Schedule II - Condensed Financial Information of Registrant - Statement of Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Revenues
Net investment income	$ 496,547	$ 389,118	$ 519,608
Net realized gains (losses)	(662,734)	379,845	823,460
Total revenues	9,614,808	9,249,980	8,508,509
Expenses
Corporate expenses	95,482	79,157	81,988
Interest expense	130,266	139,470	143,456
Net foreign exchange losses (gains)	(100,905)	(41,529)	83,634
Total expenses	8,126,315	7,146,629	6,947,726
Income (loss) from operating affiliates	73,891	264,693	16,766
Net income available to Arch	1,476,933	2,156,849	1,405,521
Preferred share dividends	(40,736)	(48,343)	(41,612)
Loss on redemption of preferred shares	0	(15,101)	0
Net income available to Arch common shareholders	1,436,197	2,093,405	1,363,909
Parent
Revenues
Net investment income	2,058	1,524	53
Net realized gains (losses)	29	0	(2,110)
Total revenues	2,087	1,524	(2,057)
Expenses
Corporate expenses	85,997	71,818	65,566
Interest expense	58,759	58,741	40,445
Net foreign exchange losses (gains)	(1)	7	3
Total expenses	144,755	130,566	106,014
Income (loss) before income taxes	(142,668)	(129,042)	(108,071)
Income (loss) from operating affiliates	(1,047)	(590)	(437)
Income (loss) before equity in net income of subsidiaries	(143,715)	(129,632)	(108,508)
Equity in net income of subsidiaries	1,592,929	2,286,481	1,514,029
Net income available to Arch	1,449,214	2,156,849	1,405,521
Preferred share dividends	(40,736)	(48,343)	(41,612)
Loss on redemption of preferred shares	0	(15,101)	0
Net income available to Arch common shareholders	$ 1,408,478	$ 2,093,405	$ 1,363,909