Guarantor Financial Information - Condensed consolidating statement of income and comprehensive income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Revenues
Net premiums earned	$ 1,240,874	$ 1,005,985	$ 2,357,891	$ 1,957,564
Net investment income	111,124	88,338	228,998	182,073
Net realized gains (losses)	21,735	68,218	55,888	105,542
Net impairment losses recognized in earnings	(1,730)	(5,343)	(3,537)	(12,982)
Other underwriting income	4,822	25,224	9,455	30,271
Equity in net income (loss) of investment funds accounted for using the equity method	32,706	8,737	80,794	15,392
Other income (loss)	(1,994)	(7)	(2,776)	(32)
Total revenues	1,407,537	1,191,152	2,726,713	2,277,828
Expenses
Losses and loss adjustment expenses	689,860	584,592	1,242,430	1,107,541
Acquisition expenses	190,436	172,677	372,725	340,515
Other operating expenses	169,981	157,314	344,700	307,462
Corporate expenses	24,876	17,200	52,668	26,583
Amortization of intangible assets	30,824	4,880	62,118	9,628
Interest expense	28,749	15,663	57,425	31,770
Net foreign exchange (gains) losses	39,543	(24,662)	58,947	(1,096)
Total expenses	1,174,269	927,664	2,191,013	1,822,403
Income (loss) before income taxes	233,268	263,488	535,700	455,425
Income tax (expense) benefit	(34,169)	(14,131)	(62,566)	(30,441)
Income (loss) before equity in net income of subsidiaries	199,099	249,357	473,134	424,984
Equity in net income of subsidiaries	0	0	0	0
Net income	199,099	249,357	473,134	424,984
Net (income) loss attributable to noncontrolling interests	(13,932)	(38,302)	(34,840)	(59,131)
Net income available to Arch	185,167	211,055	438,294	365,853
Preferred dividends	(11,349)	(5,485)	(22,567)	(10,969)
Net income (loss) available to Arch common shareholders	173,818	205,570	415,727	354,884
Comprehensive income (loss) available to Arch	279,285	273,260	631,276	546,189
Reportable Legal Entities | ACGL (Parent Guarantor)
Revenues
Net premiums earned	0	0	0	0
Net investment income	1	0	6	1
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)	(437)	(7)	(266)	199
Total revenues	(436)	(7)	(260)	200
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	21,816	17,441	39,063	26,796
Amortization of intangible assets	0	0	0	0
Interest expense	6,075	5,929	12,090	11,863
Net foreign exchange (gains) losses	0	0	0	0
Total expenses	27,891	23,370	51,153	38,659
Income (loss) before income taxes	(28,327)	(23,377)	(51,413)	(38,459)
Income tax (expense) benefit	0	0	0	0
Income (loss) before equity in net income of subsidiaries	(28,327)	(23,377)	(51,413)	(38,459)
Equity in net income of subsidiaries	213,494	234,432	489,707	404,312
Net income	185,167	211,055	438,294	365,853
Net (income) loss attributable to noncontrolling interests	0	0	0	0
Net income available to Arch	185,167	211,055	438,294	365,853
Preferred dividends	(11,349)	(5,485)	(22,567)	(10,969)
Net income (loss) available to Arch common shareholders	173,818	205,570	415,727	354,884
Comprehensive income (loss) available to Arch	279,285	273,260	631,276	546,189
Reportable Legal Entities | Arch-U.S. (Subsidiary Issuer)
Revenues
Net premiums earned	0	0	0	0
Net investment income	184	775	1,000	1,548
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	184	775	1,000	1,548
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	1,309	359	3,317	941
Amortization of intangible assets	0	0	0	0
Interest expense	11,989	6,647	23,919	13,319
Net foreign exchange (gains) losses	0	0	0	0
Total expenses	13,298	7,006	27,236	14,260
Income (loss) before income taxes	(13,114)	(6,231)	(26,236)	(12,712)
Income tax (expense) benefit	4,069	2,086	8,942	4,330
Income (loss) before equity in net income of subsidiaries	(9,045)	(4,145)	(17,294)	(8,382)
Equity in net income of subsidiaries	86,156	19,873	163,529	42,739
Net income	77,111	15,728	146,235	34,357
Net (income) loss attributable to noncontrolling interests	0	0	0	0
Net income available to Arch	77,111	15,728	146,235	34,357
Preferred dividends	0	0	0	0
Net income (loss) available to Arch common shareholders	77,111	15,728	146,235	34,357
Comprehensive income (loss) available to Arch	105,302	28,536	193,083	87,185
Reportable Legal Entities | Other ACGL Subsidiaries
Revenues
Net premiums earned	1,240,874	1,005,985	2,357,891	1,957,564
Net investment income	133,627	94,097	271,608	194,358
Net realized gains (losses)	21,735	68,218	55,888	105,542
Net impairment losses recognized in earnings	(1,730)	(5,343)	(3,537)	(12,982)
Other underwriting income	4,822	41,450	9,455	46,769
Equity in net income (loss) of investment funds accounted for using the equity method	32,706	8,737	80,794	15,392
Other income (loss)	(1,557)	0	(2,510)	(231)
Total revenues	1,430,477	1,213,144	2,769,589	2,306,412
Expenses
Losses and loss adjustment expenses	689,860	584,592	1,242,430	1,107,541
Acquisition expenses	190,436	172,677	372,725	340,515
Other operating expenses	169,981	157,314	344,700	307,462
Corporate expenses	1,751	(600)	10,288	(1,154)
Amortization of intangible assets	30,824	4,880	62,118	9,628
Interest expense	33,050	25,527	64,386	36,279
Net foreign exchange (gains) losses	29,843	(14,125)	45,191	2,370
Total expenses	1,145,745	930,265	2,141,838	1,802,641
Income (loss) before income taxes	284,732	282,879	627,751	503,771
Income tax (expense) benefit	(38,238)	(16,217)	(71,508)	(34,771)
Income (loss) before equity in net income of subsidiaries	246,494	266,662	556,243	469,000
Equity in net income of subsidiaries	0	0	0	0
Net income	246,494	266,662	556,243	469,000
Net (income) loss attributable to noncontrolling interests	(14,254)	(38,623)	(35,485)	(59,773)
Net income available to Arch	232,240	228,039	520,758	409,227
Preferred dividends	0	0	0	0
Net income (loss) available to Arch common shareholders	232,240	228,039	520,758	409,227
Comprehensive income (loss) available to Arch	475,747	300,542	699,920	592,793
Consolidating Adjustments and Eliminations
Revenues
Net premiums earned	0	0	0	0
Net investment income	(22,688)	(6,534)	(43,616)	(13,834)
Net realized gains (losses)	0	0	0	0
Net impairment losses recognized in earnings	0	0	0	0
Other underwriting income	0	(16,226)	0	(16,498)
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	(22,688)	(22,760)	(43,616)	(30,332)
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	(22,365)	(22,440)	(42,970)	(29,691)
Net foreign exchange (gains) losses	9,700	(10,537)	13,756	(3,466)
Total expenses	(12,665)	(32,977)	(29,214)	(33,157)
Income (loss) before income taxes	(10,023)	10,217	(14,402)	2,825
Income tax (expense) benefit	0	0	0	0
Income (loss) before equity in net income of subsidiaries	(10,023)	10,217	(14,402)	2,825
Equity in net income of subsidiaries	(299,650)	(254,305)	(653,236)	(447,051)
Net income	(309,673)	(244,088)	(667,638)	(444,226)
Net (income) loss attributable to noncontrolling interests	322	321	645	642
Net income available to Arch	(309,351)	(243,767)	(666,993)	(443,584)
Preferred dividends	0	0	0	0
Net income (loss) available to Arch common shareholders	(309,351)	(243,767)	(666,993)	(443,584)
Comprehensive income (loss) available to Arch	$ (581,049)	$ (329,078)	$ (893,003)	$ (679,978)