Debt Instrument, Discount Rate

No definition available.

Name:	iclr_DebtInstrumentDiscountRate
Namespace Prefix:	iclr_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

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+ Details

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

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Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
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.

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+ Details

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

- Definition

Amount of payment greater than the preceding installment payments to be paid at final maturity date of debt.

+ References

No definition available.

+ Details

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

- Definition

Percentage of principal amount of debt redeemed.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentRedemptionPricePercentageOfPrincipalAmountRedeemed
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after accumulated amortization, of debt discount.

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+ Details

Name:	us-gaap_DebtInstrumentUnamortizedDiscount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

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+ Details

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

- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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+ Details

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

Bank credit lines, loan facilities and notes - Narrative (Details) - USD ($) $ in Millions		6 Months Ended
	May 08, 2024	Jun. 30, 2025	Dec. 31, 2024	May 02, 2023	Jul. 01, 2021
Debt Instrument [Line Items]
Long-term debt at fair value		$ 3,480.4	$ 3,469.2
The New Notes \| Senior Secured Notes
Debt Instrument [Line Items]
Debt instrument, principal amount	$ 2,000.0
Debt discount paid	6.8
Senior Secured Notes (the "2027 Notes")* \| Senior Secured Notes
Debt Instrument [Line Items]
Debt instrument, principal amount	$ 750.0
Debt instrument, interest rate (in percent)	5.809%	5.809%	5.809%
Debt instrument, discount rate on underwriting (in percent)	0.00250
Senior Secured Notes (the "2029 Notes")* \| Senior Secured Notes
Debt Instrument [Line Items]
Debt instrument, principal amount	$ 750.0
Debt instrument, interest rate (in percent)	5.849%	5.849%	5.849%
Debt instrument, discount rate on underwriting (in percent)	0.00350
Senior Secured Notes (the "2034 Notes")* \| Senior Secured Notes
Debt Instrument [Line Items]
Debt instrument, principal amount	$ 500.0
Debt instrument, interest rate (in percent)	6.00%	6.00%	6.00%
Debt discount paid	$ 0.5
Debt instrument, discount rate on underwriting (in percent)	0.00450
Debt instrument, redemption price, percentage of principal amount redeemed	99.896%
Senior Secured Term Loan \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)		0.50%
Senior Secured Term Loan \| Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)		2.00%
Senior Secured Term Loan \| Senior Secured Term Loan
Debt Instrument [Line Items]
Debt instrument, principal amount					$ 5,515.0
Debt instrument, interest rate (in percent)		6.296%	6.329%		1.00%
Accelerated charge on repayments		$ 0.0
Senior Secured Revolving Loan \| Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate \| Variable rate component two \| Standard & Poor's, BB Rating
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)		1.45%
Senior Secured Revolving Loan \| Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate \| Variable rate component two \| Standard & Poor's, BB+ Rating
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)		1.10%
Senior Secured Revolving Loan \| Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate \| Variable rate component two \| Standard & Poor's, BBB- Rating
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)		0.85%
Senior Secured Revolving Loan \| Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate \| Variable rate component two \| Standard & Poor's, BBB Rating
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)		0.65%
Senior Secured Revolving Loan \| Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate \| Variable rate component two \| Standard & Poor's, BBB+ Rating
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)		0.50%
Senior Secured Revolving Loan \| Base rate \| Variable rate component one
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)		0.10%
Senior Secured Revolving Loan \| Base rate \| Minimum \| Variable rate component one
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)		0.00%
Senior Secured Revolving Loan \| Base rate \| Maximum \| Variable rate component one
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)		0.45%
Senior Secured Revolving Loan \| Senior Secured Revolving Loan \| Senior Secured Revolving Loan
Debt Instrument [Line Items]
Debt instrument, maximum borrowing capacity				$ 500.0	$ 300.0
Senior Secured Credit Facility And Senior Secured Notes \| Senior Secured Term Loan
Debt Instrument [Line Items]
Debt discount paid					27.6
Senior Secured Notes \| Senior Secured Notes
Debt Instrument [Line Items]
Debt instrument, principal amount					$ 500.0
Debt instrument, interest rate (in percent)					2.875%