Guarantor Financial Information - Condensed consolidating statement of income and comprehensive income (Details) - USD ($) $ 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
Revenues
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)
Other underwriting income									24,861	15,073	30,253
Equity in net income of investments accounted for using the equity method									123,672	45,641	142,286
Other income (loss)									2,233	2,419	(2,571)
Total revenues									6,928,200	5,450,568	5,627,375
Expenses
Losses and loss adjustment expenses									3,133,452	2,890,106	2,967,446
Acquisition expenses									840,945	805,135	775,458
Other operating expenses									800,997	677,809	684,451
Corporate expenses									80,111	78,994	83,752
Amortization of intangible assets									82,104	105,670	125,778
Interest expense									120,872	120,484	117,431
Net foreign exchange losses (gains)									20,609	(69,402)	115,782
Total expenses									5,079,090	4,608,796	4,870,098
Income (loss) before income taxes									1,849,110	841,772	757,277
Income tax (expense) benefit									(155,810)	(113,951)	(127,568)
Income (loss) before equity in net income of subsidiaries									1,693,300	727,821	629,709
Equity in net income of subsidiaries									0	0	0
Net income	326,384	392,453	468,954	448,528	136,494	227,408	243,646	150,423	1,693,300	727,821	629,709
Net (income) loss attributable to noncontrolling interests									(56,981)	30,150	(10,431)
Net income available to Arch									1,636,319	757,971	619,278
Preferred dividends	(10,403)	(10,403)	(10,403)	(10,403)	(10,403)	(10,402)	(10,403)	(10,437)	(41,612)	(41,645)	(46,041)
Loss on redemption of preferred shares									0	(2,710)	(6,735)
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
Comprehensive Income									2,027,129	611,003	851,863
Reportable legal entities | Arch Capital (Parent Guarantor)
Revenues
Net premiums earned									0	0	0
Net investment income									212	49	243
Net realized gains (losses)									0	29	0
Net impairment losses recognized in earnings									0	0	0
Other underwriting income									0	0	0
Equity in net income of investments accounted for using the equity method									0	0	0
Other income (loss)									(762)	1,918	(482)
Total revenues									(550)	1,996	(239)
Expenses
Losses and loss adjustment expenses									0	0	0
Acquisition expenses									0	0	0
Other operating expenses									0	0	0
Corporate expenses									62,701	64,279	67,450
Amortization of intangible assets									0	0	0
Interest expense									22,154	22,147	23,560
Net foreign exchange losses (gains)									1	30	2
Total expenses									84,856	86,456	91,012
Income (loss) before income taxes									(85,406)	(84,460)	(91,251)
Income tax (expense) benefit									0	0	0
Income (loss) before equity in net income of subsidiaries									(85,406)	(84,460)	(91,251)
Equity in net income of subsidiaries									1,721,725	842,431	710,529
Net income									1,636,319	757,971	619,278
Net (income) loss attributable to noncontrolling interests									0	0	0
Net income available to Arch									1,636,319	757,971	619,278
Preferred dividends									(41,612)	(41,645)	(46,041)
Loss on redemption of preferred shares										(2,710)	(6,735)
Net income available to Arch common shareholders									1,594,707	713,616	566,502
Comprehensive Income									2,027,129	611,003	851,863
Reportable legal entities | Arch-U.S. (Subsidiary Issuer)
Revenues
Net premiums earned									0	0	0
Net investment income									14,270	3,902	1,420
Net realized gains (losses)									25,313	(2,676)	0
Net impairment losses recognized in earnings									0	0	0
Other underwriting income									0	0	0
Equity in net income of investments accounted for using the equity method									779	0	0
Other income (loss)									0	0	0
Total revenues									40,362	1,226	1,420
Expenses
Losses and loss adjustment expenses									0	0	0
Acquisition expenses									0	0	0
Other operating expenses									0	0	0
Corporate expenses									7,221	2,422	4,152
Amortization of intangible assets									0	0	0
Interest expense									47,951	48,103	47,993
Net foreign exchange losses (gains)									0	0	0
Total expenses									55,172	50,525	52,145
Income (loss) before income taxes									(14,810)	(49,299)	(50,725)
Income tax (expense) benefit									3,696	13,314	10,333
Income (loss) before equity in net income of subsidiaries									(11,114)	(35,985)	(40,392)
Equity in net income of subsidiaries									564,657	388,260	303,991
Net income									553,543	352,275	263,599
Net (income) loss attributable to noncontrolling interests									0	0	0
Net income available to Arch									553,543	352,275	263,599
Preferred dividends									0	0	0
Loss on redemption of preferred shares										0	0
Net income available to Arch common shareholders									553,543	352,275	263,599
Comprehensive Income									751,210	292,973	288,752
Reportable legal entities | Other Arch Capital Subsidiaries
Revenues
Net premiums earned									5,786,498	5,231,975	4,844,532
Net investment income									706,038	649,394	559,963
Net realized gains (losses)									352,771	(410,014)	149,141
Net impairment losses recognized in earnings									(3,165)	(2,829)	(7,138)
Other underwriting income									24,861	15,073	30,253
Equity in net income of investments accounted for using the equity method									122,893	45,641	142,286
Other income (loss)									2,995	501	(2,089)
Total revenues									6,992,891	5,529,741	5,716,948
Expenses
Losses and loss adjustment expenses									3,133,452	2,890,106	2,967,446
Acquisition expenses									840,945	805,135	775,458
Other operating expenses									800,997	677,809	684,451
Corporate expenses									10,189	12,293	12,150
Amortization of intangible assets									82,104	105,670	125,778
Interest expense									142,662	138,672	135,342
Net foreign exchange losses (gains)									22,188	(59,607)	68,900
Total expenses									5,032,537	4,570,078	4,769,525
Income (loss) before income taxes									1,960,354	959,663	947,423
Income tax (expense) benefit									(159,506)	(127,265)	(137,901)
Income (loss) before equity in net income of subsidiaries									1,800,848	832,398	809,522
Equity in net income of subsidiaries									0	0	0
Net income									1,800,848	832,398	809,522
Net (income) loss attributable to noncontrolling interests									(57,868)	28,875	(11,721)
Net income available to Arch									1,742,980	861,273	797,801
Preferred dividends									0	0	0
Loss on redemption of preferred shares										0	0
Net income available to Arch common shareholders									1,742,980	861,273	797,801
Comprehensive Income									2,124,455	730,828	983,475
Consolidating adjustments and eliminations
Revenues
Net premiums earned									0	0	0
Net investment income									(92,782)	(89,712)	(90,754)
Net realized gains (losses)									(11,721)	7,317	0
Net impairment losses recognized in earnings									0	0	0
Other underwriting income									0	0	0
Equity in net income of investments accounted for using the equity method									0	0	0
Other income (loss)									0	0	0
Total revenues									(104,503)	(82,395)	(90,754)
Expenses
Losses and loss adjustment expenses									0	0	0
Acquisition expenses									0	0	0
Other operating expenses									0	0	0
Corporate expenses									0	0	0
Amortization of intangible assets									0	0	0
Interest expense									(91,895)	(88,438)	(89,464)
Net foreign exchange losses (gains)									(1,580)	(9,825)	46,880
Total expenses									(93,475)	(98,263)	(42,584)
Income (loss) before income taxes									(11,028)	15,868	(48,170)
Income tax (expense) benefit									0	0	0
Income (loss) before equity in net income of subsidiaries									(11,028)	15,868	(48,170)
Equity in net income of subsidiaries									(2,286,382)	(1,230,691)	(1,014,520)
Net income									(2,297,410)	(1,214,823)	(1,062,690)
Net (income) loss attributable to noncontrolling interests									887	1,275	1,290
Net income available to Arch									(2,296,523)	(1,213,548)	(1,061,400)
Preferred dividends									0	0	0
Loss on redemption of preferred shares										0	0
Net income available to Arch common shareholders									(2,296,523)	(1,213,548)	(1,061,400)
Comprehensive Income									$ (2,875,665)	$ (1,023,801)	$ (1,272,227)