Guarantor Financial Information - Condensed consolidating statement of cash flows (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Operating Activities
Net Cash Provided by (Used in) Operating Activities	$ 1,037,130		$ 850,868		$ 921,603
Investing Activities
Purchases of fixed maturity investments	(28,745,279)		(18,174,988)		(17,568,592)
Purchases of equity securities	(520,817)		(535,857)		(268,999)
Purchases of other investments	(2,257,481)		(1,326,729)		(1,000,049)
Proceeds from the sale of fixed maturity investments	26,823,192		17,196,614		16,366,306
Proceeds from the sale of equity securities	411,362		462,787		313,617
Proceeds from sales, redemptions and maturities of other investments	1,643,000		1,162,707		443,630
Proceeds from redemptions and maturities of fixed maturity investments	762,995		731,708		1,115,594
Net (purchases) sales of short-term investments	577,126		(750,613)		185,919
Change in investment of securities lending collateral	57,470		(55,643)		6,190
Contributions to subsidiaries	0		0		0
Intercompany loans issued	0		0
Repayments of intercompany loans	0
Purchase of business, net of cash acquired	(237,106)		0		28,948
Purchases of furniture, equipment and other assets	(19,883)		(17,499)		(18,532)
Net Cash Used For Investing Activities	(1,505,421)		(1,307,513)		(395,968)
Financing Activities
Proceeds from issuance of Series C preferred shares, net	0		0		315,763
Repurchase of Series A and B preferred shares	0		0		(325,000)
Purchases of common shares under share repurchase program	(454,137)		(57,796)		(172,056)
Proceeds from common shares issued, net	6,827		3,051		7,033
Proceeds from intercompany loans	0		0
Repayments of intercompany loans	0
Proceeds from borrowings	0		494,228		0
Repayments of borrowings	0		0		(310,868)
Change in securities lending collateral	(57,470)		55,643		(6,190)
Third party investment in non-redeemable noncontrolling interests	796,903		0		0
Third party investment in redeemable noncontrolling interests	219,233		0		0
Dividends paid to redeemable noncontrolling interests	(14,448)		0		0
Dividends paid to parent	0	[1]	0	[1]	0	[1]
Other	64,973		50,830		6,664
Preferred dividends paid	(21,938)		(21,938)		(28,381)
Net Cash Used For Financing Activities	539,943		524,018		(513,035)
Effects of exchange rate changes on foreign currency cash	(20,007)		(4,357)		6,742
Increase (decrease) in cash	51,645		63,016		19,342
Cash beginning of year	434,057		371,041		351,699
Cash end of period	485,702		434,057		371,041
Reportable legal entities | ACGL (Parent Guarantor)
Operating Activities
Net Cash Provided by (Used in) Operating Activities	467,091		68,562		209,500
Investing Activities
Purchases of fixed maturity investments	0		0		0
Purchases of equity securities	0		0		0
Purchases of other investments	0		0		0
Proceeds from the sale of fixed maturity investments	0		0		0
Proceeds from the sale of equity securities	0		0		0
Proceeds from sales, redemptions and maturities of other investments	0		0		0
Proceeds from redemptions and maturities of fixed maturity investments	0		0		0
Net (purchases) sales of short-term investments	2,423		5,799		(5,094)
Change in investment of securities lending collateral	0		0		0
Contributions to subsidiaries	0		(160)		0
Intercompany loans issued	0		0
Repayments of intercompany loans	0
Purchase of business, net of cash acquired	0				0
Purchases of furniture, equipment and other assets	(271)		(712)		(65)
Net Cash Used For Investing Activities	2,152		4,927		(5,159)
Financing Activities
Proceeds from issuance of Series C preferred shares, net					315,763
Repurchase of Series A and B preferred shares					(325,000)
Purchases of common shares under share repurchase program	(454,137)		(57,796)		(172,056)
Proceeds from common shares issued, net	6,827		3,051		7,033
Proceeds from intercompany loans	0		0
Repayments of intercompany loans	0
Proceeds from borrowings			0
Repayments of borrowings					0
Change in securities lending collateral	0		0		0
Third party investment in non-redeemable noncontrolling interests	0	[2]
Third party investment in redeemable noncontrolling interests	0	[2]
Dividends paid to redeemable noncontrolling interests	0	[2]
Dividends paid to parent	0	[1]	0	[1]	0	[1]
Other	0		0		0
Preferred dividends paid	(21,938)		(21,938)		(28,381)
Net Cash Used For Financing Activities	(469,248)		(76,683)		(202,641)
Effects of exchange rate changes on foreign currency cash	0		0		0
Increase (decrease) in cash	(5)		(3,194)		1,700
Cash beginning of year	3,223		6,417		4,717
Cash end of period	3,218		3,223		6,417
Reportable legal entities | Arch-U.S. (Subsidiary Issuer)
Operating Activities
Net Cash Provided by (Used in) Operating Activities	8,142		(6,569)		(8,086)
Investing Activities
Purchases of fixed maturity investments	(78,509)		0		0
Purchases of equity securities	0		0		0
Purchases of other investments	0		0		0
Proceeds from the sale of fixed maturity investments	16,011		0		0
Proceeds from the sale of equity securities	0		0		0
Proceeds from sales, redemptions and maturities of other investments	0		0		0
Proceeds from redemptions and maturities of fixed maturity investments	0		0		0
Net (purchases) sales of short-term investments	408,591		(400,162)		1,986
Change in investment of securities lending collateral	0		0		0
Contributions to subsidiaries	(341,707)		(97,850)		0
Intercompany loans issued	(7,500)		0
Repayments of intercompany loans	0
Purchase of business, net of cash acquired	0				0
Purchases of furniture, equipment and other assets	0		0		0
Net Cash Used For Investing Activities	(3,114)		(498,012)		1,986
Financing Activities
Proceeds from issuance of Series C preferred shares, net					0
Repurchase of Series A and B preferred shares					0
Purchases of common shares under share repurchase program	0		0		0
Proceeds from common shares issued, net	0		0		0
Proceeds from intercompany loans	7,500		10,250
Repayments of intercompany loans	(10,250)
Proceeds from borrowings			494,228
Repayments of borrowings					0
Change in securities lending collateral	0		0		0
Third party investment in non-redeemable noncontrolling interests	0	[2]
Third party investment in redeemable noncontrolling interests	0	[2]
Dividends paid to redeemable noncontrolling interests	0	[2]
Dividends paid to parent	0	[1]	0	[1]	0	[1]
Other	0		0		0
Preferred dividends paid	0		0		0
Net Cash Used For Financing Activities	(2,750)		504,478		0
Effects of exchange rate changes on foreign currency cash	0		0		0
Increase (decrease) in cash	2,278		(103)		(6,100)
Cash beginning of year	509		612		6,712
Cash end of period	2,787		509		612
Reportable legal entities | Other ACGL Subsidiaries
Operating Activities
Net Cash Provided by (Used in) Operating Activities	1,077,597		908,375		978,679
Investing Activities
Purchases of fixed maturity investments	(28,666,770)		(18,174,988)		(17,568,592)
Purchases of equity securities	(520,817)		(535,857)		(268,999)
Purchases of other investments	(2,257,481)		(1,326,729)		(1,000,049)
Proceeds from the sale of fixed maturity investments	26,807,181		17,196,614		16,366,306
Proceeds from the sale of equity securities	411,362		462,787		313,617
Proceeds from sales, redemptions and maturities of other investments	1,643,000		1,162,707		443,630
Proceeds from redemptions and maturities of fixed maturity investments	762,995		731,708		1,115,594
Net (purchases) sales of short-term investments	166,112		(356,250)		189,027
Change in investment of securities lending collateral	57,470		(55,643)		6,190
Contributions to subsidiaries	(128,825)		(20,250)		(38,576)
Intercompany loans issued	(7,500)		(10,250)
Repayments of intercompany loans	38,964
Purchase of business, net of cash acquired	(237,106)				28,948
Purchases of furniture, equipment and other assets	(19,612)		(16,787)		(18,467)
Net Cash Used For Investing Activities	(1,951,027)		(942,938)		(431,371)
Financing Activities
Proceeds from issuance of Series C preferred shares, net					0
Repurchase of Series A and B preferred shares					0
Purchases of common shares under share repurchase program	0		0		0
Proceeds from common shares issued, net	470,532		118,260		38,576
Proceeds from intercompany loans	7,500		0
Repayments of intercompany loans	(28,714)
Proceeds from borrowings			0
Repayments of borrowings					(310,868)
Change in securities lending collateral	(57,470)		55,643		(6,190)
Third party investment in non-redeemable noncontrolling interests	796,903	[2]
Third party investment in redeemable noncontrolling interests	219,233	[2]
Dividends paid to redeemable noncontrolling interests	(14,448)	[2]
Dividends paid to parent	(515,700)	[1]	(119,500)	[1]	(258,490)	[1]
Other	64,973		50,830		6,664
Preferred dividends paid	0		0		0
Net Cash Used For Financing Activities	942,809		105,233		(530,308)
Effects of exchange rate changes on foreign currency cash	(20,007)		(4,357)		6,742
Increase (decrease) in cash	49,372		66,313		23,742
Cash beginning of year	430,325		364,012		340,270
Cash end of period	479,697		430,325		364,012
Consolidating adjustments and eliminations
Operating Activities
Net Cash Provided by (Used in) Operating Activities	(515,700)		(119,500)		(258,490)
Investing Activities
Purchases of fixed maturity investments	0		0		0
Purchases of equity securities	0		0		0
Purchases of other investments	0		0		0
Proceeds from the sale of fixed maturity investments	0		0		0
Proceeds from the sale of equity securities	0		0		0
Proceeds from sales, redemptions and maturities of other investments	0		0		0
Proceeds from redemptions and maturities of fixed maturity investments	0		0		0
Net (purchases) sales of short-term investments	0		0		0
Change in investment of securities lending collateral	0		0		0
Contributions to subsidiaries	470,532		118,260		38,576
Intercompany loans issued	15,000		10,250
Repayments of intercompany loans	(38,964)
Purchase of business, net of cash acquired	0				0
Purchases of furniture, equipment and other assets	0		0		0
Net Cash Used For Investing Activities	446,568		128,510		38,576
Financing Activities
Proceeds from issuance of Series C preferred shares, net					0
Repurchase of Series A and B preferred shares					0
Purchases of common shares under share repurchase program	0		0		0
Proceeds from common shares issued, net	(470,532)		(118,260)		(38,576)
Proceeds from intercompany loans	(15,000)		(10,250)
Repayments of intercompany loans	38,964
Proceeds from borrowings			0
Repayments of borrowings					0
Change in securities lending collateral	0		0		0
Third party investment in non-redeemable noncontrolling interests	0	[2]
Third party investment in redeemable noncontrolling interests	0	[2]
Dividends paid to redeemable noncontrolling interests	0	[2]
Dividends paid to parent	515,700	[1]	119,500	[1]	258,490	[1]
Other	0		0		0
Preferred dividends paid	0		0		0
Net Cash Used For Financing Activities	69,132		(9,010)		219,914
Effects of exchange rate changes on foreign currency cash	0		0		0
Increase (decrease) in cash	0		0		0
Cash beginning of year	0		0		0
Cash end of period	$ 0		$ 0		$ 0

[1] Included in net cash provided by (used for) operating activities in the ACGL (Parent Guarantor) column.

[2] See Note 4.