity compensation attributable to the post-combination service period (20.0 ) Upfront cash paid, net 906.2 Contingent consideration 160.7 Contingent consideration, fair value of equity compensation attributable to the post-combination service period (5.7 ) Total consideration $ 1,061.2 Assets Acquired and Liabilities Assumed Cash and cash equivalents $ 68.5 Marketable securities 106.1 In-process research & development assets (IPR&D) 918.0 Goodwill 34.7 Deferred tax liabilities, net (59.8 ) Other assets and liabilities, net (6.3 ) Total net assets acquired $ 1,061.2
Summary of Pro Forma Information	The following unaudited pro forma information presents the combined results of Alexion and Achillion as if the acquisition of Achillion had been completed on January 1, 2019, with adjustments to give effect to pro forma events that are directly attributable to the acquisition. The unaudited pro forma results do not reflect operating efficiencies or potential cost savings that may have resulted from the consolidation of operations. Accordingly, the unaudited pro forma financial information is not necessarily indicative of the results of operations had we completed the transaction on January 1, 2019. Three months ended March 31, 2020 2019 Pro forma revenue $ 1,444.8 $ 1,140.4 Pro forma net income 574.9 515.7
Summary of Acquisition-Related Costs	Acquisition-related costs recorded within the condensed consolidated statement of operations associated with our business combinations for the three months ended March 31, 2020 and 2019 include the following: Three months ended March 31, 2020 2019 Transaction costs (1) $ 1.4 $ — Integration costs 0.1 — Fair value of equity compensation attributable to the post-combination service period 25.7 — Restructuring-related costs (2) 10.9 — $ 38.1 $ — (1) Transaction costs include legal fees and costs incurred to effectuate the settlement of the outstanding options (2) Restructuring-related costs include severance payments and one-time short-term retention awards agreed to in connection with the acquisition

Inventories (Tables)

Inventories (Tables)	3 Months Ended
	Mar. 31, 2020
Inventory, Net [Abstract]
Schedule of Inventory, Current	The components of inventory are as follows: March 31, December 31, 2020 2019 Raw materials $ 49.6 $ 41.2 Work-in-process 108.8 180.8 Finished goods 428.4 405.6 $ 586.8 $ 627.6

Intangible Assets and Goodwill

Intangible Assets and Goodwill (Tables)	3 Months Ended
	Mar. 31, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Intangible Assets and Goodwill	The following table summarizes the carrying amount of our intangible assets and goodwill, net of accumulated amortization: March 31, 2020 December 31, 2019 Estimated Cost Accumulated Net Cost Accumulated Net Licensing rights 3-8 $ 57.0 $ (35.6 ) $ 21.4 $ 57.0 $ (34.7 ) $ 22.3 Patents 7 10.5 (10.5 ) — 10.5 (10.5 ) — Purchased technology 6-16 4,710.5 (1,462.4 ) 3,248.1 4,710.5 (1,388.7 ) 3,321.8 Other intangibles 5 0.4 (0.3 ) 0.1 0.4 (0.2 ) 0.2 Acquired IPR&D Indefinite 918.0 — 918.0 — — — Total $ 5,696.4 $ (1,508.8 ) $ 4,187.6 $ 4,778.4 $ (1,434.1 ) $ 3,344.3 Goodwill Indefinite $ 5,075.0 $ (2.9 ) $ 5,072.1 $ 5,040.3 $ (2.9 ) $ 5,037.4
Summary of Changes in Goodwill	The following summarizes the changes in the carrying amount of goodwill: March 31, 2020 Balance at December 31, 2019 $ 5,037.4 Goodwill resulting from the acquisition of Achillion 34.7 Balance at March 31, 2020 $ 5,072.1

Earnings Per Common Share (Tabl

Earnings Per Common Share (Tables)	3 Months Ended
	Mar. 31, 2020
Earnings Per Common Share [Abstract]
Schedule Of Basic And Diluted Earnings Per Share	The following table summarizes the calculation of basic and diluted EPS for the three months ended March 31, 2020 and 2019 : Three months ended March 31, 2020 2019 Net income used for basic and diluted calculation $ 557.6 $ 587.9 Shares used in computing earnings per common share—basic 221.6 223.8 Weighted-average effect of dilutive securities: Stock awards 1.0 1.7 Shares used in computing earnings per common share—diluted 222.6 225.5 Earnings per common share: Basic $ 2.52 $ 2.63 Diluted $ 2.50 $ 2.61

Marketable Securities (Tables)

Marketable Securities (Tables)	3 Months Ended
	Mar. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Schedule of Available-for-sale Securities Reconciliation [Table Text Block]	March 31, 2020 and December 31, 2019 were as follows: March 31, 2020 Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Fair Value Commercial paper $ 134.2 $ — $ — $ 134.2 Corporate bonds 3.6 — (0.1 ) 3.5 Other government-related obligations: U.S. 60.0 — — 60.0 Bank certificates of deposit 6.5 — — 6.5 Total available-for-sale debt securities $ 204.3 $ — $ (0.1 ) $ 204.2 December 31, 2019 Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Fair Value Commercial paper $ 246.9 $ — $ — $ 246.9 Corporate bonds 24.3 — — 24.3 Other government-related obligations: U.S. 70.4 — — 70.4 Bank certificates of deposit 27.4 — — 27.4 Total available-for-sale debt securities $ 369.0 $ — $ — $ 369.0
Available-for-sale Securities by Balance Sheet Location Classification [Table Text Block]	The fair values of available-for-sale debt securities by classification in the condensed consolidated balance sheets were as follows: March 31, 2020 December 31, 2019 Cash and cash equivalents $ 179.1 $ 328.1 Marketable securities 25.1 40.9 $ 204.2 $ 369.0
Investments Classified by Contractual Maturity Date [Table Text Block]	The fair values of available-for-sale debt securities as of March 31, 2020 , by contractual maturity, are summarized as follows: March 31, 2020 Due in one year or less $ 204.2 Due after one year through three years — $ 204.2

Derivative Instruments and He_2

Derivative Instruments and Hedging Activities (Tables)	3 Months Ended
	Mar. 31, 2020
Derivative Instruments and Hedging Activities [Abstract]
Schedule of Interest Rate Contracts	The following table summarizes the total interest rate swap contracts executed as of March 31, 2020 : Type of Interest Rate Swap Contract Notional Amount Effective Date Termination Date Fixed Interest Rate or Rate Range Floating to Fixed 450.0 December 2018 December 2022 2.60% - 2.79% Floating to Fixed 1,300.0 December 2019 December 2022 2.37% - 2.83%
Schedule of Other Comprehensive Income and Earnings from Foreign Exchange Contracts	The amount of gains and (losses) recognized in the condensed consolidated statements of operations for the three months ended March 31, 2020 and 2019 from foreign exchange and interest rate swap contracts that qualified as cash flow hedges were as follows: Three months ended Three months ended March 31, 2020 March 31, 2019 Financial Statement Line Item in which the Effects of Cash Flow Hedges are Recorded Net Product Sales Interest Expense Net Product Sales Interest Expense Total amount presented in the Condensed Consolidated Statements of Operations $ 1,444.6 $ (25.8 ) $ 1,140.2 $ (19.9 ) Impact of cash flow hedging relationships: Foreign exchange forward contracts $ 11.4 $ — $ 7.0 $ — Interest rate swap contracts $ — $ (4.6 ) $ — $ 4.6 The impact on accumulated other comprehensive income (AOCI) and earnings from foreign exchange and interest rate swap contracts that qualified as cash flow hedges, for the three months ended March 31, 2020 and 2019 were as follows: Three months ended March 31, 2020 2019 Foreign Exchange Forward Contracts: Gain (loss) recognized in AOCI, net of tax $ 26.0 $ 14.2 Gain (loss) reclassified from AOCI to net product sales, net of tax $ 8.8 $ 5.4 Interest Rate Swap Contracts: Gain (loss) recognized in AOCI, net of tax $ (47.3 ) $ (14.2 ) Gain (loss) reclassified from AOCI to interest expense, net of tax $ (3.6 ) $ 3.6
Schedule of Fair Value of Outstanding Derivatives	The following tables summarize the fair value of outstanding derivatives as of March 31, 2020 and December 31, 2019 : March 31, 2020 Derivative Assets Derivative Liabilities Balance Sheet Fair Balance Sheet Fair Derivatives designated as hedging instruments: Foreign exchange forward contracts Prepaid expenses and other current assets $ 29.1 Other current liabilities $ 1.7 Foreign exchange forward contracts Other assets 1.1 Other liabilities 0.8 Interest rate swap contracts Prepaid expenses and other current assets — Other current liabilities 42.1 Interest rate swap contracts Other assets — Other liabilities 75.8 Derivatives not designated as hedging instruments: Foreign exchange forward contracts Prepaid expenses and other current assets 24.1 Other current liabilities 12.4 Total fair value of derivative instruments $ 54.3 $ 132.8 December 31, 2019 Derivative Assets Derivative Liabilities Balance Sheet Fair Balance Sheet Fair Derivatives designated as hedging instruments: Foreign exchange forward contracts Prepaid expenses and other current assets $ 12.7 Other current liabilities $ 6.2 Foreign exchange forward contracts Other assets 0.6 Other liabilities 1.1 Interest rate swap contracts Prepaid expenses and other current assets — Other current liabilities 19.5 Interest rate swap contracts Other assets — Other liabilities 41.9 Derivatives not designated as hedging instruments: Foreign exchange forward contracts Prepaid expenses and other current assets 17.2 Other current liabilities 20.4 Total fair value of derivative instruments $ 30.5 $ 89.1
Offsetting Assets and Liabilities	The following tables summarize the potential effect on our condensed consolidated balance sheets of offsetting our foreign exchange forward contracts and interest rate contracts subject to such provisions: March 31, 2020 Gross Amounts Not Offset in the Condensed Consolidated Balance Sheet Description Gross Amounts of Recognized Assets/Liabilities Gross Amounts Offset in the Condensed Consolidated Balance Sheet Net Amounts of Assets/Liabilities Presented in the Condensed Consolidated Balance Sheet Derivative Financial Instruments Cash Collateral Received (Pledged) Net Amount Derivative assets $ 54.3 $ — $ 54.3 $ (14.9 ) $ — $ 39.4 Derivative liabilities (132.8 ) — (132.8 ) 14.9 — (117.9 ) December 31, 2019 Gross Amounts Not Offset in the Condensed Consolidated Balance Sheet Description Gross Amounts of Recognized Assets/Liabilities Gross Amounts Offset in the Condensed Consolidated Balance Sheet Net Amounts of Assets/Liabilities Presented in the Condensed Consolidated Balance Sheet Derivative Financial Instruments Cash Collateral Received (Pledged) Net Amount Derivative assets $ 30.5 $ — $ 30.5 $ (21.4 ) $ — $ 9.1 Derivative liabilities (89.1 ) — (89.1 ) 21.4 — (67.7 )

Other Comprehensive Income an_2

Other Comprehensive Income and Accumulated Other Comprehensive Income (Tables)	3 Months Ended
	Mar. 31, 2020
Accumulated Other Comprehensive Income [Abstract]
Schedule of Accumulated Other Comprehensive Income (Loss)	The following tables summarize the changes in AOCI, by component, for the three months ended March 31, 2020 and 2019 : Defined Benefit Pension Plans Unrealized Gains (Losses) from Debt Securities Unrealized Gains (Losses) from Hedging Activities Foreign Currency Translation Adjustment Total Accumulated Other Comprehensive Income (Loss) Balances, December 31, 2019 $ (9.2 ) $ (0.1 ) $ (40.1 ) $ (17.4 ) $ (66.8 ) Other comprehensive income (loss) before reclassifications — (0.2 ) (21.3 ) (8.0 ) (29.5 ) Amounts reclassified from other comprehensive income — — (5.2 ) — (5.2 ) Net other comprehensive income (loss) — (0.2 ) (26.5 ) (8.0 ) (34.7 ) Balances, March 31, 2020 $ (9.2 ) $ (0.3 ) $ (66.6 ) $ (25.4 ) $ (101.5 ) Defined Benefit Pension Plans Unrealized Gains (Losses) from Debt Securities Unrealized Gains (Losses) from Hedging Activities Foreign Currency Translation Adjustment Total Accumulated Other Comprehensive Income (Loss) Balances, December 31, 2018 $ (2.6 ) $ (0.3 ) $ 9.6 $ (16.4 ) $ (9.7 ) Other comprehensive income (loss) before reclassifications — 0.2 — (2.0 ) (1.8 ) Amounts reclassified from other comprehensive income — — (9.0 ) — (9.0 ) Net other comprehensive income (loss) — 0.2 (9.0 ) (2.0 ) (10.8 ) Balances, March 31, 2019 $ (2.6 ) $ (0.1 ) $ 0.6 $ (18.4 ) $ (20.5 )
Reclassification out of Accumulated Other Comprehensive Income	The table below provides details regarding significant reclassifications from AOCI during the three months ended March 31, 2020 and 2019 : Details about Accumulated Other Comprehensive Income Components Amount Reclassified From Accumulated Other Comprehensive Income during the three months ended March 31, Affected Line Item in the Condensed Consolidated Statements of Operations 2020 2019 Unrealized Gains (Losses) on Hedging Activity Foreign exchange forward contracts $ 11.4 $ 7.0 Net product sales Interest rate swap contracts (4.6 ) 4.6 Interest expense 6.8 11.6 (1.6 ) (2.6 ) Income tax (benefit) expense $ 5.2 $ 9.0

Fair Value Measurement (Tables)

Fair Value Measurement (Tables)	3 Months Ended
	Mar. 31, 2020
Fair Value Disclosures [Abstract]
Schedule Of Assets And Liabilites Measured At Fair Value	The following tables present information about our assets and liabilities that are measured at fair value on a recurring basis as of March 31, 2020 and December 31, 2019 , and indicate the fair value hierarchy of the valuation techniques we utilized to determine such fair value. Fair Value Measurement at Balance Sheet Type of Instrument Total Level 1 Level 2 Level 3 Cash equivalents Money market funds $ 679.4 $ — $ 679.4 $ — Cash equivalents Commercial paper $ 119.1 $ — $ 119.1 $ — Cash equivalents Other government-related obligations $ 60.0 $ — $ 60.0 $ — Marketable securities Mutual funds $ 22.7 $ 22.7 $ — $ — Marketable securities Commercial paper $ 15.1 $ — $ 15.1 $ — Marketable securities Corporate bonds $ 3.5 $ — $ 3.5 $ — Marketable securities Bank certificates of deposit $ 6.5 $ — $ 6.5 $ — Other assets Equity securities $ 83.8 $ 59.7 $ 24.1 $ — Prepaid expenses and other current assets Foreign exchange forward contracts $ 53.2 $ — $ 53.2 $ — Other assets Foreign exchange forward contracts $ 1.1 $ — $ 1.1 $ — Other current liabilities Foreign exchange forward contracts $ 14.1 $ — $ 14.1 $ — Other liabilities Foreign exchange forward contracts $ 0.8 $ — $ 0.8 $ — Other current liabilities Interest rate contracts $ 42.1 $ — $ 42.1 $ — Other liabilities Interest rate contracts $ 75.8 $ — $ 75.8 $ — Contingent consideration Acquisition-related contingent consideration $ 358.9 $ — $ — $ 358.9 Other current liabilities Other contingent payments $ 26.3 $ — $ — $ 26.3 Fair Value Measurement at Balance Sheet Type of Instrument Total Level 1 Level 2 Level 3 Cash equivalents Money market funds $ 635.9 $ — $ 635.9 $ — Cash equivalents Commercial paper $ 227.9 $ — $ 227.9 $ — Cash equivalents Corporate bonds $ 20.6 $ — $ 20.6 $ — Cash equivalents Bank certificates of deposit $ 19.2 $ — $ 19.2 $ — Cash equivalents Other government-related obligations $ 60.4 $ — $ 60.4 $ — Marketable securities Mutual funds $ 23.1 $ 23.1 $ — $ — Marketable securities Commercial paper $ 19.0 $ — $ 19.0 $ — Marketable securities Corporate bonds $ 3.7 $ — $ 3.7 $ — Marketable securities Other government-related obligations $ 10.0 $ — $ 10.0 $ — Marketable securities Bank certificates of deposit $ 8.2 $ — $ 8.2 $ — Other assets Equity securities $ 79.0 $ 51.2 $ 27.8 $ — Prepaid expenses and other current assets Foreign exchange forward contracts $ 29.9 $ — $ 29.9 $ — Other assets Foreign exchange forward contracts $ 0.6 $ — $ 0.6 $ — Other current liabilities Foreign exchange forward contracts $ 26.6 $ — $ 26.6 $ — Other liabilities Foreign exchange forward contracts $ 1.1 $ — $ 1.1 $ — Other current liabilities Interest rate contracts $ 19.5 $ — $ 19.5 $ — Other liabilities Interest rate contracts $ 41.9 $ — $ 41.9 $ — Contingent consideration Acquisition-related contingent consideration $ 192.4 $ — $ — $ 192.4 Other current liabilities Other contingent payments $ 24.0 $ — $ — $ 24.0
Schedule Of Acquisition-Related Contingent Consideration	he following table represents a roll-forward of our acquisition-related contingent consideration: March 31, 2020 Balance at beginning of period $ 192.4 Amounts issued 160.7 Changes in fair value 5.8 Balance at end of period $ 358.9

Revenue Recognition (Tables)

Revenue Recognition (Tables)	3 Months Ended
	Mar. 31, 2020
Revenue from Contract with Customer [Abstract]
Disaggregation of revenue from contracts with customer	The Company disaggregates revenue from contracts with customers into product and geographical regions as summarized below. Three months ended March 31, 2020 2019 SOLIRIS United States $ 556.2 $ 463.7 Europe 263.5 264.5 Asia Pacific 87.1 100.9 Rest of World 116.1 132.9 Total $ 1,022.9 $ 962.0 ULTOMIRIS United States $ 131.5 $ 24.6 Europe 33.8 — Asia Pacific 57.1 — Rest of World 0.4 — Total $ 222.8 $ 24.6 STRENSIQ United States $ 128.1 $ 99.5 Europe 24.0 17.5 Asia Pacific 13.6 9.9 Rest of World 6.5 3.2 Total $ 172.2 $ 130.1 KANUMA United States $ 16.4 $ 13.8 Europe 7.5 6.3 Asia Pacific 0.9 0.8 Rest of World 1.9 2.6 Total $ 26.7 $ 23.5 Total Net Product Sales $ 1,444.6 $ 1,140.2
Summary of receivables and contract liabilities from contracts	March 31, 2020 December 31, 2019 Receivables, which are included in "Trade accounts receivable, net" $ 1,345.2 $ 1,243.2 Contract liabilities, which are included in "Other current liabilities" $ 22.9 $ 6.8

Income Taxes (Tables)

Income Taxes (Tables)	3 Months Ended
	Mar. 31, 2020
Income Tax Disclosure [Abstract]
Schedule of Income Tax Provision and Effective Tax Rate	The following table provides a comparative summary of our income tax expense and effective income tax rate for the three months ended March 31, 2020 and 2019 : Three months ended March 31, 2020 2019 Income tax expense (benefit) $ 106.0 $ (46.1 ) Effective income tax rate 16.0 % (8.5 )%

Acquisitions (Narrative) (Detai

Acquisitions (Narrative) (Details) - USD ($)	Jan. 28, 2020	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Schedule Of Asset Acquisition, By Acquisition [Line Items]
Contingent consideration		$ 358,900,000		$ 192,400,000
Change in fair value of contingent consideration		$ 5,800,000	$ (28,700,000)
Fair Value Inputs, Weighted Average Cost of Capital		11.50%
Acquisition costs not expensed		$ 53,300,000
Achillion
Schedule Of Asset Acquisition, By Acquisition [Line Items]
Total acquisition value, per share (in dollars per share)	$ 6.30
Upfront payment to shareholders and option holders	$ 926,200,000
Contingent consideration	160,700,000	162,400,000
Contingent consideration, post-combination service period compensation	5,700,000
Change in fair value of contingent consideration		1,700,000
Upfront payment, fair value of equity compensation attributable to the post-combination service period	(20,000,000)	(25,700,000)	$ 0
Net deferred tax liability	(59,800,000)
Deferred tax assets	$ 145,500,000
Operating loss since acquisition		$ 13,900,000
Danicopan | Achillion
Schedule Of Asset Acquisition, By Acquisition [Line Items]
Contingent value rights per share (in dollars per share)	$ 1
ACH-5528, Phase 3 | Achillion
Schedule Of Asset Acquisition, By Acquisition [Line Items]
Contingent value rights per share (in dollars per share)	$ 1
Minimum [Member] | Achillion
Schedule Of Asset Acquisition, By Acquisition [Line Items]
Estimated milestone payments	$ 0
Maximum [Member] | Achillion
Schedule Of Asset Acquisition, By Acquisition [Line Items]
Estimated milestone payments	$ 306,300,000
Acquisition Related Contingent Consideration [Member] | Minimum [Member] | Level 3 [Member]
Schedule Of Asset Acquisition, By Acquisition [Line Items]
Fair Value Inputs, Cost of Debt	2.10%	1.60%
Fair Value Inputs, Weighted Average Cost of Capital		9.00%
Acquisition Related Contingent Consideration [Member] | Maximum [Member] | Level 3 [Member]
Schedule Of Asset Acquisition, By Acquisition [Line Items]
Fair Value Inputs, Cost of Debt	2.30%	1.80%
Acquired IPRD [Member] | Achillion
Schedule Of Asset Acquisition, By Acquisition [Line Items]
In-process research & development assets (IPR&D)	$ 918,000,000
Net deferred tax liability	$ (205,300,000)

Acquisitions (Summary of Total

Acquisitions (Summary of Total Consideration) (Details) - USD ($) $ in Millions	Jan. 28, 2020	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Consideration
Contingent consideration		$ 358.9		$ 192.4
Assets Acquired and Liabilities Assumed
Goodwill		5,072.1		$ 5,037.4
Achillion
Consideration
Upfront payment to shareholders and option holders	$ 926.2
Upfront payment, fair value of equity compensation attributable to the post-combination service period	(20)	(25.7)	$ 0
Upfront cash paid, net	906.2
Contingent consideration	160.7	$ 162.4
Contingent consideration, fair value of equity compensation attributable to the post-combination service period	(5.7)
Total consideration	1,061.2
Assets Acquired and Liabilities Assumed
Cash and cash equivalents	68.5
Marketable securities	106.1
Goodwill	34.7
Deferred tax liabilities, net	(59.8)
Other assets and liabilities, net	(6.3)
Total net assets acquired	1,061.2
Acquired IPRD [Member] | Achillion
Assets Acquired and Liabilities Assumed
In-process research & development assets (IPR&D)	918
Deferred tax liabilities, net	$ (205.3)

Acquisitions (Pro Forma Informa

Acquisitions (Pro Forma Information) (Details) - Achillion - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Business Acquisition [Line Items]
Pro forma revenue	$ 1,444.8	$ 1,140.4
Pro forma net income	$ 574.9	$ 515.7

Acquisitions (Acquisition-Relat

Acquisitions (Acquisition-Related Costs) (Details) - Achillion - USD ($) $ in Millions	Jan. 28, 2020	Mar. 31, 2020	Mar. 31, 2019
Business Acquisition [Line Items]
Transaction costs		$ 1.4	$ 0
Integration costs		0.1	0
Fair value of equity compensation attributable to the post-combination service period	$ 20	25.7	0
Restructuring-related costs (2)		10.9	0
Acquisition-related costs		$ 38.1	$ 0

Inventories (Details)

Inventories (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Inventory [Line Items]
Raw materials	$ 49.6	$ 41.2
Work-in-process	108.8	180.8
Finished goods	428.4	405.6
Inventory, Net	586.8	627.6
Pre Approval Inventory
Inventory [Line Items]
Inventory, Net	$ 67.3	$ 60.5

Intangible Assets and Goodwil_2

Intangible Assets and Goodwill (Schedule of Intangible Assets and Goodwill) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Jan. 28, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Intangible Assets, Cost	$ 5,696.4		$ 4,778.4
Goodwill, Gross	5,075		5,040.3
Intangible Assets, Accumulated Amortization	(1,508.8)		(1,434.1)
Goodwill, Accumulated Amortization	(2.9)		(2.9)
Intangible assets	4,187.6		3,344.3
Goodwill	5,072.1		5,037.4
Licensing Agreements [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	57		57
Finite-Lived Intangible Assets, Accumulated Amortization	(35.6)		(34.7)
Net	$ 21.4		22.3
Patents [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	7 years
Finite-Lived Intangible Assets, Gross	$ 10.5		10.5
Finite-Lived Intangible Assets, Accumulated Amortization	(10.5)		(10.5)
Net	0		0
Purchased Technology [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	4,710.5		4,710.5
Finite-Lived Intangible Assets, Accumulated Amortization	(1,462.4)		(1,388.7)
Net	$ 3,248.1		3,321.8
Other Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	5 years
Finite-Lived Intangible Assets, Gross	$ 0.4		0.4
Finite-Lived Intangible Assets, Accumulated Amortization	(0.3)		(0.2)
Net	$ 0.1		0.2
Minimum [Member] | Licensing Agreements [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	3 years
Minimum [Member] | Purchased Technology [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	6 years
Maximum [Member] | Licensing Agreements [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	8 years
Maximum [Member] | Purchased Technology [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	16 years
Achillion
Finite-Lived Intangible Assets [Line Items]
Goodwill		$ 34.7
Synageva BioPharma Corp. [Member] | Purchased Technology [Member]
Finite-Lived Intangible Assets [Line Items]
Net	$ 2,927.4
Acquired IPRD [Member]
Finite-Lived Intangible Assets [Line Items]
Indefinite-lived Intangible Assets (Excluding Goodwill)	$ 918		$ 0
Acquired IPRD [Member] | Achillion
Finite-Lived Intangible Assets [Line Items]
In-process research & development assets (IPR&D)		$ 918

Intangible Assets and Goodwil_3

Intangible Assets and Goodwill (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Jan. 28, 2020	Dec. 31, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
Amortization of purchased intangible assets	$ 74.7	$ 80.5
2020 (remainder)	224
2021	298
2022	298
2023	298
2024	298
2025	298
Goodwill [Line Items]
Goodwill	$ 5,072.1			$ 5,037.4
Achillion
Goodwill [Line Items]
Goodwill			$ 34.7

Intangible Assets and Goodwil_4

Intangible Assets and Goodwill (Changes in Goodwill) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Jan. 28, 2020
Goodwill [Roll Forward]
Balance at December 31, 2019		$ 5,037.4
Balance at March 31, 2020	$ 5,072.1
Achillion
Goodwill [Roll Forward]
Balance at December 31, 2019	$ 34.7
Balance at March 31, 2020		$ 34.7

Debt (Details)

Debt (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2019	Jun. 07, 2018
Debt Instrument [Line Items]
Payments of financing costs			$ 53.1
Amortization of deferred financing costs	$ 1.2	$ 1.2
Remaining unamortized deferred financing costs	14.6			$ 15.8
Credit Agreement [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Letters of credit, amount outstanding	1
Credit Agreement [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity					$ 1,000
Credit Agreement [Member] | Senior Secured Term Loan [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Debt instrument, principal amount					$ 2,612.5
Debt Instrument, quarterly payment as a percent of total borrowings					5.00%
Debt Instrument, Periodic Payment	(32.6)
Outstanding debt	$ 2,481.9

Earnings Per Common Share (Summ

Earnings Per Common Share (Summary Of Calculation Of Basic And Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Earnings Per Common Share [Abstract]
Net income used for basic and diluted calculation	$ 557.6	$ 587.9
Shares used in computing earnings per common share—basic	221.6	223.8
Stock awards	1	1.7
Shares used in computing earnings per common share-diluted	222.6	225.5
Earnings (loss) Per Share, Basic	$ 2.52	$ 2.63
Earnings (loss) Per Share, Diluted	$ 2.50	$ 2.61

Earnings Per Common Share (Narr

Earnings Per Common Share (Narrative) (Details) - shares shares in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share [Abstract]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	3.2	2.6

Marketable Securities (Availabl

Marketable Securities (Available-for-sale Investments by Classification in Balance Sheet) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 204.2	$ 369
Cash and Cash Equivalents [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value	179.1	328.1
Marketable Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value	25.1	40.9
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	134.2	246.9
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	0	0
Fair Value	134.2	246.9
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3.6	24.3
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	0.1	0
Fair Value	3.5	24.3
U.S.
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	60	70.4
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	0	0
Fair Value	60	70.4
Bank certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6.5	27.4
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	0	0
Fair Value	6.5	27.4
Total available-for-sale debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	204.3	369
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	0.1	0
Fair Value	$ 204.2	$ 369

Marketable Securities (Availa_2

Marketable Securities (Available-for-sale Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Due in one year or less	$ 204.2
Due after one year through three years	0
Estimated Fair Value	$ 204.2	$ 369

Marketable Securities (Narrativ

Marketable Securities (Narrative) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 20.2	$ 21.5
Trading securities. fair value	$ 22.7	$ 23.1

Derivative Instruments and He_3

Derivative Instruments and Hedging Activities (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Derivative [Line Items]
Total revenues	$ 1,444.8	$ 1,140.4
Interest expense	$ (25.8)	(19.9)
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Foreign Exchange Forward Contracts Term	60 months
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Foreign Exchange Forward Contracts Term	7 months
Gain (loss) reclassified from AOCI to net product sales, net of tax	$ 16.2	3.2
Foreign Exchange Forward [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional amount of derivative instruments	1,096.6
Foreign Exchange Forward [Member] | Designated as Hedging Instrument [Member] | Revenue
Derivative [Line Items]
Estimated gains (losses) to be reclassified from other comprehenisve income in next 12 months	27.8
Foreign Exchange Forward [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional amount of derivative instruments	1,066.7
Foreign Exchange Forward, Open Expense [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional amount of derivative instruments	12.7
Interest Rate Swap Two [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional amount of derivative instruments	$ 450
Interest Rate Swap Two [Member] | Designated as Hedging Instrument [Member] | Minimum [Member]
Derivative [Line Items]
Fixed interest borrowing spreads	2.60%
Interest Rate Swap Two [Member] | Designated as Hedging Instrument [Member] | Maximum [Member]
Derivative [Line Items]
Fixed interest borrowing spreads	2.79%
Interest Rate Contract [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Estimated gains (losses) to be reclassified from other comprehenisve income in next 12 months	$ 42.1
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Gain (Loss), Cash Flow Hedge, Including Noncontrolling Interest [Member]
Derivative [Line Items]
Total revenues	11.4	7
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Gain (Loss), Cash Flow Hedge, Including Noncontrolling Interest [Member] | Foreign Exchange Forward [Member]
Derivative [Line Items]
Total revenues	11.4	7
Interest expense	0	0
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Gain (Loss), Cash Flow Hedge, Including Noncontrolling Interest [Member] | Interest Rate Contract [Member]
Derivative [Line Items]
Total revenues	0	0
Interest expense	(4.6)	4.6
Net Product Sales
Derivative [Line Items]
Total revenues	$ 1,444.6	$ 1,140.2

Derivative Instruments and He_4

Derivative Instruments and Hedging Activities (Schedule of Interest Rate Contracts) (Details) - Designated as Hedging Instrument [Member] $ in Millions	Mar. 31, 2020USD ($)
Interest Rate Swap Two [Member]
Derivative [Line Items]
Derivative, notional amount	$ 450
Interest Rate Swap Four [Member]
Derivative [Line Items]
Derivative, notional amount	$ 1,300
Minimum [Member] | Interest Rate Swap Two [Member]
Derivative [Line Items]
Derivative, fixed interest rate	2.60%
Minimum [Member] | Interest Rate Swap Four [Member]
Derivative [Line Items]
Derivative, fixed interest rate	2.37%
Maximum [Member] | Interest Rate Swap Two [Member]
Derivative [Line Items]
Derivative, fixed interest rate	2.79%
Maximum [Member] | Interest Rate Swap Four [Member]
Derivative [Line Items]
Derivative, fixed interest rate	2.83%

Derivative Instruments and He_5

Derivative Instruments and Hedging Activities (Schedule Of Other Comprehensive Income And Earnings From Foreign Exchange Contracts) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Derivative Instruments, Gain (Loss) [Line Items]
Total revenues	$ 1,444.8	$ 1,140.4
Interest expense	(25.8)	(19.9)
Foreign Exchange Forward [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in AOCI, net of tax	26	14.2
Foreign Exchange Forward [Member] | Sales | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax	8.8	5.4
Interest Rate Contract [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in AOCI, net of tax	(47.3)	(14.2)
Interest Rate Contract [Member] | Interest Expense | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax	(3.6)	3.6
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Gain (Loss), Cash Flow Hedge, Including Noncontrolling Interest [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total revenues	11.4	7
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Gain (Loss), Cash Flow Hedge, Including Noncontrolling Interest [Member] | Foreign Exchange Forward [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total revenues	11.4	7
Interest expense	0	0
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Gain (Loss), Cash Flow Hedge, Including Noncontrolling Interest [Member] | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total revenues	0	0
Interest expense	$ (4.6)	$ 4.6

Derivative Instruments and He_6

Derivative Instruments and Hedging Activities (Schedule Of Fair Value Of Outstanding Derivatives) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Interest Rate Contract [Member] | Other Current Liabilities | Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives, Fair Value	$ 42.1	$ 19.5
Interest Rate Contract [Member] | Other Assets | Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value		0
Interest Rate Contract [Member] | Other Liabilities | Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives, Fair Value	75.8	41.9
Interest Rate Contract [Member] | Prepaid Expenses and Other Current Assets [Member] | Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value		0
Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value	54.3	30.5
Liability Derivatives, Fair Value	132.8	89.1
Foreign Exchange Forward [Member] | Other Current Liabilities | Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives, Fair Value	1.7	6.2
Foreign Exchange Forward [Member] | Other Assets | Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value	1.1	0.6
Foreign Exchange Forward [Member] | Other Liabilities | Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives, Fair Value	0.8	1.1
Foreign Exchange Forward [Member] | Prepaid Expenses and Other Current Assets [Member] | Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value	29.1	12.7
Foreign Exchange Forward [Member] | Other Current Liabilities | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives, Fair Value	12.4	20.4
Foreign Exchange Forward [Member] | Prepaid Expenses and Other Current Assets [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value	24.1	$ 17.2
Interest Rate Contract [Member] | Level 2 [Member] | Other Assets
Derivatives, Fair Value [Line Items]
Interest Rate Derivative Assets, at Fair Value	0
Interest Rate Contract [Member] | Level 2 [Member] | Prepaid Expenses and Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Interest Rate Derivative Assets, at Fair Value	$ 0

Derivative Instruments and He_7

Derivative Instruments and Hedging Activities (Offsetting Assets and Liabilities) (Details) - Foreign Exchange Forward [Member] - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Derivative Asset, Fair Value, Amount Offset Against Collateral [Abstract]
Gross Amounts of Recognized Assets	$ 54.3	$ 30.5
Gross Amounts Offset in the Condensed Consolidated Balance Sheet	0	0
Amounts of Assets Presented in the Condensed Consolidated Balance Sheet	54.3	30.5
Derivative Financial Instruments	(14.9)	(21.4)
Cash Collateral Received	0	0
Net Amount	39.4	9.1
Derivative Liability, Fair Value, Amount Offset Against Collateral [Abstract]
Gross Amounts of Recognized Liabilities	(132.8)	(89.1)
Gross Amounts Offset in the Condensed Consolidated Balance Sheet	0	0
Amounts of Liabilities Presented in the Condensed Consolidated Balance Sheet	(132.8)	(89.1)
Derivative Financial Instruments	14.9	21.4
Cash Collateral Pledged	0	0
Net Amount	$ (117.9)	$ (67.7)

Other Investments (Details)

Other Investments (Details) - USD ($) $ in Millions	Dec. 09, 2019	Mar. 31, 2020	Oct. 31, 2019	Sep. 30, 2019	Mar. 31, 2019	Jan. 31, 2019	Oct. 31, 2018	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2014
Schedule of Equity Method Investments [Line Items]
Payments to Acquire Other Investments								$ 34.5		$ 43.8
Other income and (expense)								(0.9)		2.4
Research and development								200.9		195.9
Unrealized Gain (Loss) on Investments								(9.2)		33.8
Zealand [Member]
Schedule of Equity Method Investments [Line Items]
Unrealized Gain (Loss) on Investments								(0.2)	$ (0.7)
Equity Securities, FV-NI		$ 27.7						27.7	28.5		$ 28.5
Payments to Acquire Equity Method Investments					$ 13.8
Moderna LLC [Member]
Schedule of Equity Method Investments [Line Items]
Payments to Acquire Other Investments												$ 37.5
Unrealized Gain (Loss) on Investments										29.8
Proceeds from Sale of Equity Method Investments	$ 114.7
Equity Method Investment, Realized Gain (Loss) on Disposal	$ 77.2
Dicerna [Member]
Schedule of Equity Method Investments [Line Items]
Unrealized Gain (Loss) on Investments								(3.1)		$ (3.3)
Equity Securities, FV-NI		15.3						15.3	18.4		18.4
Payments to Acquire Equity Method Investments							$ 10.3
Caelum Biosciences [Member]
Schedule of Equity Method Investments [Line Items]
Payments to Acquire Other Investments						$ 41
Option To Acquire Remaining Equity, Amount						$ 16.1					64
Other income and (expense)											32
Eidos [Member]
Schedule of Equity Method Investments [Line Items]
Unrealized Gain (Loss) on Investments								(3.7)
Equity Securities, FV-NI		24.1						24.1	27.8		27.8
Payments to Acquire Equity Method Investments				$ 19.9
Stealth
Schedule of Equity Method Investments [Line Items]
Unrealized Gain (Loss) on Investments								(2.8)
Equity Securities, FV-NI		1.6						1.6	$ 4.4		4.4
Payments to Acquire Equity Method Investments			$ 9.6
Common Stock [Member]
Schedule of Equity Method Investments [Line Items]
Unrealized Gain (Loss) on Investments								0.6
Equity Securities, FV-NI		15.1						$ 15.1
Payments to Acquire Equity Method Investments		$ 14.5
Collaborative Arrangement [Member] | Caelum Biosciences [Member]
Schedule of Equity Method Investments [Line Items]
Collaboration Agreement, Potential Future Payments											20
Research and development											2.3
Collaborative Agreement, Amendment [Member] | Caelum Biosciences [Member]
Schedule of Equity Method Investments [Line Items]
Research and development											$ 4.1

Stockholders' Equity (Details)

Stockholders' Equity (Details) - USD ($) shares in Millions, $ in Millions	1 Months Ended	3 Months Ended
	May 06, 2020	Mar. 31, 2020	Mar. 31, 2019	May 04, 2020	Oct. 22, 2019	Feb. 28, 2017
Class of Stock [Line Items]
Treasury Stock, Shares, Acquired		1.3	0.1
Treasury Stock, Value, Acquired, Cost Method		$ 107.1	$ 11.3
Common Stock [Member]
Class of Stock [Line Items]
Repurchase authorization amount					$ 1,000	$ 1,000
Subsequent Event
Class of Stock [Line Items]
Treasury Stock, Shares, Acquired	0.4
Treasury Stock, Value, Acquired, Cost Method	$ 35
Subsequent Event | Common Stock [Member]
Class of Stock [Line Items]
Remaining authorized repurchase amount				$ 893.3

Other Comprehensive Income an_3

Other Comprehensive Income and Accumulated Other Comprehensive Income (Changes in AOCI) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ (66.8)	$ (9.7)
Other comprehensive income (loss) before reclassifications	(29.5)	(1.8)
Amounts reclassified from other comprehensive income	(5.2)	(9)
Other comprehensive income (loss), net of tax	(34.7)	(10.8)
Ending Balance	(101.5)	(20.5)
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(9.2)	(2.6)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from other comprehensive income	0	0
Other comprehensive income (loss), net of tax	0	0
Ending Balance	(9.2)	(2.6)
Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(0.1)	(0.3)
Other comprehensive income (loss) before reclassifications	(0.2)	0.2
Amounts reclassified from other comprehensive income	0	0
Other comprehensive income (loss), net of tax	(0.2)	0.2
Ending Balance	(0.3)	(0.1)
Accumulated Gain (Loss), Cash Flow Hedge, Including Noncontrolling Interest [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(40.1)	9.6
Other comprehensive income (loss) before reclassifications	(21.3)	0
Amounts reclassified from other comprehensive income	(5.2)	(9)
Other comprehensive income (loss), net of tax	(26.5)	(9)
Ending Balance	(66.6)	0.6
Accumulated Translation Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(17.4)	(16.4)
Other comprehensive income (loss) before reclassifications	(8)	(2)
Amounts reclassified from other comprehensive income	0	0
Other comprehensive income (loss), net of tax	(8)	(2)
Ending Balance	$ (25.4)	$ (18.4)

Other Comprehensive Income an_4

Other Comprehensive Income and Accumulated Other Comprehensive Income (Significant Reclassifications from AOCI) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total revenues	$ 1,444.8	$ 1,140.4
Other income (expense)	(7.1)	(1.9)
Income before income taxes	663.6	541.8
Income tax provision	(106)	46.1
Net income	557.6	587.9
Accumulated Gain (Loss), Cash Flow Hedge, Including Noncontrolling Interest [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total revenues	11.4	7
Other income (expense)	(4.6)	4.6
Income before income taxes	6.8	11.6
Income tax provision	(1.6)	(2.6)
Net income	$ 5.2	$ 9

Fair Value Measurement (Schedul

Fair Value Measurement (Schedule Of Assets And Liabilites Measured At Fair Value) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Other Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities
Foreign Exchange Forward [Member] | Other Current Liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign exchange forward contracts, liability	14.1	$ 26.6
Foreign Exchange Forward [Member] | Other Current Liabilities | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign exchange forward contracts, liability	0	0
Foreign Exchange Forward [Member] | Other Current Liabilities | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign exchange forward contracts, liability	14.1	26.6
Foreign Exchange Forward [Member] | Other Current Liabilities | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign exchange forward contracts, liability	0	0
Foreign Exchange Forward [Member] | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign exchange forward contracts, liability	0.8	1.1
Foreign Exchange Forward [Member] | Other Liabilities | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign exchange forward contracts, liability	0	0
Foreign Exchange Forward [Member] | Other Liabilities | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign exchange forward contracts, liability	0.8	1.1
Foreign Exchange Forward [Member] | Other Liabilities | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign exchange forward contracts, liability	0	0
Acquisition Related Contingent Consideration [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Business Combination, Contingent Consideration, Liability	358.9
Acquisition Related Contingent Consideration [Member] | Other Current Liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Business Combination, Contingent Consideration, Liability		24
Acquisition Related Contingent Consideration [Member] | Other Current Liabilities | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Business Combination, Contingent Consideration, Liability		0
Acquisition Related Contingent Consideration [Member] | Other Current Liabilities | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Business Combination, Contingent Consideration, Liability		0
Acquisition Related Contingent Consideration [Member] | Other Current Liabilities | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Business Combination, Contingent Consideration, Liability		24
Acquisition Related Contingent Consideration [Member] | Contingent Consideration [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Business Combination, Contingent Consideration, Liability	358.9	192.4
Acquisition Related Contingent Consideration [Member] | Contingent Consideration [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Business Combination, Contingent Consideration, Liability	0	0
Acquisition Related Contingent Consideration [Member] | Contingent Consideration [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Business Combination, Contingent Consideration, Liability	0	0
Acquisition Related Contingent Consideration [Member] | Contingent Consideration [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Business Combination, Contingent Consideration, Liability	358.9	192.4
Other Contingent Payments [Member] | Other Current Liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Business Combination, Contingent Consideration, Liability	26.3
Other Contingent Payments [Member] | Other Current Liabilities | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Business Combination, Contingent Consideration, Liability	0
Other Contingent Payments [Member] | Other Current Liabilities | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Business Combination, Contingent Consideration, Liability	0
Other Contingent Payments [Member] | Other Current Liabilities | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Business Combination, Contingent Consideration, Liability	26.3
Interest Rate Contract [Member] | Prepaid Expenses and Other Current Assets [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest Rate Derivative Assets, at Fair Value	0
Interest Rate Contract [Member] | Other Assets | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest Rate Derivative Assets, at Fair Value	0
Interest Rate Contract [Member] | Other Current Liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest Rate Derivative Liabilities, at Fair Value	42.1	19.5
Interest Rate Contract [Member] | Other Current Liabilities | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest Rate Derivative Assets, at Fair Value		0
Interest Rate Derivative Liabilities, at Fair Value	0
Interest Rate Contract [Member] | Other Current Liabilities | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest Rate Derivative Liabilities, at Fair Value	42.1	19.5
Interest Rate Contract [Member] | Other Current Liabilities | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest Rate Derivative Assets, at Fair Value		0
Interest Rate Derivative Liabilities, at Fair Value	0
Interest Rate Contract [Member] | Other Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest Rate Derivative Assets, at Fair Value	75.8	41.9
Interest Rate Contract [Member] | Other Liabilities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest Rate Derivative Assets, at Fair Value	0	0
Interest Rate Contract [Member] | Other Liabilities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest Rate Derivative Assets, at Fair Value	75.8	41.9
Interest Rate Contract [Member] | Other Liabilities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest Rate Derivative Assets, at Fair Value	0	0
Money Market Funds [Member] | Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	679.4	635.9
Money Market Funds [Member] | Cash Equivalents [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Money Market Funds [Member] | Cash Equivalents [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	679.4	635.9
Money Market Funds [Member] | Cash Equivalents [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Mutual Funds [Member] | Marketable Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	22.7	23.1
Mutual Funds [Member] | Marketable Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	22.7	23.1
Mutual Funds [Member] | Marketable Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Mutual Funds [Member] | Marketable Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Commercial paper | Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	119.1	227.9
Commercial paper | Cash Equivalents [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Commercial paper | Cash Equivalents [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	119.1	227.9
Commercial paper | Cash Equivalents [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Commercial paper | Marketable Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	15.1	19
Commercial paper | Marketable Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Commercial paper | Marketable Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	15.1	19
Commercial paper | Marketable Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Corporate bonds | Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		20.6
Corporate bonds | Cash Equivalents [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		0
Corporate bonds | Cash Equivalents [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		20.6
Corporate bonds | Cash Equivalents [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		0
Corporate bonds | Marketable Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	3.5	3.7
Corporate bonds | Marketable Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Corporate bonds | Marketable Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	3.5	3.7
Corporate bonds | Marketable Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	0	0
Other Government Obligations [Member] | Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	60	60.4
Other Government Obligations [Member] | Cash Equivalents [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		0
Marketable securities	0
Other Government Obligations [Member] | Cash Equivalents [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	60	60.4
Other Government Obligations [Member] | Cash Equivalents [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		0
Marketable securities	0
Other Government Obligations [Member] | Marketable Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities		10
Other Government Obligations [Member] | Marketable Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities		0
Other Government Obligations [Member] | Marketable Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities		10
Other Government Obligations [Member] | Marketable Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities		0
Bank certificates of deposit | Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		19.2
Bank certificates of deposit | Cash Equivalents [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		0
Bank certificates of deposit | Cash Equivalents [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		19.2
Bank certificates of deposit | Cash Equivalents [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		0
Bank certificates of deposit | Marketable Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	6.5	8.2
Bank certificates of deposit | Marketable Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0
Marketable securities		0
Bank certificates of deposit | Marketable Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	6.5	8.2
Bank certificates of deposit | Marketable Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0
Marketable securities		0
Foreign Exchange Forward [Member] | Prepaid Expenses and Other Current Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign exchange forward contracts, asset	53.2	29.9
Foreign Exchange Forward [Member] | Prepaid Expenses and Other Current Assets [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign exchange forward contracts, asset	0	0
Foreign Exchange Forward [Member] | Prepaid Expenses and Other Current Assets [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign exchange forward contracts, asset	53.2	29.9
Foreign Exchange Forward [Member] | Prepaid Expenses and Other Current Assets [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign exchange forward contracts, asset	0	0
Foreign Exchange Forward [Member] | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign exchange forward contracts, asset	1.1	0.6
Foreign Exchange Forward [Member] | Other Assets | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign exchange forward contracts, asset	0	0
Foreign Exchange Forward [Member] | Other Assets | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign exchange forward contracts, asset	1.1	0.6
Foreign Exchange Forward [Member] | Other Assets | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign exchange forward contracts, asset	0	0
Equity Securities [Member] | Other Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	83.8	79
Equity Securities [Member] | Other Assets [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	59.7	51.2
Equity Securities [Member] | Other Assets [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	24.1	$ 27.8
Equity Securities [Member] | Other Assets [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	$ 0

Fair Value Measurement (Sched_2

Fair Value Measurement (Schedule Of Acquisition-Related Contingent Consideration) (Details) - USD ($) $ in Millions	Jan. 28, 2020	Mar. 31, 2020
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value Inputs, Weighted Average Cost of Capital		11.50%
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, Low		$ 0
Business Combination, Contingent Consideration, Arrangements, Range of Outcomes, Value, High		908.3
Acquisition-Related Contingent Consideration [Roll Forward]
Asset Acquisition, Contingent Consideration, Milestone Payments		160.7
Acquisition Related Contingent Consideration [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Business Combination, Contingent Consideration, Liability		358.9
Acquisition Related Contingent Consideration [Member] | Level 3 [Member]
Acquisition-Related Contingent Consideration [Roll Forward]
Balance at beginning of period		192.4
Change in fair value		5.8
Balance at end of period		$ 358.9
Acquisition Related Contingent Consideration [Member] | Minimum [Member] | Level 3 [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value Inputs, Cost of Debt	2.10%	1.60%
Fair Value Inputs, Weighted Average Cost of Capital		9.00%
Acquisition Related Contingent Consideration [Member] | Maximum [Member] | Level 3 [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value Inputs, Cost of Debt	2.30%	1.80%

Fair Value Measurement (Narrati

Fair Value Measurement (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended		3 Months Ended		12 Months Ended
	Apr. 30, 2020	Jan. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Research and development			$ 200.9	$ 195.9
Caelum Biosciences [Member] | Collaborative Arrangement [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Collaboration Agreement, Upfront Payment		$ 30
Collaboration Agreement, Potential Future Payments, Milestone Achievement		$ 30
Derivative Liability			$ 26.3		$ 27.1
Derivative Fair Value, Discounted Cost Of Debt			2.10%
Collaboration Agreement, Potential Future Payments					20
Collaboration Agreement, Payment For Additional Equity Interest					60
Research and development					$ 2.3
Minimum [Member] | Caelum Biosciences [Member] | Collaborative Arrangement [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Derivative Fair Value, Discounted Cost Of Debt		3.30%
Maximum [Member] | Caelum Biosciences [Member] | Collaborative Arrangement [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Derivative Fair Value, Discounted Cost Of Debt		3.50%
Subsequent Event | Caelum Biosciences [Member] | Collaborative Arrangement [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Collaboration Agreement, Milestone Payment	$ 15

Revenue Recognition (Disaggrega

Revenue Recognition (Disaggregation of Revenue by Product and Geographical Region) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	$ 1,444.8	$ 1,140.4
Soliris
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	1,022.9	962
Ultomiris [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	222.8	24.6
Strensiq Product Line
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	172.2	130.1
Kanuma Product Line
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	26.7	23.5
United States | Soliris
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	556.2	463.7
United States | Ultomiris [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	131.5	24.6
United States | Strensiq Product Line
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	128.1	99.5
United States | Kanuma Product Line
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	16.4	13.8
Europe | Soliris
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	263.5	264.5
Europe | Ultomiris [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	33.8	0
Europe | Strensiq Product Line
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	24	17.5
Europe | Kanuma Product Line
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	7.5	6.3
Asia Pacific | Soliris
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	87.1	100.9
Asia Pacific | Ultomiris [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	57.1	0
Asia Pacific | Strensiq Product Line
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	13.6	9.9
Asia Pacific | Kanuma Product Line
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	0.9	0.8
Rest of World | Soliris
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	116.1	132.9
Rest of World | Ultomiris [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	0.4	0
Rest of World | Strensiq Product Line
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	6.5	3.2
Rest of World | Kanuma Product Line
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	1.9	2.6
Product [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Net Product Sales	$ 1,444.6	$ 1,140.2

Revenue Recognition (Summary of

Revenue Recognition (Summary of Receivables and Contract Liabilities from Contracts) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Revenue from Contract with Customer [Abstract]
Receivables, which are included in Trade accounts receivable, net	$ 1,345.2	$ 1,243.2
Contract liabilities, which are included in Other current liabilities	$ 22.9	$ 6.8

Income Taxes (Schedule of Incom

Income Taxes (Schedule of Income Tax Provision and Effective Tax Rate) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Income Tax Disclosure [Abstract]
Provision for income taxes	$ 106	$ (46.1)
Effective tax rate	16.00%	(8.50%)

Income Taxes (Narrative) (Detai

Income Taxes (Narrative) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019USD ($)
Schedule Of Asset Acquisition, By Acquisition [Line Items]
Effective Income Tax Rate Reconciliation, Deduction, Percent	23.30%
Intellectual Property Election [Member]
Schedule Of Asset Acquisition, By Acquisition [Line Items]
Deferred Income Tax Expense (Benefit)	$ (95.7)
Valuation Allowance Release [Member]
Schedule Of Asset Acquisition, By Acquisition [Line Items]
Deferred Income Tax Expense (Benefit)	$ (30.3)

Commitments and Contingencies (

Commitments and Contingencies (Narrative) (Details) - USD ($) $ in Millions

1 Months Ended

3 Months Ended

12 Months Ended

Apr. 30, 2020

Sep. 30, 2019

Mar. 31, 2019

Jan. 31, 2019

Nov. 30, 2018

Oct. 31, 2018

Mar. 31, 2020

Dec. 31, 2019

Sep. 30, 2019

Mar. 31, 2019

Dec. 31, 2018

Dec. 31, 2017

Dec. 31, 2019

Other Commitments [Line Items]

Research and development

$ 200.9

$ 195.9

Asset Acquisition, Contingent Consideration, Milestone Payments

160.7

Lonza Group AG

Other Commitments [Line Items]

Remaining total commitments

1,053.2

Unrecorded Unconditional Purchase Obligation, Undiscounted Fixed Payment Amount

97.7

Other Third Party Manufacturers

Other Commitments [Line Items]

Remaining total commitments

64.6

Collaborative Arrangement [Member] | Caelum Biosciences [Member]

Other Commitments [Line Items]

Collaboration Agreement, Upfront Payment

$ 30

Research and development

$ 2.3

Collaboration Agreement, Potential Future Payments, Milestone Achievement

30

Derivative Liability

26.3

$ 27.1

27.1

Collaboration Agreement, Acquisition Option, Maximum Potential Future Payment

500

Collaboration Agreement, Potential Future Payments

20

Collaboration Agreement, Payment For Additional Equity Interest

$ 60

Zealand Pharma [Member] | Zealand [Member]

Other Commitments [Line Items]

Payments To Acquire Licenses And Equity Investment

$ 40

Prepaid Research And Development Expense

5

5

Research and development

$ 21.2

Collaboration Agreement, Potential Future Payments, Milestone Achievement

610

Collaboration Agreement, Potential Future Payments, Additional Milestone Achievement

115

Collaboration Agreement, Potential Future Payments, Option Fee

15

Affibody AB [Domain] | Affibody [Member]

Other Commitments [Line Items]

Payments To Acquire Licenses And Equity Investment

25

Affibody AB [Domain] | Eidos [Member]

Other Commitments [Line Items]

Payments To Acquire Licenses And Equity Investment

$ 50

Research and development

$ 30.1

Collaboration Agreement, Potential Future Payments, Milestone Achievement

30

Collaboration and License Agreement [Member]

Other Commitments [Line Items]

Collaboration Agreement, Potential Future Payments, Milestone Achievement

42.5

Dicerna Pharmaceutical Collaboration Agreement [Member] | Dicerna [Member]

Other Commitments [Line Items]

Payments To Acquire Licenses And Equity Investment

$ 37

Research and development

$ 20

$ 26.7

Research and Development Arrangement, Potential Payment, Maximum

614.1

License Agreement 1 [Member] | Halozyme Therapeutics, Inc [Member]

Other Commitments [Line Items]

Research and Development Arrangement, Potential Payment, Maximum

160

Research and Development Expense (Excluding Acquired in Process Cost)

$ 40

Eidos [Member]

Other Commitments [Line Items]

Payments to Acquire Equity Method Investments

$ 19.9

Caelum Biosciences [Member] | Collaborative Arrangement [Member]

Other Commitments [Line Items]

Equity Method Investment, Aggregate Cost

$ 57.1

Zealand [Member]

Other Commitments [Line Items]

Payments to Acquire Equity Method Investments

$ 13.8

Dicerna [Member]

Other Commitments [Line Items]

Payments to Acquire Equity Method Investments

$ 10.3

DOJ And OIG [Member]

Other Commitments [Line Items]

Loss Contingency Accrual

13.1

Canadian Patented Medicine Prices Review Board [Member]

Other Commitments [Line Items]

Escrow Deposit

45.4

Loss Contingency, Estimate of Possible Loss

$ 31.4

Syntimmune, Inc [Member]

Other Commitments [Line Items]

Asset Acquisition, Contingent Consideration, Milestone Payments

$ 800

Subsequent Event | Collaborative Arrangement [Member] | Caelum Biosciences [Member]

Other Commitments [Line Items]

Collaboration Agreement, Milestone Payment

$ 15

Subsequent Events (Details)

Subsequent Events (Details) - Scenario, Forecast - Portola Pharmaceuticals [Member] $ / shares in Units, $ in Millions	3 Months Ended
	Sep. 30, 2020USD ($)$ / shares
Subsequent Event [Line Items]
Total acquisition value, per share (in dollars per share) | $ / shares	$ 18
Upfront payment to shareholders and option holders | $	$ 1,380