Unaudited Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Comprehensive Income
Net income (loss)	$ 1,350	$ 988	$ 3,161	$ 3,377
Unrealized appreciation (decline) in value of available-for-sale investments:
Unrealized holding gains (losses) arising during period	88	645	625	478
Reclassification of net realized (gains) losses, included in net income (loss)	(41)	(60)	4	22
Foreign currency translation adjustments	1	25	91	(24)
Comprehensive income (loss) available to Arch	$ 1,398	$ 1,598	$ 3,881	$ 3,853