Segment Information - Summary of underwriting income or loss by segment (Details) - USD ($) $ in Thousands		3 Months Ended			6 Months Ended
		Jun. 30, 2018		Jun. 30, 2017	Jun. 30, 2018		Jun. 30, 2017		Dec. 31, 2017
Segment Reporting Information [Line Items]
Gross premiums written		$ 1,696,544		$ 1,609,659	$ 3,534,758		$ 3,267,649
Premiums ceded		(397,648)		(360,964)	(823,318)		(742,694)
Net premiums written		1,298,896		1,248,695	2,711,440		2,524,955
Change in unearned premiums		37,867		(7,821)	(139,778)		(167,064)
Net premiums earned		1,336,763		1,240,874	2,571,662		2,357,891
Other underwriting income		3,874		4,822	9,223		9,455
Losses and loss adjustment expenses		(726,153)		(689,860)	(1,363,013)		(1,242,430)
Acquisition expenses, net		(202,838)		(190,436)	(394,214)		(372,725)
Other operating expenses		(176,181)		(169,981)	(351,196)		(344,700)
Underwriting income (loss)		235,465		195,419	472,462		407,491
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)
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)
Corporate expenses		(15,604)	[1]	(22,201)	(30,086)	[1]	(34,409)	[1]
UGC transaction costs and other		(6,908)	[1]	(2,675)	(7,738)	[1]	(18,259)	[1]
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)
Income before income taxes		280,015		233,268	468,314		535,700
Income tax expense		(23,668)		(34,169)	(45,583)		(62,566)
Net income (loss)		256,347		199,099	422,731		473,134
Dividends attributable to redeemable noncontrolling interests		(4,585)		(4,586)	(9,170)		(9,170)
Amounts attributable to non-redeemable noncontrolling interests		(8,116)		(9,346)	(19,492)		(25,670)
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
Underwriting Ratios
Loss ratio		54.30%		55.60%	53.00%		52.70%
Acquisition expense ratio		15.20%		15.30%	15.30%		15.80%
Other operating expense ratio		13.20%		13.70%	13.70%		14.60%
Combined ratio		82.70%		84.60%	82.00%		83.10%
Goodwill and intangible assets		$ 593,008		$ 712,975	$ 593,008		$ 712,975		$ 652,611
Other
Segment Reporting Information [Line Items]
Gross premiums written	[2]	175,175		152,813	389,045		306,933
Premiums ceded		(34,589)		(12,410)	(68,907)		(22,844)
Net premiums written		140,586		140,403	320,138		284,089
Change in unearned premiums		18,932		10,351	(23,872)		(11,338)
Net premiums earned		159,518		150,754	296,266		272,751
Other underwriting income		688		824	1,389		1,640
Losses and loss adjustment expenses		(117,141)		(110,621)	(215,130)		(196,031)
Acquisition expenses, net		(34,289)		(34,747)	(65,610)		(67,255)
Other operating expenses		(9,094)		(8,853)	(17,793)		(16,043)
Underwriting income (loss)		(318)		(2,643)	(878)		(4,938)
Net investment income		27,907		18,604	54,388		40,666
Net realized gains (losses)		(17,066)		3,689	(16,205)		9,330
Net impairment losses recognized in earnings		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
Corporate expenses		0	[1]	0	0	[1]	0	[1]
UGC transaction costs and other		0	[1]	0	0	[1]	0	[1]
Amortization of intangible assets		0		0	0		0
Interest expense		(4,286)		(2,837)	(9,015)		(5,757)
Net foreign exchange gains (losses)		7,495		(1,722)	2,813		(1,281)
Income before income taxes		13,732		15,091	31,103		38,020
Income tax expense		(24)		0	(27)		0
Net income (loss)		13,708		15,091	31,076		38,020
Dividends attributable to redeemable noncontrolling interests		(4,585)		(4,586)	(9,170)		(9,170)
Amounts attributable to non-redeemable noncontrolling interests		(8,116)		(9,346)	(19,492)		(25,670)
Net income available to Arch		1,007		1,159	2,414		3,180
Preferred dividends		0		0	0		0
Loss on redemption of preferred shares					0
Net income available to Arch common shareholders		$ 1,007		$ 1,159	$ 2,414		$ 3,180
Underwriting Ratios
Loss ratio		73.40%		73.40%	72.60%		71.90%
Acquisition expense ratio		21.50%		23.00%	22.10%		24.70%
Other operating expense ratio		5.70%		5.90%	6.00%		5.90%
Combined ratio		100.60%		102.30%	100.70%		102.50%
Goodwill and intangible assets		$ 7,650		$ 7,650	$ 7,650		$ 7,650
Sub-Total
Segment Reporting Information [Line Items]
Gross premiums written	[2]	1,591,202		1,533,142	3,312,807		3,139,828
Premiums ceded		(432,892)		(424,850)	(921,505)		(898,962)
Net premiums written		1,158,310		1,108,292	2,391,302		2,240,866
Change in unearned premiums		18,935		(18,172)	(115,906)		(155,726)
Net premiums earned		1,177,245		1,090,120	2,275,396		2,085,140
Other underwriting income		3,186		3,998	7,834		7,815
Losses and loss adjustment expenses		(609,012)		(579,239)	(1,147,883)		(1,046,399)
Acquisition expenses, net		(168,549)		(155,689)	(328,604)		(305,470)
Other operating expenses		(167,087)		(161,128)	(333,403)		(328,657)
Underwriting income (loss)		235,783		198,062	473,340		412,429
Net investment income		107,761		92,520	208,004		188,332
Net realized gains (losses)		(59,545)		18,046	(171,404)		46,558
Net impairment losses recognized in earnings		(470)		(1,730)	(632)		(3,537)
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)
Corporate expenses		(15,604)	[1]	(22,201)	(30,086)	[1]	(34,409)	[1]
UGC transaction costs and other		(6,908)	[1]	(2,675)	(7,738)	[1]	(18,259)	[1]
Amortization of intangible assets		(26,472)		(30,824)	(53,208)		(62,118)
Interest expense		(26,058)		(25,912)	(51,965)		(51,668)
Net foreign exchange gains (losses)		46,211		(37,821)	31,172		(57,666)
Income before income taxes		266,283		218,177	437,211		497,680
Income tax expense		(23,644)		(34,169)	(45,556)		(62,566)
Net income (loss)		242,639		184,008	391,655		435,114
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		242,639		184,008	391,655		435,114
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		$ 232,236		$ 172,659	$ 368,105		$ 412,547
Underwriting Ratios
Loss ratio		51.70%		53.10%	50.40%		50.20%
Acquisition expense ratio		14.30%		14.30%	14.40%		14.60%
Other operating expense ratio		14.20%		14.80%	14.70%		15.80%
Combined ratio		80.20%		82.20%	79.50%		80.60%
Goodwill and intangible assets		$ 585,358		$ 705,325	$ 585,358		$ 705,325
Sub-Total | Insurance
Segment Reporting Information [Line Items]
Gross premiums written	[2]	769,372		743,902	1,592,750		1,526,183
Premiums ceded		(245,265)		(247,446)	(492,445)		(481,541)
Net premiums written		524,107		496,456	1,100,305		1,044,642
Change in unearned premiums		22,342		21,118	(15,119)		(21,422)
Net premiums earned		546,449		517,574	1,085,186		1,023,220
Other underwriting income		0		0	0		0
Losses and loss adjustment expenses		(357,465)		(350,939)	(711,195)		(683,580)
Acquisition expenses, net		(90,670)		(78,872)	(175,839)		(153,740)
Other operating expenses		(92,680)		(92,267)	(184,654)		(180,393)
Underwriting income (loss)		$ 5,634		$ (4,504)	$ 13,498		$ 5,507
Underwriting Ratios
Loss ratio		65.40%		67.80%	65.50%		66.80%
Acquisition expense ratio		16.60%		15.20%	16.20%		15.00%
Other operating expense ratio		17.00%		17.80%	17.00%		17.60%
Combined ratio		99.00%		100.80%	98.70%		99.40%
Goodwill and intangible assets		$ 20,724		$ 24,480	$ 20,724		$ 24,480
Sub-Total | Reinsurance
Segment Reporting Information [Line Items]
Gross premiums written	[2]	490,327		453,186	1,067,810		928,968
Premiums ceded		(136,247)		(115,262)	(331,977)		(281,354)
Net premiums written		354,080		337,924	735,833		647,614
Change in unearned premiums		(13,762)		(23,222)	(116,343)		(88,061)
Net premiums earned		340,318		314,702	619,490		559,553
Other underwriting income		(129)		(279)	1,103		(585)
Losses and loss adjustment expenses		(229,956)		(207,606)	(371,631)		(313,060)
Acquisition expenses, net		(50,142)		(51,151)	(98,461)		(97,298)
Other operating expenses		(35,678)		(36,711)	(71,249)		(74,244)
Underwriting income (loss)		$ 24,413		$ 18,955	$ 79,252		$ 74,366
Underwriting Ratios
Loss ratio		67.60%		66.00%	60.00%		55.90%
Acquisition expense ratio		14.70%		16.30%	15.90%		17.40%
Other operating expense ratio		10.50%		11.70%	11.50%		13.30%
Combined ratio		92.80%		94.00%	87.40%		86.60%
Goodwill and intangible assets		$ 0		$ 609	$ 0		$ 609
Sub-Total | Mortgage
Segment Reporting Information [Line Items]
Gross premiums written	[2]	330,990		336,226	652,168		684,849
Premiums ceded		(50,867)		(62,314)	(97,004)		(136,239)
Net premiums written		280,123		273,912	555,164		548,610
Change in unearned premiums		10,355		(16,068)	15,556		(46,243)
Net premiums earned		290,478		257,844	570,720		502,367
Other underwriting income		3,315		4,277	6,731		8,400
Losses and loss adjustment expenses		(21,591)		(20,694)	(65,057)		(49,759)
Acquisition expenses, net		(27,737)		(25,666)	(54,304)		(54,432)
Other operating expenses		(38,729)		(32,150)	(77,500)		(74,020)
Underwriting income (loss)		$ 205,736		$ 183,611	$ 380,590		$ 332,556
Underwriting Ratios
Loss ratio		7.40%		8.00%	11.40%		9.90%
Acquisition expense ratio		9.50%		10.00%	9.50%		10.80%
Other operating expense ratio		13.30%		12.50%	13.60%		14.70%
Combined ratio		30.20%		30.50%	34.50%		35.40%
Goodwill and intangible assets		$ 564,634		$ 680,236	$ 564,634		$ 680,236

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