Number of persons employed by the Entity

No definition available.

Name:	dei_EntityNumberOfEmployees
Namespace Prefix:	dei_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Business Combination, Consideration Transferred, Cash Payment To Shareholders, Per Share Held

+ References

No definition available.

+ Details

Name:	iclr_BusinessCombinationConsiderationTransferredCashPaymentToShareholdersPerShareHeld
Namespace Prefix:	iclr_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Business Combination, Consideration Transferred, Shares Issued, Per Current Share Held To Shareholders

+ References

No definition available.

+ Details

Name:	iclr_BusinessCombinationConsiderationTransferredSharesIssuedToShareholdersPerCurrentShareHeld
Namespace Prefix:	iclr_
Data Type:	xbrli:sharesItemType
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_BusinessAcquisitionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 23
-URI http://asc.fasb.org/extlink&oid=123586518&loc=d3e1043-128460

+ Details

Name:	us-gaap_BusinessCombinationAcquisitionRelatedCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Costs incurred to effect a business combination which have been expensed during the period. Such costs could include business integration costs, systems integration and conversion costs, and severance and other employee-related costs.

+ References

No definition available.

+ Details

Name:	us-gaap_BusinessCombinationIntegrationRelatedCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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

Business combinations - Narrative (Details) $ / shares in Units, $ in Thousands		3 Months Ended
	Jul. 01, 2021 Employee $ / shares shares	Mar. 31, 2022 USD ($)	Mar. 31, 2021 USD ($)
Business Acquisition [Line Items]
Entity number of employees \| Employee	38,000
Acquisition-related costs		$ 12,085	$ 12,501
PRA Health Sciences, Inc
Business Acquisition [Line Items]
Business acquisition, share price (in USD per share) \| $ / shares	$ 80
Business acquisition, number of shares issued (in shares) \| shares	0.4125
Integration-related costs		$ 12,100
Acquisition-related costs			$ 12,500