Schedule II - Condensed Financial Information of Registrant - Statement of Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Revenues
Net investment income	$ 389,118	$ 519,608	$ 627,738
Net realized gains (losses)	379,845	823,460	363,198
Total revenues	9,249,980	8,508,509	6,925,967
Expenses
Corporate expenses	79,157	81,988	80,111
Interest expense	139,470	143,456	120,872
Net foreign exchange gains (losses)	41,529	(83,634)	(20,609)
Total expenses	7,146,629	6,947,726	5,079,090
Income (loss) from operating affiliates	264,693	16,766	2,233
Net income available to Arch	2,156,849	1,405,521	1,636,319
Preferred share dividends	(48,343)	(41,612)	(41,612)
Loss on redemption of preferred shares	(15,101)	0	0
Net income available to Arch common shareholders	2,093,405	1,363,909	1,594,707
Parent
Revenues
Net investment income	1,524	53	212
Net realized gains (losses)	0	(2,110)	0
Total revenues	1,524	(2,057)	212
Expenses
Corporate expenses	71,818	65,566	62,701
Interest expense	58,741	40,445	22,154
Net foreign exchange gains (losses)	7	3	1
Total expenses	130,566	106,014	84,856
Income (loss) before income taxes	(129,042)	(108,071)	(84,644)
Income (loss) from operating affiliates	(590)	(437)	(762)
Income (loss) before equity in net income of subsidiaries	(129,632)	(108,508)	(85,406)
Equity in net income of subsidiaries	2,286,481	1,514,029	1,721,725
Net income available to Arch	2,156,849	1,405,521	1,636,319
Preferred share dividends	(48,343)	(41,612)	(41,612)
Loss on redemption of preferred shares	(15,101)	0	0
Net income available to Arch common shareholders	$ 2,093,405	$ 1,363,909	$ 1,594,707