Unaudited Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Comprehensive Income
Net income (loss)	$ 18,258	$ 421,415	$ 619,635	$ 1,615,787
Unrealized appreciation (decline) in value of available-for-sale investments:
Unrealized holding gains (losses) arising during period	(609,759)	(95,923)	(1,892,119)	(279,102)
Reclassification of net realized (gains) losses, included in net income (loss)	46,585	(62,654)	206,573	(120,504)
Foreign currency translation adjustments	(70,388)	(31,710)	(141,681)	(54,089)
Comprehensive income (loss)	(615,304)	231,128	(1,207,592)	1,162,092
Net (income) loss attributable to noncontrolling interests	(1,157)	(1,473)	(2,390)	(82,203)
Other comprehensive (income) loss attributable to noncontrolling interests	0	9,423	0	13,983
Comprehensive income (loss) available to Arch	$ (616,461)	$ 239,078	$ (1,209,982)	$ 1,093,872