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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, 2022 | Dec. 31, 2021 | Dec. 31, 2020
Segment Reporting Information [Line Items] |  |  |
Income from operations, excluding restructuring and other items | $ 795,237 | $ 378,529 | $ 391,500
Ireland |  |  |
Segment Reporting Information [Line Items] |  |  |
Income from operations, excluding restructuring and other items | 218,088 | 131,961 | 276,478
Rest of Europe |  |  |
Segment Reporting Information [Line Items] |  |  |
Income from operations, excluding restructuring and other items | 253,799 | 177,863 | 35,765
U.S. |  |  |
Segment Reporting Information [Line Items] |  |  |
Income from operations, excluding restructuring and other items | 254,849 | 39,132 | 58,018
Other |  |  |
Segment Reporting Information [Line Items] |  |  |
Income from operations, excluding restructuring and other items | $ 68,501 | $ 29,573 | $ 21,239

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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: |