Cover Page - shares	9 Months Ended
	Sep. 30, 2020	Oct. 27, 2020
Document and Entity Information [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 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		218,845,432
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3