Fair Value Measurement (Schedule Of Acquisition-Related Contingent Consideration) (Details) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($)
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, Low	$ 0	$ 0
Business Combination, Contingent Consideration, Arrangements, Range of Outcomes, Value, High	741	741
Acquisition Related Contingent Consideration [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Business Combination, Contingent Consideration, Liability	160	160
Acquisition Related Contingent Consideration [Member] \| Level 3 [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Payments for contingent consideration		25
Acquisition-Related Contingent Consideration [Roll Forward]
Balance at beginning of period		(153)
Milestone payments		25
Change in fair value		(32)
Balance at end of period	(160)	$ (160)
Acquisition Related Contingent Consideration [Member] \| Minimum [Member] \| Level 3 [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value Inputs, Cost of Debt		4.70%
Fair Value Inputs, Weighted Average Cost of Capital		10.00%
Acquisition Related Contingent Consideration [Member] \| Maximum [Member] \| Level 3 [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value Inputs, Weighted Average Cost of Capital		21.00%
Enobia Pharma Corp. [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Payments for contingent consideration	25
Acquisition-Related Contingent Consideration [Roll Forward]
Milestone payments	$ 25

Fair Value Inputs, Cost of Debt

No definition available.

Name:	alxn_FairValueInputsCostOfDebt
Namespace Prefix:	alxn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair Value Inputs, Weighted Average Cost of Capital

+ References

No definition available.

+ Details

Name:	alxn_FairValueInputsWeightedAverageCostOfCapital
Namespace Prefix:	alxn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Payment for Contingent Consideration

+ References

No definition available.

+ Details

Name:	alxn_PaymentforContingentConsideration
Namespace Prefix:	alxn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

For contingent consideration arrangements recognized in connection with a business combination, this element represents an estimate of the high-end of the potential range (undiscounted) of the consideration which may be paid.

+ References

+ Details

Name:	us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

For contingent consideration arrangements and indemnification assets recognized in connection with a business combination, this element represents an estimate of the low-end of the potential range (undiscounted) of the consideration which may be paid.

+ References

+ Details

Name:	us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueLow
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of liability recognized arising from contingent consideration in a business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 35
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6911338&loc=d3e6819-128478

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 25
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6408-128476

+ Details

Name:	us-gaap_BusinessCombinationContingentConsiderationLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

Name:	us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FairValueInputsLiabilitiesQuantitativeInformationLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of gain (loss) recognized on the income statement for financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_FairValueByLiabilityClassAxis=alxn_AcquisitionRelatedContingentConsiderationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=alxn_EnobiaPharmaCorpMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: