Average number of quotes obtained per investment from pricing services in determining the fair values used.

Name:	acgl_AverageQuotesPerInvestment
Namespace Prefix:	acgl
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

- Definition

This item represents the amount of the total realized and unrealized gains or losses for the period which are included in the statement of income (or changes in net assets) in net realized gains (losses); the fair value of which assets was or is measured on a recurring basis using significant unobservable inputs (Level 3).

+ Details

Name:	acgl_FairValueAssetsMeasuredOnRecurringBasisGainLossIncludedInNetRealizedGainsOrLosses
Namespace Prefix:	acgl
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	acgl_FairValueDisclosuresTextItemsAbstract
Namespace Prefix:	acgl
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	acgl_FairValueTransfersBetweenLevel1AndLevel2Abstract
Namespace Prefix:	acgl
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the percentage of aggregate assets and liabilities reported on the balance sheet at period end measured at fair value which were priced using non-binding broker quotes.

+ Details

Name:	acgl_PercentageOfTotalAssetsAndLiabilitiesMeasuredAtFairValuePricedUsingNonBindingBrokerQuotes
Namespace Prefix:	acgl
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the aggregate of the assets and liabilities reported on the balance sheet at period end measured at fair value.

+ Details

Name:	acgl_TotalAssetsAndLiabilitiesMeasuredAtFairValue
Namespace Prefix:	acgl
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the aggregate of the assets and liabilities reported on the balance sheet at period end measured at fair value which were priced using non-binding broker quotes.

+ Details

Name:	acgl_TotalAssetsAndLiabilitiesMeasuredAtFairValuePricedUsingNonBindingBrokerQuotes
Namespace Prefix:	acgl
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6947722&loc=d3e13433-108611

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 12

-URI http://asc.fasb.org/extlink&oid=6947722&loc=d3e13476-108611

+ Details

Name:	us-gaap_DebtInstrumentFairValue
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ Details

Name:	us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems
Namespace Prefix:	us-gaap
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of the transfers of assets or liabilities measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (bb)

-URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueLevel2ToLevel1TransfersAmount
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer), excluding current portion. Senior note holders are paid off in full before any payments are made to junior note holders.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_SeniorLongTermNotes
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Fair Value (Details - Text Items) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
Fair Value (Details - Text Items) (USD $)	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Fair Value Disclosures Text Items [Abstract]
Average quotes per investment			2.8		2.7
Total assets and liabilities measured at fair value	$ 11,980,000,000		$ 11,980,000,000		$ 11,520,000,000
Total assets and liabilities measured at fair value priced using non binding broker quotes	1,590,000,000		1,590,000,000		1,810,000,000
Percentage of total assets and liabilities measured at fair value priced using non-binding broker quotes			13.30%		15.70%
Fair value, assets measured on recurring basis, gain (loss) included in net realized gains (losses)	5,700,000	4,800,000	1,900,000	10,200,000
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying value of senior notes	300,000,000		300,000,000		300,000,000
Fair Value Transfers Between Level 1 And Level 2 [Abstract]
Fair value, level 2 to level 1 transfers, amount					1,090,000,000
Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of senior notes	$ 343,600,000		$ 343,600,000		$ 310,900,000