Reserve for Losses and Loss Adjustment Expenses - Reconciliation of beginning and ending balances of losses and loss adjustment reserves (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense [Abstract]
Reserve for losses and loss adjustment expenses at beginning of period	$ 21,268	$ 18,194	$ 20,032	$ 17,757
Unpaid losses and loss adjustment expenses recoverable	6,394	5,686	6,280	5,599
Net reserve for losses and loss adjustment expenses at beginning of period	14,874	12,508	13,752	12,158
Net incurred losses and loss adjustment expenses relating to losses occurring in:
Current year	1,793	1,863	5,012	4,279
Prior years	(146)	(180)	(403)	(492)
Total net incurred losses and loss adjustment expenses	1,647	1,683	4,609	3,787
Net foreign exchange (gains) losses and other	(123)	(245)	(24)	(527)
Net paid losses and loss adjustment expenses relating to losses occurring in:
Current year	(345)	(227)	(700)	(464)
Prior years	(713)	(538)	(2,297)	(1,773)
Total net paid losses and loss adjustment expenses	(1,058)	(765)	(2,997)	(2,237)
Net reserve for losses and loss adjustment expenses at end of period	15,340	13,181	15,340	13,181
Unpaid losses and loss adjustment expenses recoverable	6,496	6,107	6,496	6,107
Reserve for losses and loss adjustment expenses at end of period	$ 21,836	$ 19,288	$ 21,836	$ 19,288