Guarantor Financial Information - Condensed consolidating statement of income and comprehensive income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues
Net premiums earned	$ 1,336,763	$ 1,240,874	$ 2,571,662	$ 2,357,891
Net investment income	135,668	111,124	262,392	228,998
Net realized gains (losses)	(76,611)	21,735	(187,609)	55,888
Net impairment losses recognized in earnings	(470)	(1,730)	(632)	(3,537)
Other underwriting income	3,874	4,822	9,223	9,455
Equity in net income (loss) of investment funds accounted for using the equity method	8,472	32,706	36,541	80,794
Other income (loss)	3,113	(1,994)	3,187	(2,776)
Total revenues	1,410,809	1,407,537	2,694,764	2,726,713
Expenses
Losses and loss adjustment expenses	726,153	689,860	1,363,013	1,242,430
Acquisition expenses	202,838	190,436	394,214	372,725
Other operating expenses	176,181	169,981	351,196	344,700
Corporate expenses	22,512	24,876	37,824	52,668
Amortization of intangible assets	26,472	30,824	53,208	62,118
Interest expense	30,344	28,749	60,980	57,425
Net foreign exchange (gains) losses	(53,706)	39,543	(33,985)	58,947
Total expenses	1,130,794	1,174,269	2,226,450	2,191,013
Income (loss) before income taxes	280,015	233,268	468,314	535,700
Income tax (expense) benefit	(23,668)	(34,169)	(45,583)	(62,566)
Income (loss) before equity in net income of subsidiaries	256,347	199,099	422,731	473,134
Equity in net income of subsidiaries	0	0	0	0
Net income	256,347	199,099	422,731	473,134
Net (income) loss attributable to noncontrolling interests	(12,701)	(13,932)	(28,662)	(34,840)
Net income available to Arch	243,646	185,167	394,069	438,294
Preferred dividends	(10,403)	(11,349)	(20,840)	(22,567)
Loss on redemption of preferred shares	0	0	(2,710)	0
Net income available to Arch common shareholders	233,243	173,818	370,519	415,727
Comprehensive income available to Arch	183,500	279,285	231,662	631,276
Reportable Legal Entities | Arch Capital (Parent Guarantor)
Revenues
Net premiums earned	0	0	0	0
Net investment income	15	1	35	6
Net realized gains (losses)	0	0	29	0
Net impairment losses recognized in earnings	0	0	0	0
Other underwriting income	0	0	0	0
Equity in net income (loss) of investment funds accounted for using the equity method	0	0	0	0
Other income (loss)	2,339	(437)	2,261	(266)
Total revenues	2,354	(436)	2,325	(260)
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	16,642	21,816	32,811	39,063
Amortization of intangible assets	0	0	0	0
Interest expense	5,537	6,075	11,073	12,090
Net foreign exchange (gains) losses	0	0	29	0
Total expenses	22,179	27,891	43,913	51,153
Income (loss) before income taxes	(19,825)	(28,327)	(41,588)	(51,413)
Income tax (expense) benefit	0	0	0	0
Income (loss) before equity in net income of subsidiaries	(19,825)	(28,327)	(41,588)	(51,413)
Equity in net income of subsidiaries	263,471	213,494	435,657	489,707
Net income	243,646	185,167	394,069	438,294
Net (income) loss attributable to noncontrolling interests	0	0	0	0
Net income available to Arch	243,646	185,167	394,069	438,294
Preferred dividends	(10,403)	(11,349)	(20,840)	(22,567)
Loss on redemption of preferred shares			(2,710)
Net income available to Arch common shareholders	233,243	173,818	370,519	415,727
Comprehensive income available to Arch	183,500	279,285	231,662	631,276
Reportable Legal Entities | Arch-U.S. (Subsidiary Issuer)
Revenues
Net premiums earned	0	0	0	0
Net investment income	560	184	818	1,000
Net realized gains (losses)	0	0	(7)	0
Net impairment losses recognized in earnings	0	0	0	0
Other underwriting income	0	0	0	0
Equity in net income (loss) of investment funds accounted for using the equity method	0	0	0	0
Other income (loss)	0	0	0	0
Total revenues	560	184	811	1,000
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	470	1,309	759	3,317
Amortization of intangible assets	0	0	0	0
Interest expense	12,013	11,989	23,939	23,919
Net foreign exchange (gains) losses	0	0	0	0
Total expenses	12,483	13,298	24,698	27,236
Income (loss) before income taxes	(11,923)	(13,114)	(23,887)	(26,236)
Income tax (expense) benefit	2,477	4,069	5,428	8,942
Income (loss) before equity in net income of subsidiaries	(9,446)	(9,045)	(18,459)	(17,294)
Equity in net income of subsidiaries	86,727	86,156	173,147	163,529
Net income	77,281	77,111	154,688	146,235
Net (income) loss attributable to noncontrolling interests	0	0	0	0
Net income available to Arch	77,281	77,111	154,688	146,235
Preferred dividends	0	0	0	0
Loss on redemption of preferred shares			0
Net income available to Arch common shareholders	77,281	77,111	154,688	146,235
Comprehensive income available to Arch	70,066	105,302	76,603	193,083
Reportable Legal Entities | Other Arch Capital Subsidiaries
Revenues
Net premiums earned	1,336,763	1,240,874	2,571,662	2,357,891
Net investment income	157,532	133,627	306,299	271,608
Net realized gains (losses)	(76,611)	21,735	(187,631)	55,888
Net impairment losses recognized in earnings	(470)	(1,730)	(632)	(3,537)
Other underwriting income	3,874	4,822	9,223	9,455
Equity in net income (loss) of investment funds accounted for using the equity method	8,472	32,706	36,541	80,794
Other income (loss)	774	(1,557)	926	(2,510)
Total revenues	1,430,334	1,430,477	2,736,388	2,769,589
Expenses
Losses and loss adjustment expenses	726,153	689,860	1,363,013	1,242,430
Acquisition expenses	202,838	190,436	394,214	372,725
Other operating expenses	176,181	169,981	351,196	344,700
Corporate expenses	5,400	1,751	4,254	10,288
Amortization of intangible assets	26,472	30,824	53,208	62,118
Interest expense	34,911	33,050	70,083	64,386
Net foreign exchange (gains) losses	(43,357)	29,843	(26,921)	45,191
Total expenses	1,128,598	1,145,745	2,209,047	2,141,838
Income (loss) before income taxes	301,736	284,732	527,341	627,751
Income tax (expense) benefit	(26,145)	(38,238)	(51,011)	(71,508)
Income (loss) before equity in net income of subsidiaries	275,591	246,494	476,330	556,243
Equity in net income of subsidiaries	0	0	0	0
Net income	275,591	246,494	476,330	556,243
Net (income) loss attributable to noncontrolling interests	(13,023)	(14,254)	(29,307)	(35,485)
Net income available to Arch	262,568	232,240	447,023	520,758
Preferred dividends	0	0	0	0
Loss on redemption of preferred shares			0
Net income available to Arch common shareholders	262,568	232,240	447,023	520,758
Comprehensive income available to Arch	212,802	475,747	291,883	699,920
Consolidating Adjustments and Eliminations
Revenues
Net premiums earned	0	0	0	0
Net investment income	(22,439)	(22,688)	(44,760)	(43,616)
Net realized gains (losses)	0	0	0	0
Net impairment losses recognized in earnings	0	0	0	0
Other underwriting income	0	0	0	0
Equity in net income (loss) of investment funds accounted for using the equity method	0	0	0	0
Other income (loss)	0	0	0	0
Total revenues	(22,439)	(22,688)	(44,760)	(43,616)
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	(22,117)	(22,365)	(44,115)	(42,970)
Net foreign exchange (gains) losses	(10,349)	9,700	(7,093)	13,756
Total expenses	(32,466)	(12,665)	(51,208)	(29,214)
Income (loss) before income taxes	10,027	(10,023)	6,448	(14,402)
Income tax (expense) benefit	0	0	0	0
Income (loss) before equity in net income of subsidiaries	10,027	(10,023)	6,448	(14,402)
Equity in net income of subsidiaries	(350,198)	(299,650)	(608,804)	(653,236)
Net income	(340,171)	(309,673)	(602,356)	(667,638)
Net (income) loss attributable to noncontrolling interests	322	322	645	645
Net income available to Arch	(339,849)	(309,351)	(601,711)	(666,993)
Preferred dividends	0	0	0	0
Loss on redemption of preferred shares			0
Net income available to Arch common shareholders	(339,849)	(309,351)	(601,711)	(666,993)
Comprehensive income available to Arch	$ (282,868)	$ (581,049)	$ (368,486)	$ (893,003)