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, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense [Abstract]
Reserve for losses and loss adjustment expenses at beginning of period		$ 24,466	$ 21,268	$ 22,752	$ 20,032
Unpaid losses and loss adjustment expenses recoverable		7,083	6,394	6,690	6,280
Net reserve for losses and loss adjustment expenses at beginning of period		17,383	14,874	16,062	13,752
Net incurred losses and loss adjustment expenses relating to losses occurring in:
Current year		2,524	1,793	6,324	5,012
Prior years		(121)	(146)	(366)	(403)
Total net incurred losses and loss adjustment expenses		2,403	1,647	5,958	4,609
Net losses and loss adjustment expense reserves of acquired business	[1]	2,413	0	2,463	0
Net foreign exchange (gains) losses and other		246	(123)	152	(24)
Net paid losses and loss adjustment expenses relating to losses occurring in:
Current year		(362)	(345)	(647)	(700)
Prior years		(967)	(713)	(2,872)	(2,297)
Total net paid losses and loss adjustment expenses		(1,329)	(1,058)	(3,519)	(2,997)
Net reserve for losses and loss adjustment expenses at end of period		21,116	15,340	21,116	15,340
Unpaid losses and loss adjustment expenses recoverable		7,563	6,496	7,563	6,496
Reserve for losses and loss adjustment expenses at end of period		$ 28,679	$ 21,836	$ 28,679	$ 21,836

[1] Activity for the 2024 third quarter related to the acquisition of MCE (see note 2) and Watford Insurance Company (see note 16), while activity for the nine months ended September 30, 2024 also reflects the Company’s acquisition of RMIC Companies, Inc. and its wholly-owned subsidiaries (“RMIC”) that, together, comprise the run-off mortgage insurance business of Old Republic International Corporation