Unaudited Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Comprehensive Income
Net income (loss)	$ 403,945	$ 718,664	$ 601,377	$ 1,194,372
Unrealized appreciation (decline) in value of available-for-sale investments:
Unrealized holding gains (losses) arising during period	(598,005)	78,571	(1,282,360)	(183,179)
Reclassification of net realized (gains) losses, included in net income (loss)	57,710	(60,547)	159,988	(57,850)
Foreign currency translation adjustments	(68,525)	6,205	(71,293)	(22,379)
Comprehensive income (loss)	(204,875)	742,893	(592,288)	930,964
Net (income) loss attributable to noncontrolling interests	399	(43,178)	(1,233)	(80,730)
Other comprehensive (income) loss attributable to noncontrolling interests	0	(10)	0	4,560
Comprehensive income (loss) available to Arch	$ (204,476)	$ 699,705	$ (593,521)	$ 854,794