Unaudited Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Comprehensive Income
Net income (loss)	$ 421,415	$ 488,682	$ 1,615,787	$ 866,397
Unrealized appreciation (decline) in value of available-for-sale investments:
Unrealized holding gains (losses) arising during period	(95,923)	110,782	(279,102)	546,291
Reclassification of net realized (gains) losses, included in net income (loss)	(62,654)	(79,803)	(120,504)	(368,423)
Foreign currency translation adjustments	(31,710)	16,709	(54,089)	(5,729)
Comprehensive income (loss)	231,128	536,370	1,162,092	1,038,536
Net (income) loss attributable to noncontrolling interests	(1,473)	(69,643)	(82,203)	(4,420)
Other comprehensive (income) loss attributable to noncontrolling interests	9,423	(10,820)	13,983	2,127
Comprehensive income (loss) available to Arch	$ 239,078	$ 455,907	$ 1,093,872	$ 1,036,243