Segment Information - Summary of underwriting income or loss by segment (Details) $ in Thousands		3 Months Ended			9 Months Ended
		Sep. 30, 2019 USD ($)		Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) segment		Sep. 30, 2018 USD ($)		Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Gross premiums written		$ 2,181,121		$ 1,731,328	$ 6,196,809		$ 5,266,086
Premiums ceded		(567,664)		(397,775)	(1,613,195)		(1,221,093)
Net premiums written		1,613,457		1,333,553	4,583,614		4,044,993
Change in unearned premiums		(175,434)		(42,675)	(312,998)		(182,453)
Net premiums earned		1,438,023		1,290,878	4,270,616		3,862,540
Other underwriting income		3,326		5,823	18,104		15,046
Losses and loss adjustment expenses		(802,455)		(699,420)	(2,288,530)		(2,062,433)
Acquisition expenses, net		(211,120)		(201,602)	(619,057)		(595,816)
Other operating expenses		(196,512)		(161,098)	(596,589)		(512,294)
Underwriting income (loss)		231,262		234,581	784,544		707,043
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)
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
Corporate expenses		(15,066)	[1]	(13,244)	(47,911)	[1]	(43,330)	[1]
Transaction costs and other		(1,995)	[1]	(1,091)	(5,363)	[1]	(8,829)	[1]
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
Income before income taxes		437,305		282,122	1,509,006		750,436
Income tax expense		(38,116)		(33,356)	(128,474)		(78,939)
Net income		399,189		248,766	1,380,532		671,497
Dividends attributable to redeemable noncontrolling interests		(6,600)		(4,599)	(15,778)		(13,769)
Amounts attributable to non-redeemable noncontrolling interests		(136)		(16,759)	(54,819)		(36,251)
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
Underwriting Ratios
Loss ratio		55.80%		54.20%	53.60%		53.40%
Acquisition expense ratio		14.70%		15.60%	14.50%		15.40%
Other operating expense ratio		13.70%		12.50%	14.00%		13.30%
Combined ratio		84.20%		82.30%	82.10%		82.10%
Goodwill and intangible assets		$ 624,500		$ 566,662	$ 624,500		$ 566,662		$ 634,920
Other
Segment Reporting Information [Line Items]
Number of segments | segment					2
Gross premiums written	[2]	249,960		185,033	$ 598,627		574,078
Premiums ceded		(94,208)		(33,356)	(178,118)		(102,263)
Net premiums written		155,752		151,677	420,509		471,815
Change in unearned premiums		(29,920)		(16,054)	2,735		(39,926)
Net premiums earned		125,832		135,623	423,244		431,889
Other underwriting income		579		703	1,844		2,092
Losses and loss adjustment expenses		(96,214)		(96,957)	(318,480)		(312,087)
Acquisition expenses, net		(23,072)		(29,619)	(81,654)		(95,229)
Other operating expenses		(11,568)		(9,959)	(39,368)		(27,752)
Underwriting income (loss)		(4,443)		(209)	(14,414)		(1,087)
Net investment income		34,614		29,696	102,314		84,084
Net realized gains (losses)		(18,659)		(4,695)	6,167		(20,900)
Net impairment losses recognized in earnings		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
Corporate expenses		0	[1]	0	0	[1]	0	[1]
Transaction costs and other		0	[1]	0	0	[1]	0	[1]
Amortization of intangible assets		0		0	0		0
Interest expense		(8,091)		(5,064)	(19,579)		(14,079)
Net foreign exchange gains (losses)		3,330		3,708	2,918		6,521
Income before income taxes		6,751		23,436	77,406		54,539
Income tax expense		0		0	(20)		(27)
Net income		6,751		23,436	77,386		54,512
Dividends attributable to redeemable noncontrolling interests		(6,600)		(4,599)	(15,778)		(13,769)
Amounts attributable to non-redeemable noncontrolling interests		(136)		(16,759)	(54,819)		(36,251)
Net income available to Arch		15		2,078	6,789		4,492
Preferred dividends		0		0	0		0
Loss on redemption of preferred shares							0
Net income available to Arch common shareholders		$ 15		$ 2,078	$ 6,789		$ 4,492
Underwriting Ratios
Loss ratio		76.50%		71.50%	75.20%		72.30%
Acquisition expense ratio		18.30%		21.80%	19.30%		22.00%
Other operating expense ratio		9.20%		7.30%	9.30%		6.40%
Combined ratio		104.00%		100.60%	103.80%		100.70%
Goodwill and intangible assets		$ 7,650		$ 7,650	$ 7,650		$ 7,650
Sub-Total
Segment Reporting Information [Line Items]
Number of segments | segment					3
Gross premiums written	[2]	2,043,292		1,622,532	$ 5,853,574		4,935,339
Premiums ceded		(585,587)		(440,656)	(1,690,469)		(1,362,161)
Net premiums written		1,457,705		1,181,876	4,163,105		3,573,178
Change in unearned premiums		(145,514)		(26,621)	(315,733)		(142,527)
Net premiums earned		1,312,191		1,155,255	3,847,372		3,430,651
Other underwriting income		2,747		5,120	16,260		12,954
Losses and loss adjustment expenses		(706,241)		(602,463)	(1,970,050)		(1,750,346)
Acquisition expenses, net		(188,048)		(171,983)	(537,403)		(500,587)
Other operating expenses		(184,944)		(151,139)	(557,221)		(484,542)
Underwriting income (loss)		235,705		234,790	798,958		708,130
Net investment income		126,874		114,328	371,161		322,332
Net realized gains (losses)		81,177		(47,010)	318,722		(218,414)
Net impairment losses recognized in earnings		(1,163)		(492)	(2,521)		(1,124)
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
Corporate expenses		(15,066)	[1]	(13,244)	(47,911)	[1]	(43,330)	[1]
Transaction costs and other		(1,995)	[1]	(1,091)	(5,363)	[1]	(8,829)	[1]
Amortization of intangible assets		(20,003)		(26,315)	(60,214)		(79,523)
Interest expense		(23,237)		(24,666)	(70,094)		(76,631)
Net foreign exchange gains (losses)		29,794		7,130	28,779		38,302
Income before income taxes		430,554		258,686	1,431,600		695,897
Income tax expense		(38,116)		(33,356)	(128,454)		(78,912)
Net income		392,438		225,330	1,303,146		616,985
Dividends attributable to redeemable noncontrolling interests		0		0	0		0
Amounts attributable to non-redeemable noncontrolling interests		0		0	0		0
Net income available to Arch		392,438		225,330	1,303,146		616,985
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		$ 382,035		$ 214,928	$ 1,271,937		$ 583,033
Underwriting Ratios
Loss ratio		53.80%		52.10%	51.20%		51.00%
Acquisition expense ratio		14.30%		14.90%	14.00%		14.60%
Other operating expense ratio		14.10%		13.10%	14.50%		14.10%
Combined ratio		82.20%		80.10%	79.70%		79.70%
Goodwill and intangible assets		$ 616,850		$ 559,012	$ 616,850		$ 559,012
Sub-Total | Insurance
Segment Reporting Information [Line Items]
Gross premiums written	[2]	1,005,874		836,820	2,867,753		2,429,570
Premiums ceded		(302,034)		(259,968)	(914,751)		(752,413)
Net premiums written		703,840		576,852	1,953,002		1,677,157
Change in unearned premiums		(98,504)		(15,794)	(201,719)		(30,913)
Net premiums earned		605,336		561,058	1,751,283		1,646,244
Other underwriting income		0		0	0		0
Losses and loss adjustment expenses		(422,782)		(409,435)	(1,168,677)		(1,120,630)
Acquisition expenses, net		(91,259)		(88,255)	(265,177)		(264,094)
Other operating expenses		(115,408)		(90,081)	(338,327)		(274,735)
Underwriting income (loss)		$ (24,113)		$ (26,713)	$ (20,898)		$ (13,215)
Underwriting Ratios
Loss ratio		69.80%		73.00%	66.70%		68.10%
Acquisition expense ratio		15.10%		15.70%	15.10%		16.00%
Other operating expense ratio		19.10%		16.10%	19.30%		16.70%
Combined ratio		104.00%		104.80%	101.10%		100.80%
Goodwill and intangible assets		$ 158,990		$ 20,141	$ 158,990		$ 20,141
Sub-Total | Reinsurance
Segment Reporting Information [Line Items]
Gross premiums written	[2]	662,572		435,396	1,890,974		1,503,206
Premiums ceded		(226,096)		(123,705)	(627,120)		(455,682)
Net premiums written		436,476		311,691	1,263,854		1,047,524
Change in unearned premiums		(72,621)		(18,418)	(186,450)		(134,761)
Net premiums earned		363,855		293,273	1,077,404		912,763
Other underwriting income		(1,208)		1,387	4,393		2,490
Losses and loss adjustment expenses		(270,379)		(183,413)	(751,147)		(555,044)
Acquisition expenses, net		(62,393)		(50,367)	(173,504)		(148,828)
Other operating expenses		(32,533)		(29,936)	(102,197)		(101,185)
Underwriting income (loss)		$ (2,658)		$ 30,944	$ 54,949		$ 110,196
Underwriting Ratios
Loss ratio		74.30%		62.50%	69.70%		60.80%
Acquisition expense ratio		17.10%		17.20%	16.10%		16.30%
Other operating expense ratio		8.90%		10.20%	9.50%		11.10%
Combined ratio		100.30%		89.90%	95.30%		88.20%
Goodwill and intangible assets		$ 0		$ 0	$ 0		$ 0
Sub-Total | Mortgage
Segment Reporting Information [Line Items]
Gross premiums written	[2]	375,092		350,559	1,095,607		1,002,727
Premiums ceded		(57,703)		(57,226)	(149,358)		(154,230)
Net premiums written		317,389		293,333	946,249		848,497
Change in unearned premiums		25,611		7,591	72,436		23,147
Net premiums earned		343,000		300,924	1,018,685		871,644
Other underwriting income		3,955		3,733	11,867		10,464
Losses and loss adjustment expenses		(13,080)		(9,615)	(50,226)		(74,672)
Acquisition expenses, net		(34,396)		(33,361)	(98,722)		(87,665)
Other operating expenses		(37,003)		(31,122)	(116,697)		(108,622)
Underwriting income (loss)		$ 262,476		$ 230,559	$ 764,907		$ 611,149
Underwriting Ratios
Loss ratio		3.80%		3.20%	4.90%		8.60%
Acquisition expense ratio		10.00%		11.10%	9.70%		10.10%
Other operating expense ratio		10.80%		10.30%	11.50%		12.50%
Combined ratio		24.60%		24.60%	26.10%		31.20%
Goodwill and intangible assets		$ 457,860		$ 538,871	$ 457,860		$ 538,871

[1] Certain expenses have been excluded from ‘corporate expenses’ and reflected in ‘transaction costs and other.’

[2] Certain amounts included in the gross premiums written of each segment are related to intersegment transactions. Accordingly, the sum of gross premiums written for each segment does not agree to the total gross premiums written as shown in the table above due to the elimination of intersegment transactions in the total.