Reinsurance - Effects of reinsurance (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Premiums Written
Direct	$ 10,056	$ 9,652	$ 8,542
Assumed	11,455	8,751	6,785
Ceded	(5,779)	(4,935)	(4,249)
Net premiums written	15,732	13,468	11,078
Premiums Earned
Direct	9,721	9,131	8,058
Assumed	10,880	7,890	5,768
Ceded	(5,501)	(4,581)	(4,147)
Net premiums earned	15,100	12,440	9,679
Losses and Loss Adjustment Expenses
Direct	5,676	4,739	3,991
Assumed	6,137	3,975	3,558
Ceded	(3,471)	(2,468)	(2,521)
Net	$ 8,342	$ 6,246	$ 5,028

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