Unaudited Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Comprehensive Income
Net income (loss)	$ 475,708	$ (88,674)
Unrealized appreciation (decline) in value of available-for-sale investments:
Unrealized holding gains (losses) arising during period	(261,750)	(57,287)
Reclassification of net realized (gains) losses, included in net income (loss)	2,697	(121,229)
Foreign currency translation adjustments	(28,584)	(44,689)
Comprehensive income (loss)	188,071	(311,879)
Net (income) loss attributable to noncontrolling interests	(37,552)	232,791
Other comprehensive (income) loss attributable to noncontrolling interests	4,570	33,058
Comprehensive income (loss) available to Arch	$ 155,089	$ (46,030)