Unaudited Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Comprehensive Income
Net income (loss)	$ 466,389	$ 485,845	$ 377,715	$ 981,343
Unrealized appreciation (decline) in value of available-for-sale investments:
Unrealized holding gains (losses) arising during period	492,796	222,473	435,509	448,360
Reclassification of net realized (gains) losses, included in net income (loss)	(167,391)	(55,345)	(288,620)	(65,566)
Foreign currency translation adjustments	22,251	4,267	(22,438)	9,783
Comprehensive income (loss)	814,045	657,240	502,166	1,373,920
Net (income) loss attributable to noncontrolling interests	(167,568)	(16,891)	65,223	(63,861)
Other comprehensive (income) loss attributable to noncontrolling interests	(20,111)	(2,891)	12,947	(7,030)
Comprehensive income (loss) available to Arch	$ 626,366	$ 637,458	$ 580,336	$ 1,303,029