Guarantor Financial Information - Condensed consolidating statement of income and comprehensive income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenues
Net premiums earned	$ 1,438,023	$ 1,290,878	$ 4,270,616	$ 3,862,540
Net investment income	161,488	144,024	473,475	406,416
Net realized gains (losses)	62,518	(51,705)	324,889	(239,314)
Net impairment losses recognized in earnings	(1,163)	(492)	(2,521)	(1,124)
Other underwriting income	3,326	5,823	18,104	15,046
Equity in net income of investment funds accounted for using the equity method	17,130	15,982	96,533	52,523
Other income (loss)	1,338	(726)	3,550	2,461
Total revenues	1,682,660	1,403,784	5,184,646	4,098,548
Expenses
Losses and loss adjustment expenses	802,455	699,420	2,288,530	2,062,433
Acquisition expenses	211,120	201,602	619,057	595,816
Other operating expenses	196,512	161,098	596,589	512,294
Corporate expenses	17,061	14,335	53,274	52,159
Amortization of intangible assets	20,003	26,315	60,214	79,523
Interest expense	31,328	29,730	89,673	90,710
Net foreign exchange (gains) losses	(33,124)	(10,838)	(31,697)	(44,823)
Total expenses	1,245,355	1,121,662	3,675,640	3,348,112
Income (loss) before income taxes	437,305	282,122	1,509,006	750,436
Income tax (expense) benefit	(38,116)	(33,356)	(128,474)	(78,939)
Income (loss) before equity in net income of subsidiaries	399,189	248,766	1,380,532	671,497
Equity in net income of subsidiaries	0	0	0	0
Net income	399,189	248,766	1,380,532	671,497
Net (income) loss attributable to noncontrolling interests	(6,736)	(21,358)	(70,597)	(50,020)
Net income available to Arch	392,453	227,408	1,309,935	621,477
Preferred dividends	(10,403)	(10,402)	(31,209)	(31,242)
Loss on redemption of preferred shares	0	0	0	(2,710)
Net income available to Arch common shareholders	382,050	217,006	1,278,726	587,525
Comprehensive income available to Arch	397,340	200,524	1,700,369	432,186
Reportable Legal Entities | Arch Capital (Parent Guarantor)
Revenues
Net premiums earned	0	0	0	0
Net investment income	81	2	165	37
Net realized gains (losses)	0	0	0	29
Net impairment losses recognized in earnings	0	0	0	0
Other underwriting income	0	0	0	0
Equity in net income of investment funds accounted for using the equity method	0	0	0	0
Other income (loss)	(153)	(195)	(634)	2,066
Total revenues	(72)	(193)	(469)	2,132
Expenses
Losses and loss adjustment expenses	0	0	0	0
Acquisition expenses	0	0	0	0
Other operating expenses	0	0	0	0
Corporate expenses	15,066	15,170	46,666	47,981
Amortization of intangible assets	0	0	0	0
Interest expense	5,539	5,536	16,615	16,609
Net foreign exchange (gains) losses	1	0	3	29
Total expenses	20,606	20,706	63,284	64,619
Income (loss) before income taxes	(20,678)	(20,899)	(63,753)	(62,487)
Income tax (expense) benefit	0	0	0	0
Income (loss) before equity in net income of subsidiaries	(20,678)	(20,899)	(63,753)	(62,487)
Equity in net income of subsidiaries	413,131	248,307	1,373,688	683,964
Net income	392,453	227,408	1,309,935	621,477
Net (income) loss attributable to noncontrolling interests	0	0	0	0
Net income available to Arch	392,453	227,408	1,309,935	621,477
Preferred dividends	(10,403)	(10,402)	(31,209)	(31,242)
Loss on redemption of preferred shares				(2,710)
Net income available to Arch common shareholders	382,050	217,006	1,278,726	587,525
Comprehensive income available to Arch	397,340	200,524	1,700,369	432,186
Reportable Legal Entities | Arch-U.S. (Subsidiary Issuer)
Revenues
Net premiums earned	0	0	0	0
Net investment income	2,899	1,202	9,425	2,020
Net realized gains (losses)	1,880	(64)	16,223	(71)
Net impairment losses recognized in earnings	0	0	0	0
Other underwriting income	0	0	0	0
Equity in net income of investment funds accounted for using the equity method	600	0	536	0
Other income (loss)	0	0	0	0
Total revenues	5,379	1,138	26,184	1,949
Expenses
Losses and loss adjustment expenses	0	0	0	0
Acquisition expenses	0	0	0	0
Other operating expenses	0	0	0	0
Corporate expenses	1,631	446	5,912	1,205
Amortization of intangible assets	0	0	0	0
Interest expense	12,019	12,075	35,966	36,014
Net foreign exchange (gains) losses	0	0	0	0
Total expenses	13,650	12,521	41,878	37,219
Income (loss) before income taxes	(8,271)	(11,383)	(15,694)	(35,270)
Income tax (expense) benefit	1,647	2,276	3,491	7,704
Income (loss) before equity in net income of subsidiaries	(6,624)	(9,107)	(12,203)	(27,566)
Equity in net income of subsidiaries	124,814	92,906	410,115	266,053
Net income	118,190	83,799	397,912	238,487
Net (income) loss attributable to noncontrolling interests	0	0	0	0
Net income available to Arch	118,190	83,799	397,912	238,487
Preferred dividends	0	0	0	0
Loss on redemption of preferred shares				0
Net income available to Arch common shareholders	118,190	83,799	397,912	238,487
Comprehensive income available to Arch	138,832	77,389	608,108	153,992
Reportable Legal Entities | Other Arch Capital Subsidiaries
Revenues
Net premiums earned	1,438,023	1,290,878	4,270,616	3,862,540
Net investment income	181,621	165,289	532,031	471,588
Net realized gains (losses)	66,683	(51,641)	320,356	(239,272)
Net impairment losses recognized in earnings	(1,163)	(492)	(2,521)	(1,124)
Other underwriting income	3,326	5,823	18,104	15,046
Equity in net income of investment funds accounted for using the equity method	16,530	15,982	95,997	52,523
Other income (loss)	1,491	(531)	4,184	395
Total revenues	1,706,511	1,425,308	5,238,767	4,161,696
Expenses
Losses and loss adjustment expenses	802,455	699,420	2,288,530	2,062,433
Acquisition expenses	211,120	201,602	619,057	595,816
Other operating expenses	196,512	161,098	596,589	512,294
Corporate expenses	364	(1,281)	696	2,973
Amortization of intangible assets	20,003	26,315	60,214	79,523
Interest expense	36,712	34,276	104,430	104,359
Net foreign exchange (gains) losses	(25,405)	(10,426)	(26,715)	(37,347)
Total expenses	1,241,761	1,111,004	3,642,801	3,320,051
Income (loss) before income taxes	464,750	314,304	1,595,966	841,645
Income tax (expense) benefit	(39,763)	(35,632)	(131,965)	(86,643)
Income (loss) before equity in net income of subsidiaries	424,987	278,672	1,464,001	755,002
Equity in net income of subsidiaries	0	0	0	0
Net income	424,987	278,672	1,464,001	755,002
Net (income) loss attributable to noncontrolling interests	(6,906)	(21,669)	(71,405)	(50,976)
Net income available to Arch	418,081	257,003	1,392,596	704,026
Preferred dividends	0	0	0	0
Loss on redemption of preferred shares				0
Net income available to Arch common shareholders	418,081	257,003	1,392,596	704,026
Comprehensive income available to Arch	427,238	230,565	1,779,463	522,448
Consolidating Adjustments and Eliminations
Revenues
Net premiums earned	0	0	0	0
Net investment income	(23,113)	(22,469)	(68,146)	(67,229)
Net realized gains (losses)	(6,045)	0	(11,690)	0
Net impairment losses recognized in earnings	0	0	0	0
Other underwriting income	0	0	0	0
Equity in net income of investment funds accounted for using the equity method	0	0	0	0
Other income (loss)	0	0	0	0
Total revenues	(29,158)	(22,469)	(79,836)	(67,229)
Expenses
Losses and loss adjustment expenses	0	0	0	0
Acquisition expenses	0	0	0	0
Other operating expenses	0	0	0	0
Corporate expenses	0	0	0	0
Amortization of intangible assets	0	0	0	0
Interest expense	(22,942)	(22,157)	(67,338)	(66,272)
Net foreign exchange (gains) losses	(7,720)	(412)	(4,985)	(7,505)
Total expenses	(30,662)	(22,569)	(72,323)	(73,777)
Income (loss) before income taxes	1,504	100	(7,513)	6,548
Income tax (expense) benefit	0	0	0	0
Income (loss) before equity in net income of subsidiaries	1,504	100	(7,513)	6,548
Equity in net income of subsidiaries	(537,945)	(341,213)	(1,783,803)	(950,017)
Net income	(536,441)	(341,113)	(1,791,316)	(943,469)
Net (income) loss attributable to noncontrolling interests	170	311	808	956
Net income available to Arch	(536,271)	(340,802)	(1,790,508)	(942,513)
Preferred dividends	0	0	0	0
Loss on redemption of preferred shares				0
Net income available to Arch common shareholders	(536,271)	(340,802)	(1,790,508)	(942,513)
Comprehensive income available to Arch	$ (566,070)	$ (307,954)	$ (2,387,571)	$ (676,440)