Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) - Schedule of Unbilled Services and Unearned Revenue or Payments on Account (Details) - USD ($) $ in Thousands |
Sep. 30, 2025 |
Jun. 30, 2025 |
Mar. 31, 2025 |
Sep. 30, 2024 |
Jun. 30, 2024 |
Mar. 31, 2024 |
Sep. 30, 2023 |
Jun. 30, 2023 |
Mar. 31, 2023 |
|---|---|---|---|---|---|---|---|---|---|
| Revenue from Contract with Customer [Abstract] | |||||||||
| Unbilled services (unbilled revenue) | $ 1,195,747 | $ 1,173,021 | $ 1,101,511 | $ 1,118,860 | $ 1,021,519 | $ 949,567 | $ 828,453 | $ 902,321 | $ 880,412 |
| Unearned revenue (Payments on account ) | $ (1,546,887) | $ (1,515,422) | $ (1,495,847) | $ (1,477,952) | $ (1,544,275) | $ (1,585,694) | $ (1,563,948) | $ (1,493,403) | $ (1,480,275) |