Allowance for Expected Credit Losses (Details) - Changes in the Allowance for Expected Credit Losses on Premiums Receivable $ in Millions |
3 Months Ended |
|---|---|
|
Mar. 31, 2020
USD ($)
| |
| Credit Loss [Abstract] | |
| Premiums receivable (net of allowance for expected credit losses), balance, beginning of year | $ 7,909 |
| Premiums receivable (net of allowance for expected credit losses), balance, end of period | 8,202 |
| Premiums receivable, allowance for expected credit losses [Roll Forward] | |
| Premiums receivable, allowance for expected credit losses, beginning balance | 49 |
| Current period change for expected credit losses | 43 |
| Write-offs of uncollectible premiums receivable | 14 |
| Premiums receivable, allowance for expected credit losses, ending balance | $ 78 |