Unaudited Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Comprehensive Income
Net income (loss)	$ 488,682	$ 399,189	$ 866,397	$ 1,380,532
Unrealized appreciation (decline) in value of available-for-sale investments:
Unrealized holding gains (losses) arising during period	110,782	59,290	546,291	507,650
Reclassification of net realized (gains) losses, included in net income (loss)	(79,803)	(38,743)	(368,423)	(104,309)
Foreign currency translation adjustments	16,709	(16,424)	(5,729)	(6,641)
Comprehensive income (loss)	536,370	403,312	1,038,536	1,777,232
Net (income) loss attributable to noncontrolling interests	(69,643)	(6,736)	(4,420)	(70,597)
Other comprehensive (income) loss attributable to noncontrolling interests	(10,820)	764	2,127	(6,266)
Comprehensive income (loss) available to Arch	$ 455,907	$ 397,340	$ 1,036,243	$ 1,700,369