Segment Information - Summary of underwriting income or loss by segment (Details) - USD ($) $ in Thousands		3 Months Ended			6 Months Ended
		Jun. 30, 2017		Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Gross premiums written		$ 1,609,659		$ 1,329,936	$ 3,267,649	$ 2,767,902
Premiums ceded		(360,964)		(306,373)	(742,694)	(623,104)
Net premiums written		1,248,695		1,023,563	2,524,955	2,144,798
Change in unearned premiums		(7,821)		(17,578)	(167,064)	(187,234)
Net premiums earned		1,240,874		1,005,985	2,357,891	1,957,564
Other underwriting income		4,822		25,224	9,455	30,271
Losses and loss adjustment expenses		(689,860)		(584,592)	(1,242,430)	(1,107,541)
Acquisition expenses, net		(190,436)		(172,677)	(372,725)	(340,515)
Other operating expenses		(169,981)		(157,314)	(344,700)	(307,462)
Underwriting income (loss)		195,419		116,626	407,491	232,317
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)
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)
Corporate expenses		(22,201)	[1]	(17,200)	(34,409)	(26,583)
UGC transaction costs and other	[1]	(2,675)			(18,259)
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
Income before income taxes		233,268		263,488	535,700	455,425
Income tax expense		(34,169)		(14,131)	(62,566)	(30,441)
Net income (loss)		199,099		249,357	473,134	424,984
Dividends attributable to redeemable noncontrolling interests		(4,586)		(4,586)	(9,170)	(9,173)
Amounts attributable to non-redeemable noncontrolling interests		(9,346)		(33,716)	(25,670)	(49,958)
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
Underwriting Ratios
Loss ratio		55.60%		58.10%	52.70%	56.60%
Acquisition expense ratio		15.30%		17.20%	15.80%	17.40%
Other operating expense ratio		13.70%		15.60%	14.60%	15.70%
Combined ratio		84.60%		90.90%	83.10%	89.70%
Goodwill and intangible assets		$ 712,975		$ 88,327	$ 712,975	$ 88,327	$ 781,553
Other
Segment Reporting Information [Line Items]
Gross premiums written	[2]	152,813		109,285	306,933	257,891
Premiums ceded		(12,410)		(4,457)	(22,844)	(8,929)
Net premiums written		140,403		104,828	284,089	248,962
Change in unearned premiums		10,351		15,739	(11,338)	(12,878)
Net premiums earned		150,754		120,567	272,751	236,084
Other underwriting income		824		969	1,640	1,898
Losses and loss adjustment expenses		(110,621)		(83,502)	(196,031)	(162,615)
Acquisition expenses, net		(34,747)		(33,645)	(67,255)	(66,584)
Other operating expenses		(8,853)		(6,113)	(16,043)	(11,451)
Underwriting income (loss)		(2,643)		(1,724)	(4,938)	(2,668)
Net investment income		18,604		17,941	40,666	41,267
Net realized gains (losses)		3,689		27,291	9,330	32,753
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	0
UGC transaction costs and other	[1]	0			0
Amortization of intangible assets		0		0	0	0
Interest expense		(2,837)		(3,231)	(5,757)	(6,711)
Net foreign exchange gains (losses)		(1,722)		2,201	(1,281)	676
Income before income taxes		15,091		42,478	38,020	65,317
Income tax expense		0		0	0	0
Net income (loss)		15,091		42,478	38,020	65,317
Dividends attributable to redeemable noncontrolling interests		(4,586)		(4,586)	(9,170)	(9,173)
Amounts attributable to non-redeemable noncontrolling interests		(9,346)		(33,716)	(25,670)	(49,958)
Net income available to Arch		1,159		4,176	3,180	6,186
Preferred dividends		0		0	0	0
Net income (loss) available to Arch common shareholders		$ 1,159		$ 4,176	$ 3,180	$ 6,186
Underwriting Ratios
Loss ratio		73.40%		69.30%	71.90%	68.90%
Acquisition expense ratio		23.00%		27.90%	24.70%	28.20%
Other operating expense ratio		5.90%		5.10%	5.90%	4.90%
Combined ratio		102.30%		102.30%	102.50%	102.00%
Goodwill and intangible assets		$ 7,650		$ 0	$ 7,650	$ 0
Sub-Total
Segment Reporting Information [Line Items]
Gross premiums written	[2]	1,533,142		1,292,199	3,139,828	2,683,260
Premiums ceded		(424,850)		(373,464)	(898,962)	(787,424)
Net premiums written		1,108,292		918,735	2,240,866	1,895,836
Change in unearned premiums		(18,172)		(33,317)	(155,726)	(174,356)
Net premiums earned		1,090,120		885,418	2,085,140	1,721,480
Other underwriting income		3,998		24,255	7,815	28,373
Losses and loss adjustment expenses		(579,239)		(501,090)	(1,046,399)	(944,926)
Acquisition expenses, net		(155,689)		(139,032)	(305,470)	(273,931)
Other operating expenses		(161,128)		(151,201)	(328,657)	(296,011)
Underwriting income (loss)		198,062		118,350	412,429	234,985
Net investment income		92,520		70,397	188,332	140,806
Net realized gains (losses)		18,046		40,927	46,558	72,789
Net impairment losses recognized in earnings		(1,730)		(5,343)	(3,537)	(12,982)
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)
Corporate expenses		(22,201)	[1]	(17,200)	(34,409)	(26,583)
UGC transaction costs and other	[1]	(2,675)			(18,259)
Amortization of intangible assets		(30,824)		(4,880)	(62,118)	(9,628)
Interest expense		(25,912)		(12,432)	(51,668)	(25,059)
Net foreign exchange gains (losses)		(37,821)		22,461	(57,666)	420
Income before income taxes		218,177		221,010	497,680	390,108
Income tax expense		(34,169)		(14,131)	(62,566)	(30,441)
Net income (loss)		184,008		206,879	435,114	359,667
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		184,008		206,879	435,114	359,667
Preferred dividends		(11,349)		(5,485)	(22,567)	(10,969)
Net income (loss) available to Arch common shareholders		$ 172,659		$ 201,394	$ 412,547	$ 348,698
Underwriting Ratios
Loss ratio		53.10%		56.60%	50.20%	54.90%
Acquisition expense ratio		14.30%		15.70%	14.60%	15.90%
Other operating expense ratio		14.80%		17.10%	15.80%	17.20%
Combined ratio		82.20%		89.40%	80.60%	88.00%
Goodwill and intangible assets		$ 705,325		$ 88,327	$ 705,325	$ 88,327
Sub-Total | Insurance
Segment Reporting Information [Line Items]
Gross premiums written	[2]	743,902		762,043	1,526,183	1,560,596
Premiums ceded		(247,446)		(246,875)	(481,541)	(495,664)
Net premiums written		496,456		515,168	1,044,642	1,064,932
Change in unearned premiums		21,118		12,482	(21,422)	(24,193)
Net premiums earned		517,574		527,650	1,023,220	1,040,739
Other underwriting income		0		0	0	0
Losses and loss adjustment expenses		(350,939)		(354,633)	(683,580)	(678,242)
Acquisition expenses, net		(78,872)		(77,312)	(153,740)	(151,660)
Other operating expenses		(92,267)		(91,440)	(180,393)	(176,498)
Underwriting income (loss)		$ (4,504)		$ 4,265	$ 5,507	$ 34,339
Underwriting Ratios
Loss ratio		67.80%		67.20%	66.80%	65.20%
Acquisition expense ratio		15.20%		14.70%	15.00%	14.60%
Other operating expense ratio		17.80%		17.30%	17.60%	17.00%
Combined ratio		100.80%		99.20%	99.40%	96.80%
Goodwill and intangible assets		$ 24,480		$ 27,457	$ 24,480	$ 27,457
Sub-Total | Reinsurance
Segment Reporting Information [Line Items]
Gross premiums written	[2]	453,186		412,053	928,968	893,443
Premiums ceded		(115,262)		(119,951)	(281,354)	(280,517)
Net premiums written		337,924		292,102	647,614	612,926
Change in unearned premiums		(23,222)		(846)	(88,061)	(60,462)
Net premiums earned		314,702		291,256	559,553	552,464
Other underwriting income		(279)		20,118	(585)	20,443
Losses and loss adjustment expenses		(207,606)		(146,091)	(313,060)	(257,689)
Acquisition expenses, net		(51,151)		(55,756)	(97,298)	(110,514)
Other operating expenses		(36,711)		(36,914)	(74,244)	(73,172)
Underwriting income (loss)		$ 18,955		$ 72,613	$ 74,366	$ 131,532
Underwriting Ratios
Loss ratio		66.00%		50.20%	55.90%	46.60%
Acquisition expense ratio		16.30%		19.10%	17.40%	20.00%
Other operating expense ratio		11.70%		12.70%	13.30%	13.20%
Combined ratio		94.00%		82.00%	86.60%	79.80%
Goodwill and intangible assets		$ 609		$ 1,440	$ 609	$ 1,440
Sub-Total | Mortgage
Segment Reporting Information [Line Items]
Gross premiums written	[2]	336,226		118,434	684,849	229,714
Premiums ceded		(62,314)		(6,969)	(136,239)	(11,736)
Net premiums written		273,912		111,465	548,610	217,978
Change in unearned premiums		(16,068)		(44,953)	(46,243)	(89,701)
Net premiums earned		257,844		66,512	502,367	128,277
Other underwriting income		4,277		4,137	8,400	7,930
Losses and loss adjustment expenses		(20,694)		(366)	(49,759)	(8,995)
Acquisition expenses, net		(25,666)		(5,964)	(54,432)	(11,757)
Other operating expenses		(32,150)		(22,847)	(74,020)	(46,341)
Underwriting income (loss)		$ 183,611		$ 41,472	$ 332,556	$ 69,114
Underwriting Ratios
Loss ratio		8.00%		0.60%	9.90%	7.00%
Acquisition expense ratio		10.00%		9.00%	10.80%	9.20%
Other operating expense ratio		12.50%		34.40%	14.70%	36.10%
Combined ratio		30.50%		44.00%	35.40%	52.30%
Goodwill and intangible assets		$ 680,236		$ 59,430	$ 680,236	$ 59,430

[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.