Unaudited Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Revenues
Net premiums earned	$ 4,337	$ 3,565	$ 8,525	$ 6,987
Net investment income	405	364	783	691
Net realized gains (losses)	229	122	232	189
Other underwriting income	62	3	115	15
Equity in net income of investments accounted for using the equity method	162	167	215	266
Other income (loss)	18	8	16	22
Total revenues	5,213	4,229	9,886	8,170
Expenses
Losses and loss adjustment expenses	2,303	1,827	4,890	3,555
Acquisition expenses	824	633	1,588	1,240
Other operating expenses	454	346	927	709
Corporate expenses	47	41	107	94
Amortization of intangible assets	48	27	97	48
Interest expense	38	35	73	69
Net foreign exchange (gains) losses	88	(1)	115	(32)
Total expenses	3,802	2,908	7,797	5,683
Income (loss) before income taxes and income (loss) from operating affiliates	1,411	1,321	2,089	2,487
Income tax (expense) benefit	(214)	(97)	(335)	(198)
Income (loss) from operating affiliates	40	45	57	100
Net income (loss) available to Arch	1,237	1,269	1,811	2,389
Preferred dividends	(10)	(10)	(20)	(20)
Net income (loss) available to Arch common shareholders	$ 1,227	$ 1,259	$ 1,791	$ 2,369
Net income per common share and common share equivalent
Basic (per share)	$ 3.30	$ 3.38	$ 4.81	$ 6.37
Diluted (per share)	$ 3.23	$ 3.30	$ 4.70	$ 6.22
Weighted average common shares and common share equivalents outstanding
Basic (shares)	372.2	372.7	372.6	371.8
Diluted (shares)	379.9	381.6	380.8	380.9