Fair value measurements - Reconciliation of Investments Measured at Fair Value using Level 3 Inputs (Details) - USD ($) $ in Millions |
3 Months Ended | 9 Months Ended | ||
|---|---|---|---|---|
Sep. 30, 2026 |
Sep. 30, 2025 |
Sep. 30, 2026 |
Sep. 30, 2025 |
|
| Other long-term investments | ||||
| Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] | ||||
| Beginning balance | $ 57.5 | $ 87.0 | $ 82.9 | $ 86.6 |
| Transfers in to (out of) Level 3 | 0.0 | 0.0 | 0.0 | 0.0 |
| Purchases | 0.0 | 0.0 | 0.0 | 0.0 |
| Sales & Settlements | 0.0 | 0.0 | (22.7) | 0.0 |
| Realized and Unrealized Gains (Losses) | (0.6) | 1.1 | (3.3) | 1.5 |
| Ending balance | 56.9 | 88.1 | 56.9 | 88.1 |
| Net derivatives | ||||
| Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] | ||||
| Beginning balance | (22.9) | 6.8 | 5.4 | (13.4) |
| Transfers in to (out of) Level 3 | 0.0 | 0.0 | 0.0 | 0.0 |
| Purchases | 0.0 | 0.0 | 0.0 | 0.0 |
| Sales & Settlements | 3.4 | 0.0 | 2.1 | 5.4 |
| Realized and Unrealized Gains (Losses) | (2.5) | (6.8) | (29.5) | 8.0 |
| Ending balance | $ (22.0) | $ 0.0 | $ (22.0) | $ 0.0 |