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Fair value measurements - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2024 | Dec. 31, 2023 | Mar. 31, 2023
Available-for-sale securities [Line Items] |  |  |
Investments used to satisfy nonqualified benefit plans obligations | $ 62.7 | $ 66.2 | $ 83.5
Net unrealized gain (loss) on investments used to satisfy obligations under nonqualified benefit plans | 2.9 |  | $ 2.9
Investment withdrawal, reduction in cost basis | $ 9.0 | $ 20.0 |

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