Guarantor Financial Information - Condensed consolidating statement of cash flows (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Condensed Cash Flow Statements, Captions [Line Items]
Net Cash Provided by (Used in) Operating Activities	$ 198,169		$ 205,659
Purchases of fixed maturity investments	(7,131,071)		(3,970,320)
Purchases of equity securities	(89,227)		(63,353)
Purchases of other investments	(304,454)		(250,442)
Proceeds from the sale of fixed maturity investments	7,014,281		3,796,638
Proceeds from the sale of equity securities	49,614		81,513
Proceeds from the sale of other investments	331,176		280,010
Proceeds from redemptions and maturities of fixed maturity investments	168,484		181,727
Net (purchases) sales of short-term investments	156,262		(221,444)
Change in investment of securities lending collateral	4,669		(41,019)
Contributions to subsidiaries	0		0
Intercompany loans issued	0		0
Purchase of business, net of cash acquired	(235,578)	[1]	0	[1]
Purchases of furniture, equipment and other assets	(5,382)		(3,742)
Net Cash Used For Investing Activities	(41,226)		(210,432)
Purchases of common shares under share repurchase program	0		(40,964)
Proceeds from common shares issued, net	3,021		1,280
Proceeds from intercompany borrowings			0
Repayments of intercompany borrowings	0
Change in securities lending collateral	(4,669)		41,019
Third party investment in non-redeemable noncontrolling interests	796,903	[2]	0	[2]
Third party investment in redeemable noncontrolling interests	186,893	[2]	0	[2]
Dividends paid to parent	0		0
Other	1,700		1,084
Preferred dividends paid	(5,484)		(5,484)
Net Cash Provided By (Used For) Financing Activities	978,364		(3,065)
Effects of exchange rate changes on foreign currency cash	241		(6,436)
Cash and Cash Equivalents, Period Increase (Decrease)	1,135,548		(14,274)
Cash	1,569,605		356,767
Reportable Legal Entities | ACGL (Parent Guarantor)
Condensed Cash Flow Statements, Captions [Line Items]
Net Cash Provided by (Used in) Operating Activities	11,685		34,432
Purchases of fixed maturity investments	0		0
Purchases of equity securities	0		0
Purchases of other investments	0		0
Proceeds from the sale of fixed maturity investments	0		0
Proceeds from the sale of equity securities	0		0
Proceeds from the sale of other investments	0		0
Proceeds from redemptions and maturities of fixed maturity investments	0		0
Net (purchases) sales of short-term investments	(16)		7,964
Change in investment of securities lending collateral	0		0
Contributions to subsidiaries	0		0
Intercompany loans issued	0		0
Purchase of business, net of cash acquired	0
Purchases of furniture, equipment and other assets	(76)		(31)
Net Cash Used For Investing Activities	(92)		7,933
Purchases of common shares under share repurchase program			(40,964)
Proceeds from common shares issued, net	3,021		1,280
Proceeds from intercompany borrowings			0
Repayments of intercompany borrowings	0
Change in securities lending collateral	0		0
Third party investment in non-redeemable noncontrolling interests	0	[2]
Third party investment in redeemable noncontrolling interests	0	[2]
Dividends paid to parent	0		0
Other	0		0
Preferred dividends paid	(5,484)		(5,484)
Net Cash Provided By (Used For) Financing Activities	(2,463)		(45,168)
Effects of exchange rate changes on foreign currency cash	0		0
Cash and Cash Equivalents, Period Increase (Decrease)	9,130		(2,803)
Cash	12,353		3,614
Reportable Legal Entities | Arch-U.S. (Subsidiary Issuer)
Condensed Cash Flow Statements, Captions [Line Items]
Net Cash Provided by (Used in) Operating Activities	7,151		(254)
Purchases of fixed maturity investments	0		0
Purchases of equity securities	0		0
Purchases of other investments	0		0
Proceeds from the sale of fixed maturity investments	0		0
Proceeds from the sale of equity securities	0		0
Proceeds from the sale of other investments	0		0
Proceeds from redemptions and maturities of fixed maturity investments	0		0
Net (purchases) sales of short-term investments	360,269		(2)
Change in investment of securities lending collateral	0		0
Contributions to subsidiaries	(312,207)		(10,250)
Intercompany loans issued	0		0
Purchase of business, net of cash acquired	0
Purchases of furniture, equipment and other assets	0		0
Net Cash Used For Investing Activities	48,062		(10,252)
Purchases of common shares under share repurchase program			0
Proceeds from common shares issued, net	0		0
Proceeds from intercompany borrowings			10,250
Repayments of intercompany borrowings	(10,250)
Change in securities lending collateral	0		0
Third party investment in non-redeemable noncontrolling interests	0	[2]
Third party investment in redeemable noncontrolling interests	0	[2]
Dividends paid to parent	0		0
Other	0		0
Preferred dividends paid	0		0
Net Cash Provided By (Used For) Financing Activities	(10,250)		10,250
Effects of exchange rate changes on foreign currency cash	0		0
Cash and Cash Equivalents, Period Increase (Decrease)	44,963		(256)
Cash	45,472		356
Reportable Legal Entities | Other ACGL Subsidiaries
Condensed Cash Flow Statements, Captions [Line Items]
Net Cash Provided by (Used in) Operating Activities	194,833		216,481
Purchases of fixed maturity investments	(7,131,071)		(3,970,320)
Purchases of equity securities	(89,227)		(63,353)
Purchases of other investments	(304,454)		(250,442)
Proceeds from the sale of fixed maturity investments	7,014,281		3,796,638
Proceeds from the sale of equity securities	49,614		81,513
Proceeds from the sale of other investments	331,176		280,010
Proceeds from redemptions and maturities of fixed maturity investments	168,484		181,727
Net (purchases) sales of short-term investments	(203,991)		(229,406)
Change in investment of securities lending collateral	4,669		(41,019)
Contributions to subsidiaries	(100,000)		(250)
Intercompany loans issued	10,250		(10,250)
Purchase of business, net of cash acquired	(235,578)
Purchases of furniture, equipment and other assets	(5,306)		(3,711)
Net Cash Used For Investing Activities	(491,153)		(228,863)
Purchases of common shares under share repurchase program			0
Proceeds from common shares issued, net	412,207		10,500
Proceeds from intercompany borrowings			0
Repayments of intercompany borrowings	0
Change in securities lending collateral	(4,669)		41,019
Third party investment in non-redeemable noncontrolling interests	796,903	[2]
Third party investment in redeemable noncontrolling interests	186,893	[2]
Dividends paid to parent	(15,500)		(45,000)
Other	1,700		1,084
Preferred dividends paid	0		0
Net Cash Provided By (Used For) Financing Activities	1,377,534		7,603
Effects of exchange rate changes on foreign currency cash	241		(6,436)
Cash and Cash Equivalents, Period Increase (Decrease)	1,081,455		(11,215)
Cash	1,511,780		352,797
Consolidating Adjustments and Eliminations
Condensed Cash Flow Statements, Captions [Line Items]
Net Cash Provided by (Used in) Operating Activities	(15,500)		(45,000)
Purchases of fixed maturity investments	0		0
Purchases of equity securities	0		0
Purchases of other investments	0		0
Proceeds from the sale of fixed maturity investments	0		0
Proceeds from the sale of equity securities	0		0
Proceeds from the sale of other investments	0		0
Proceeds from redemptions and maturities of fixed maturity investments	0		0
Net (purchases) sales of short-term investments	0		0
Change in investment of securities lending collateral	0		0
Contributions to subsidiaries	412,207		10,500
Intercompany loans issued	(10,250)		10,250
Purchase of business, net of cash acquired	0
Purchases of furniture, equipment and other assets	0		0
Net Cash Used For Investing Activities	401,957		20,750
Purchases of common shares under share repurchase program			0
Proceeds from common shares issued, net	(412,207)		(10,500)
Proceeds from intercompany borrowings			(10,250)
Repayments of intercompany borrowings	10,250
Change in securities lending collateral	0		0
Third party investment in non-redeemable noncontrolling interests	0	[2]
Third party investment in redeemable noncontrolling interests	0	[2]
Dividends paid to parent	15,500		45,000
Other	0		0
Preferred dividends paid	0		0
Net Cash Provided By (Used For) Financing Activities	(386,457)		24,250
Effects of exchange rate changes on foreign currency cash	0		0
Cash and Cash Equivalents, Period Increase (Decrease)	0		0
Cash	$ 0		$ 0

[1] See Note 2.

[2] See Note 4.