Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) - Narrative (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2026	Dec. 31, 2025
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Increase (decrease) in unbilled services (unbilled revenue)	$ (73,422)
Increase in unearned revenue (Payments on account)	176,884
Revenue, remaining performance obligation	$ 16,400,000	$ 14,900,000
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation percentage		48.00%
Revenue, remaining performance obligation, expected timing of satisfaction, period		12 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation percentage	47.00%
Revenue, remaining performance obligation, expected timing of satisfaction, period	12 months

Increase (Decrease) In Contract With Customer, Asset, Gross, Current

No definition available.

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Increase (Decrease) In Contract With Customer, Liability, Gross, Current

+ References

No definition available.

+ Details

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Namespace Prefix:	iclr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of transaction price allocated to performance obligation that has not been recognized as revenue.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (a)
-SubTopic 10
-Topic 606
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-13

+ Details

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

- 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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-13

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

Period in which remaining performance obligation is expected to be recognized as revenue, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of remaining performance obligation to total remaining performance obligation not recognized as revenue.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)(1)
-SubTopic 10
-Topic 606
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-13

+ Details

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Balance Type:	na
Period Type:	instant

- Details

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- Details

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