Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Acquisition Costs		$ 1,734	$ 1,531	$ 1,263
Reserves for Losses and Loss Adjustment Expenses		29,369	22,752	20,032
Unearned Premiums		10,218	8,808	7,337
Net Premiums Earned		15,100	12,440	9,679
Net Losses and Loss Adjustment Expenses Incurred		8,342	6,246	5,028
Amortization of Deferred Acquisition Costs		2,651	2,312	1,740
Other Operating Expenses	[1]	1,472	1,301	1,128
Net Premiums Written		15,732	13,468	11,078
Operating segments | Insurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Acquisition Costs		696	566	301
Reserves for Losses and Loss Adjustment Expenses		16,277	12,250	11,017
Unearned Premiums		4,857	3,917	3,382
Net Premiums Earned		6,627	5,446	4,560
Net Losses and Loss Adjustment Expenses Incurred		4,070	3,122	2,784
Amortization of Deferred Acquisition Costs		1,217	1,055	887
Other Operating Expenses	[1]	995	819	665
Net Premiums Written		6,874	5,862	5,021
Operating segments | Reinsurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Acquisition Costs		981	901	992
Reserves for Losses and Loss Adjustment Expenses		12,567	9,924	8,306
Unearned Premiums		4,891	4,254	3,206
Net Premiums Earned		7,242	5,836	3,959
Net Losses and Loss Adjustment Expenses Incurred		4,327	3,227	2,568
Amortization of Deferred Acquisition Costs		1,432	1,240	813
Other Operating Expenses	[1]	270	288	268
Net Premiums Written		7,746	6,554	4,924
Operating segments | Mortgage
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Acquisition Costs		57	64	(30)
Reserves for Losses and Loss Adjustment Expenses		525	578	709
Unearned Premiums		470	637	749
Net Premiums Earned		1,231	1,158	1,160
Net Losses and Loss Adjustment Expenses Incurred		(55)	(103)	(324)
Amortization of Deferred Acquisition Costs		2	17	40
Other Operating Expenses	[1]	207	194	195
Net Premiums Written		$ 1,112	$ 1,052	$ 1,133

[1] Certain other operating expenses relate to the Company’s corporate items. Such amounts are not reflected in the table above. See note 4, “Segment Information,” to our consolidated financial statements in Item 8 for information