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Business Segment and Geographical Information - Distribution of Income from Operations, Excluding Restructuring, by Geographical Area (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Segment Reporting Information [Line Items] |  |  |
Income from operations, excluding restructuring and other items | $ 956,152 | $ 795,237 | $ 378,529
Ireland |  |  |
Segment Reporting Information [Line Items] |  |  |
Income from operations, excluding restructuring and other items | 432,026 | 218,088 | 131,961
Rest of Europe |  |  |
Segment Reporting Information [Line Items] |  |  |
Income from operations, excluding restructuring and other items | 153,450 | 253,799 | 177,863
U.S. |  |  |
Segment Reporting Information [Line Items] |  |  |
Income from operations, excluding restructuring and other items | 310,343 | 254,849 | 39,132
Other |  |  |
Segment Reporting Information [Line Items] |  |  |
Income from operations, excluding restructuring and other items | $ 60,333 | $ 68,501 | $ 29,573

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- Definition Operating Income (Loss) Before Restructuring Charges, Transaction And Integration Expenses + References No definition available. + Details
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Namespace Prefix: | iclr_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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

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- Details
Name: | srt_StatementGeographicalAxis=country_IE
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_StatementGeographicalAxis=country_US
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_StatementGeographicalAxis=iclr_OtherCountriesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |