Reinsurance - Effects of reinsurance (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Premiums Written
Direct	$ 9,652	$ 8,542	$ 7,707
Assumed	8,751	6,785	5,045
Ceded	(4,935)	(4,249)	(3,735)
Net premiums written	13,468	11,078	9,017
Premiums Earned
Direct	9,131	8,058	7,150
Assumed	7,890	5,768	4,334
Ceded	(4,581)	(4,147)	(3,402)
Net premiums earned	12,440	9,679	8,082
Losses and Loss Adjustment Expenses
Direct	4,739	3,991	4,267
Assumed	3,975	3,558	2,826
Ceded	(2,468)	(2,521)	(2,508)
Net	$ 6,246	$ 5,028	$ 4,585

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