Segment Information - Summary of underwriting income or loss by segment (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013		Dec. 31, 2013
Segment Reporting Information [Line Items]
Gross premiums written	$ 1,295,136	[1]	$ 1,163,699	[1]
Premiums ceded	(230,146)		(210,923)
Net premiums written	1,064,990		952,776
Change in unearned premiums	(205,210)		(200,006)
Net premiums earned	859,780		752,770
Other underwriting income	1,582		538
Losses and loss adjustment expenses	(436,240)		(399,403)
Acquisition expenses, net	(160,342)		(127,592)
Other operating expenses	(132,324)		(109,915)
Underwriting income (loss)	132,456		116,398
Net investment income	66,994		65,672
Net realized gains	19,697		58,340
Net impairment losses recognized in earnings	(2,971)		(2,246)
Equity in net income (loss) of investment funds accounted for using the equity method	3,253		13,823
Other income (loss)	(2,104)		1,244
Other expenses	(13,475)		(10,268)
Interest expense	(14,404)		(5,898)
Net foreign exchange gains (losses)	(6,563)		24,264
Income before income taxes	182,883		261,329
Income tax expense (benefit)	(3,738)		(4,853)
Net income	179,145		256,476
Net (income) loss attributable to noncontrolling interests	3,355	[2]	0	[2]
Net income available to Arch	182,500		256,476
Preferred dividends	(5,484)		(5,484)
Net income available to Arch common shareholders	177,016		250,992
Underwriting Ratios
Loss ratio	50.70%		53.10%
Acquisition expense ratio	18.60%		16.90%
Other operating expense ratio	15.40%	[3]	14.60%	[3]
Combined ratio	84.70%		84.60%
Investable assets	15,344,386		13,132,116
Total assets	21,800,358		18,757,061		19,566,094
Total liabilities	14,895,495		13,421,531		13,918,598
Underwriting segments
Segment Reporting Information [Line Items]
Gross premiums written	1,295,136		1,163,699
Premiums ceded	(262,340)		(210,923)
Net premiums written	1,032,796		952,776
Change in unearned premiums	(175,182)		(200,006)
Net premiums earned	857,614		752,770
Other underwriting income	1,582		538
Losses and loss adjustment expenses	(434,884)		(399,403)
Acquisition expenses, net	(159,519)		(127,592)
Other operating expenses	(131,215)		(109,915)
Underwriting income (loss)	133,578		116,398
Net investment income	66,993		65,672
Net realized gains	19,697		58,340
Net impairment losses recognized in earnings	(2,971)		(2,246)
Equity in net income (loss) of investment funds accounted for using the equity method	3,253		13,823
Other income (loss)	(2,104)		1,244
Other expenses	(10,799)		(10,268)
Interest expense	(14,404)		(5,898)
Net foreign exchange gains (losses)	(6,656)		24,264
Income before income taxes	186,587		261,329
Income tax expense (benefit)	(3,738)		(4,853)
Net income	182,849		256,476
Net (income) loss attributable to noncontrolling interests	0		0
Net income available to Arch	182,849		256,476
Preferred dividends	(5,484)		(5,484)
Net income available to Arch common shareholders	177,365		250,992
Underwriting Ratios
Loss ratio	50.70%		53.10%
Acquisition expense ratio	18.50%		16.90%
Other operating expense ratio	15.30%		14.60%
Combined ratio	84.50%		84.60%
Investable assets	14,261,106		13,132,116
Total assets	20,653,897		18,757,061
Total liabilities	14,860,124		13,421,531
Underwriting segments | Insurance
Segment Reporting Information [Line Items]
Gross premiums written	730,646	[1]	688,817	[1]
Premiums ceded	(185,044)		(184,267)
Net premiums written	545,602		504,550
Change in unearned premiums	(68,101)		(59,585)
Net premiums earned	477,501		444,965
Other underwriting income	500		525
Losses and loss adjustment expenses	(286,770)		(283,467)
Acquisition expenses, net	(76,932)		(70,758)
Other operating expenses	(81,144)		(76,315)
Underwriting income (loss)	33,155		14,950
Underwriting Ratios
Loss ratio	60.10%		63.70%
Acquisition expense ratio	16.00%		15.80%
Other operating expense ratio	17.00%	[3]	17.20%	[3]
Combined ratio	93.10%		96.70%
Underwriting segments | Reinsurance
Segment Reporting Information [Line Items]
Gross premiums written	517,053	[1]	450,447	[1]
Premiums ceded	(73,127)		(27,979)
Net premiums written	443,926		422,468
Change in unearned premiums	(102,578)		(126,315)
Net premiums earned	341,348		296,153
Other underwriting income	316		13
Losses and loss adjustment expenses	(139,636)		(113,857)
Acquisition expenses, net	(73,433)		(53,143)
Other operating expenses	(36,195)		(32,197)
Underwriting income (loss)	92,400		96,969
Underwriting Ratios
Loss ratio	40.90%		38.40%
Acquisition expense ratio	21.50%		17.90%
Other operating expense ratio	10.60%	[3]	10.90%	[3]
Combined ratio	73.00%		67.20%
Underwriting segments | Mortgage
Segment Reporting Information [Line Items]
Gross premiums written	47,907		25,758
Premiums ceded	(4,639)		0
Net premiums written	43,268		25,758
Change in unearned premiums	(4,503)		(14,106)
Net premiums earned	38,765		11,652
Other underwriting income	766		0
Losses and loss adjustment expenses	(8,478)		(2,079)
Acquisition expenses, net	(9,154)		(3,691)
Other operating expenses	(13,876)		(1,403)
Underwriting income (loss)	8,023		4,479
Underwriting Ratios
Loss ratio	21.90%		17.80%
Acquisition expense ratio	23.60%		31.70%
Other operating expense ratio	35.80%		12.00%
Combined ratio	81.30%		61.50%
Other
Segment Reporting Information [Line Items]
Gross premiums written	32,194	[1]	0	[1]
Premiums ceded	0		0
Net premiums written	32,194		0
Change in unearned premiums	(30,028)		0
Net premiums earned	2,166		0
Other underwriting income	0		0
Losses and loss adjustment expenses	(1,356)		0
Acquisition expenses, net	(823)		0
Other operating expenses	(1,109)		0
Underwriting income (loss)	(1,122)		0
Net investment income	1		0
Net realized gains	0		0
Net impairment losses recognized in earnings	0		0
Equity in net income (loss) of investment funds accounted for using the equity method	0		0
Other income (loss)	0		0
Other expenses	(2,676)		0
Interest expense	0		0
Net foreign exchange gains (losses)	93		0
Income before income taxes	(3,704)		0
Income tax expense (benefit)	0		0
Net income	(3,704)		0
Net (income) loss attributable to noncontrolling interests	3,355		0
Net income available to Arch	(349)		0
Preferred dividends	0		0
Net income available to Arch common shareholders	(349)		0
Underwriting Ratios
Loss ratio	62.60%		0.00%
Acquisition expense ratio	38.00%		0.00%
Other operating expense ratio	51.20%	[3]	0.00%	[3]
Combined ratio	151.80%		0.00%
Investable assets	1,083,280		0
Total assets	1,146,461		0
Total liabilities	$ 35,371		$ 0

[1] 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.

[2] See Note 4.

[3] The acquisition expense ratio is adjusted to include certain other underwriting income.