Allowance for Expected Credit Losses (Details) - Changes in the Allowance for Expected Credit Losses on Premiums Receivable - USD ($) $ in Millions |
3 Months Ended | 9 Months Ended | ||
|---|---|---|---|---|
Sep. 30, 2025 |
Sep. 30, 2024 |
Sep. 30, 2025 |
Sep. 30, 2024 |
|
| Credit Loss [Abstract] | ||||
| Premiums receivable (net of allowance for expected credit losses), balance, beginning of period | $ 12,042 | $ 11,491 | $ 11,110 | $ 10,282 |
| Premiums receivable (net of allowance for expected credit losses), balance, end of period | 11,621 | 11,271 | 11,621 | 11,271 |
| Premiums receivable, allowance for expected credit losses [Roll Forward] | ||||
| Premiums receivable, allowance for expected credit losses, beginning balance | 61 | 69 | 58 | 69 |
| Current period change for expected credit losses | 13 | 16 | 48 | 41 |
| Write-offs of uncollectible premiums receivable | 16 | 15 | 48 | 40 |
| Premiums receivable, allowance for expected credit losses, ending balance | $ 58 | $ 70 | $ 58 | $ 70 |