Allowance for Expected Credit Losses - Ceded credit risk (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Concentration Risk [Line Items]
Reinsurance recoverable for paid and unpaid claims and claims adjustments		$ 7,064	$ 6,564	$ 5,881
Shareholders' equity | Reinsurer concentration risk | Largest balance due from any one carrier
Concentration Risk [Line Items]
Concentration risk percentage		7.20%	9.00%
AM Best "A-" Or Better Rating | Reinsurance recoverable | Reinsurer concentration risk
Concentration Risk [Line Items]
Concentration risk percentage		66.80%	68.80%
Rating Below A- | Reinsurance recoverable | Reinsurer concentration risk | All Other Carriers
Concentration Risk [Line Items]
Concentration risk percentage		0.10%	0.10%
No AM Best rating | Reinsurance recoverable | Reinsurer concentration risk | Reinsurance trusts or letters of credit
Concentration Risk [Line Items]
Concentration risk percentage		95.00%	95.00%
No AM Best rating | Reinsurance recoverable | Reinsurer concentration risk | All Other Carriers
Concentration Risk [Line Items]
Concentration risk percentage	[1]	33.10%	31.10%

[1] At December 31, 2023 and 2022 period, over 95% of such amount were collateralized through reinsurance trusts, funds withheld arrangements, letters of credit or other, respectively