Unaudited Condensed Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Net premiums written	$ 1,455,453	$ 1,613,457	$ 1,444,898	$ 1,525,259	$ 1,301,754	$ 1,333,553	$ 1,298,896	$ 1,412,544	$ 6,039,067	$ 5,346,747	$ 4,961,373
Net premiums earned	1,515,882	1,438,023	1,463,727	1,368,866	1,369,435	1,290,878	1,336,763	1,234,899	5,786,498	5,231,975	4,844,532
Net investment income	154,263	161,488	155,038	156,949	157,217	144,024	135,668	126,724	627,738	563,633	470,872
Net realized gains (losses)	41,474	62,518	120,806	141,565	(166,030)	(51,705)	(76,611)	(110,998)	366,363	(405,344)	149,141
Net impairment losses recognized in earnings	(644)	(1,163)	(49)	(1,309)	(1,705)	(492)	(470)	(162)	(3,165)	(2,829)	(7,138)
Underwriting income (loss)	251,421	231,262	293,134	260,148	166,955	234,581	235,465	236,997	1,035,965	873,998	447,430
Net income (loss) attributable to Arch	326,384	392,453	468,954	448,528	136,494	227,408	243,646	150,423	1,693,300	727,821	629,709
Preferred dividends	(10,403)	(10,403)	(10,403)	(10,403)	(10,403)	(10,402)	(10,403)	(10,437)	(41,612)	(41,645)	(46,041)
Net income available to Arch common shareholders	$ 315,981	$ 382,050	$ 458,551	$ 438,125	$ 126,091	$ 217,006	$ 233,243	$ 137,276	$ 1,594,707	$ 713,616	$ 566,502
Basic (per share)	$ 0.78	$ 0.95	$ 1.14	$ 1.09	$ 0.31	$ 0.54	$ 0.58	$ 0.34	$ 3.97	$ 1.76	$ 1.40
Diluted (per share)	$ 0.76	$ 0.92	$ 1.12	$ 1.07	$ 0.31	$ 0.53	$ 0.56	$ 0.33	$ 3.87	$ 1.73	$ 1.36