Foreign Exchange Forward Contracts Term

No definition available.

Name:	alxn_ForeignExchangeForwardContractsTerm
Namespace Prefix:	alxn_
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- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

The estimated value of gains (losses), net anticipated to be transferred in the future from accumulated other comprehensive income into earnings.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=112277219&loc=d3e80845-113994

+ Details

Name:	us-gaap_DerivativeInstrumentsGainLossReclassificationFromAccumulatedOCIToIncomeEstimatedNetAmountToBeTransferred
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_DerivativeLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Total amount of gain (loss) derived from fair value hedges recognized in earnings in the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624171-113959

+ Details

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

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix:
Data Type:	na
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- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember
Namespace Prefix:
Data Type:	na
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Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeForwardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=alxn_ForeignExchangeForwardOpenExpenseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Derivative Instruments and Hedging Activities (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Estimated amount to be reclassified from other comprehenisve income in next year	$ 6.7
Gain in other income and expense	$ (0.4)	$ 23.0	$ (14.7)
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Foreign exchange forward contracts durations (months)	60 months
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Foreign exchange forward contracts durations (months)	8 months
Interest Rate Contract [Member] \| Designated as Hedging Instrument [Member]
Derivative [Line Items]
Estimated amount to be reclassified from other comprehenisve income in next year	$ (19.5)
Foreign Exchange Forward [Member] \| Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional amount of foreign exchange contracts	1,089.9
Foreign Exchange Forward [Member] \| Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional amount of foreign exchange contracts	1,974.6
Foreign Exchange Forward, Open Expense [Member] \| Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional amount of foreign exchange contracts	$ 14.0