Reserve for Losses and Loss Adjustment Expenses - Reconciliation of beginning and ending balances of losses and loss adjustment reserves (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense [Abstract]
Reserve for losses and loss adjustment expenses at beginning of year		$ 22,752	$ 20,032	$ 17,757
Unpaid losses and loss adjustment expenses recoverable		6,690	6,280	5,599
Net reserve for losses and loss adjustment expenses at beginning of year		16,062	13,752	12,158
Net incurred losses and loss adjustment expenses relating to losses occurring in:
Current year		8,849	6,784	5,797
Prior years		(507)	(538)	(769)
Total net incurred losses and loss adjustment expenses		8,342	6,246	5,028
Net losses and loss adjustment expense reserves of acquired business	[1]	2,477	0	0
Foreign exchange (gains) losses and other		(260)	157	(293)
Net paid losses and loss adjustment expenses relating to losses occurring in:
Current year		(1,176)	(1,081)	(888)
Prior years		(3,897)	(3,012)	(2,253)
Total net paid losses and loss adjustment expenses		(5,073)	(4,093)	(3,141)
Net reserve for losses and loss adjustment expenses at end of year		21,548	16,062	13,752
Unpaid losses and loss adjustment expenses recoverable		7,821	6,690	6,280
Reserve for losses and loss adjustment expenses at end of year		$ 29,369	$ 22,752	$ 20,032

[1] Activity in the 2024 period primarily related to the MCE Acquisition (see note 2), along with acquisitions of Watford Insurance Company (see note 16) and RMIC Companies, Inc. and its wholly-owned subsidiaries (“RMIC”) that, together, comprise the run-off mortgage insurance business of Old Republic International Corporation