Segment Information - Analysis of underwriting income by segment and reconciliation to net income available to common shareholders (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Segment Reporting Information [Line Items]
Gross premiums written									$ 4,840,616		$ 4,196,623		$ 3,869,161
Premiums ceded									(948,678)		(845,256)		(816,926)
Net premiums written	895,481	959,539	971,928	1,064,990	748,921	839,135	810,535	952,776	3,891,938		3,351,367		3,052,235
Change in unearned premiums									(298,190)		(205,415)		(117,095)
Net premiums earned	923,165	903,651	907,152	859,780	839,366	795,000	758,816	752,770	3,593,748		3,145,952		2,935,140
Other underwriting income									10,142		7,639		8,090
Losses and loss adjustment expenses									(1,919,250)		(1,679,424)		(1,861,277)
Acquisition expenses, net									(657,262)		(564,103)		(508,884)
Other operating expenses									(556,280)		(458,327)		(430,035)
Underwriting income (loss)	113,464	101,087	124,091	132,456	128,318	110,992	96,029	116,398	471,098		451,737		143,034
Net investment income	82,496	80,105	72,990	66,994	67,095	66,083	68,369	65,672	302,585		267,219		294,895
Net realized gains	10,561	18,515	54,144	19,697	9,048	(6,022)	12,652	58,340	102,917		74,018		194,228
Net impairment losses recognized in earnings	(3,837)	(8,593)	(14,749)	(2,971)	(88)	(728)	(724)	(2,246)	(30,150)		(3,786)		(11,388)
Equity in net income (loss) of investment funds accounted for using the equity method									19,883		35,701		73,510
Other income (loss)									(10,252)		(586)		(12,094)
Other expenses									(49,944)		(42,403)		(35,318)
Interest expense									(45,634)		(27,060)		(28,525)
Net foreign exchange (gains) losses									83,744		(12,335)		(28,955)
Income before income taxes									844,247		742,505		589,387
Income tax (expense) benefit									(22,987)		(32,774)		4,010
Net income	207,134	223,264	211,717	179,145	161,490	114,825	176,940	256,476	821,260		709,731		593,397
Dividends attributable to redeemable noncontrolling interests									(14,728)		0		0
Amounts attributable to noncontrolling interests									27,823		0		0
Net income available to Arch									834,355		709,731		593,397
Preferred dividends	(5,485)	(5,484)	(5,485)	(5,484)	(5,485)	(5,484)	(5,485)	(5,484)	(21,938)		(21,938)		(25,079)
Loss on repurchase of preferred shares									0		0		(10,612)
Net income available to common shareholders	209,679	223,191	202,531	177,016	156,005	109,341	171,455	250,992	812,417		687,793		557,706
Underwriting Ratios
Loss ratio									53.40%		53.40%		63.40%
Acquisition expense ratio									18.30%		17.90%		17.30%
Other operating expense ratio									15.50%		14.60%		14.70%
Combined ratio									87.20%		85.90%		95.40%
Goodwill and intangible assets	109,539				27,319				109,539		27,319		38,259
Investable assets	15,773,209				14,049,525				15,773,209		14,049,525		13,045,134
Total assets	22,009,543				19,566,094				22,009,543		19,566,094		17,816,762
Total liabilities	14,890,897				13,918,598				14,890,897		13,918,598		12,647,884
Insurance
Segment Reporting Information [Line Items]
Premiums ceded									(862,015)	[1]	(763,713)	[1]	(768,625)	[1]
Net premiums written									2,146,654		1,948,796		1,825,334
Reinsurance
Segment Reporting Information [Line Items]
Premiums ceded									(261,255)	[1]	(86,620)	[1]	(55,099)	[1]
Net premiums written									1,265,991		1,313,001		1,158,790
Mortgage
Segment Reporting Information [Line Items]
Premiums ceded									(22,519)	[1]	0	[1]	0	[1]
Net premiums written									204,837		89,570		68,111
Other
Segment Reporting Information [Line Items]
Number of segments									2
Gross premiums written									288,627	[2]	0	[2]	0	[2]
Premiums ceded									(14,171)	[1]	0		0
Net premiums written									274,456		0		0
Change in unearned premiums									(170,979)		0		0
Net premiums earned									103,477		0		0
Other underwriting income									0		0		0
Losses and loss adjustment expenses									(70,173)		0		0
Acquisition expenses, net									(30,116)		0		0
Other operating expenses									(6,268)		0		0
Underwriting income (loss)									(3,080)		0		0
Net investment income									18,249		0		0
Net realized gains									(30,467)		0		0
Net impairment losses recognized in earnings									0		0		0
Equity in net income (loss) of investment funds accounted for using the equity method									0		0		0
Other income (loss)									0		0		0
Other expenses									(2,329)		0		0
Interest expense									0		0		0
Net foreign exchange (gains) losses									1,086		0		0
Income before income taxes									(16,541)		0		0
Income tax (expense) benefit									0		0		0
Net income									(16,541)		0		0
Dividends attributable to redeemable noncontrolling interests									(14,728)		0		0
Amounts attributable to noncontrolling interests									27,823		0		0
Net income available to Arch									(3,446)		0		0
Preferred dividends									0		0		0
Loss on repurchase of preferred shares													0
Net income available to common shareholders									(3,446)		0		0
Underwriting Ratios
Loss ratio									67.80%		0.00%		0.00%
Acquisition expense ratio									29.10%		0.00%		0.00%
Other operating expense ratio									6.10%		0.00%		0.00%
Combined ratio									103.00%		0.00%		0.00%
Goodwill and intangible assets	0				0				0		0		0
Investable assets	1,163,240				0				1,163,240		0		0
Total assets	1,482,516				0				1,482,516		0		0
Total liabilities	398,660				0				398,660		0		0
Underwriting segments
Segment Reporting Information [Line Items]
Number of segments									3
Gross premiums written									4,760,394	[2]	4,196,623	[2]	3,869,161	[2]
Premiums ceded									(1,142,912)		(845,256)		(816,926)
Net premiums written									3,617,482		3,351,367		3,052,235
Change in unearned premiums									(127,211)		(205,415)		(117,095)
Net premiums earned									3,490,271		3,145,952		2,935,140
Other underwriting income									10,142		7,639		8,090
Losses and loss adjustment expenses									(1,849,077)		(1,679,424)		(1,861,277)
Acquisition expenses, net									(627,146)		(564,103)		(508,884)
Other operating expenses									(550,012)		(458,327)		(430,035)
Underwriting income (loss)									474,178		451,737		143,034
Net investment income									284,336		267,219		294,895
Net realized gains									133,384		74,018		194,228
Net impairment losses recognized in earnings									(30,150)		(3,786)		(11,388)
Equity in net income (loss) of investment funds accounted for using the equity method									19,883		35,701		73,510
Other income (loss)									(10,252)		(586)		(12,094)
Other expenses									(47,615)		(42,403)		(35,318)
Interest expense									(45,634)		(27,060)		(28,525)
Net foreign exchange (gains) losses									82,658		(12,335)		(28,955)
Income before income taxes									860,788		742,505		589,387
Income tax (expense) benefit									(22,987)		(32,774)		4,010
Net income									837,801		709,731		593,397
Dividends attributable to redeemable noncontrolling interests									0		0		0
Amounts attributable to noncontrolling interests									0		0		0
Net income available to Arch									837,801		709,731		593,397
Preferred dividends									(21,938)		(21,938)		(25,079)
Loss on repurchase of preferred shares													(10,612)
Net income available to common shareholders									815,863		687,793		557,706
Underwriting Ratios
Loss ratio									53.00%		53.40%		63.40%
Acquisition expense ratio									18.00%		17.90%		17.30%
Other operating expense ratio									15.80%		14.60%		14.70%
Combined ratio									86.80%		85.90%		95.40%
Goodwill and intangible assets	109,539				27,319				109,539		27,319		38,259
Investable assets	14,609,969				14,049,525				14,609,969		14,049,525		13,045,134
Total assets	20,527,027				19,566,094				20,527,027		19,566,094		17,816,762
Total liabilities	14,492,237				13,918,598				14,492,237		13,918,598		12,647,884
Underwriting segments | Insurance
Segment Reporting Information [Line Items]
Gross premiums written									3,008,669	[2]	2,712,509	[2]	2,593,959	[2]
Premiums ceded									(862,015)		(763,713)		(768,625)
Net premiums written									2,146,654		1,948,796		1,825,334
Change in unearned premiums									(129,284)		(72,782)		(24,991)
Net premiums earned									2,017,370		1,876,014		1,800,343
Other underwriting income									2,135		2,122		2,335
Losses and loss adjustment expenses									(1,260,953)		(1,188,445)		(1,283,841)
Acquisition expenses, net									(316,308)		(311,904)		(298,983)
Other operating expenses									(335,157)		(315,387)		(307,489)
Underwriting income (loss)									107,087		62,400		(87,635)
Underwriting Ratios
Loss ratio									62.50%		63.30%		71.30%
Acquisition expense ratio									15.70%		16.60%		16.60%
Other operating expense ratio									16.60%		16.80%		17.10%
Combined ratio									94.80%		96.70%		105.00%
Goodwill and intangible assets	28,331				20,419				28,331		20,419		21,357
Underwriting segments | Reinsurance
Segment Reporting Information [Line Items]
Gross premiums written									1,527,245	[2]	1,399,621	[2]	1,213,889	[2]
Premiums ceded									(261,254)		(86,620)		(55,099)
Net premiums written									1,265,991		1,313,001		1,158,790
Change in unearned premiums									13,337		(94,329)		(40,663)
Net premiums earned									1,279,328		1,218,672		1,118,127
Other underwriting income									3,167		5,258		5,755
Losses and loss adjustment expenses									(532,450)		(486,236)		(574,821)
Acquisition expenses, net									(261,438)		(234,373)		(204,903)
Other operating expenses									(147,964)		(134,563)		(118,245)
Underwriting income (loss)									340,643		368,758		225,913
Underwriting Ratios
Loss ratio									41.60%		39.90%		51.40%
Acquisition expense ratio									20.40%		19.20%		18.30%
Other operating expense ratio									11.60%		11.00%		10.60%
Combined ratio									73.60%		70.10%		80.30%
Goodwill and intangible assets	3,333				6,900				3,333		6,900		16,902
Underwriting segments | Mortgage
Segment Reporting Information [Line Items]
Gross premiums written									227,356	[2]	89,570	[2]	68,111	[2]
Premiums ceded									(22,519)		0		0
Net premiums written									204,837		89,570		68,111
Change in unearned premiums									(11,264)		(38,304)		(51,441)
Net premiums earned									193,573		51,266		16,670
Other underwriting income									4,840		259		0
Losses and loss adjustment expenses									(55,674)		(4,743)		(2,615)
Acquisition expenses, net									(49,400)		(17,826)		(4,998)
Other operating expenses									(66,891)		(8,377)		(4,301)
Underwriting income (loss)									26,448		20,579		4,756
Underwriting Ratios
Loss ratio									28.80%		9.30%		15.70%
Acquisition expense ratio									25.50%		34.80%		30.00%
Other operating expense ratio									34.60%		16.30%		25.80%
Combined ratio									88.90%		60.40%		71.50%
Goodwill and intangible assets	$ 77,875				$ 0				$ 77,875		$ 0		$ 0

[1] Certain amounts included in the gross premiums written of each segment are related to intersegment transactions and are included in the gross premiums written of each segment. 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.

[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.