Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) - Schedule of Accounts Receivables and Unbilled Revenue (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 |
|---|---|---|---|---|---|---|---|---|---|
| Contract Assets [Abstract] | |||||||||
| Billed services (accounts receivable) | $ 1,427,376 | $ 1,411,329 | $ 1,426,035 | $ 1,431,581 | $ 1,608,774 | $ 1,737,822 | $ 1,883,296 | $ 1,788,140 | $ 1,838,875 |
| Allowance for credit losses | (39,686) | (44,074) | (38,573) | (35,501) | (40,122) | (35,303) | (32,934) | (29,029) | (36,149) |
| Accounts receivable (net) | 1,387,690 | 1,367,255 | 1,387,462 | 1,396,080 | 1,568,652 | 1,702,519 | 1,850,362 | 1,759,111 | 1,802,726 |
| 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 |
| Accounts receivable and unbilled revenue, net | $ 2,583,437 | $ 2,540,276 | $ 2,488,973 | $ 2,514,940 | $ 2,590,171 | $ 2,652,086 | $ 2,678,815 | $ 2,661,432 | $ 2,683,138 |