Common stock dividend rate percentage.

No definition available.

Name:	agii_CommonStockDividendRatePercentage
Namespace Prefix:	agii_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Treasury shares purchased on the open market during the period.

+ References

No definition available.

+ Details

Name:	agii_StockRepurchasedDuringPeriodOnOpenMarket
Namespace Prefix:	agii_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Equity impact of the cost of repurchasing common stock on the on the open market during the period. Recorded using the cost method.

+ References

No definition available.

+ Details

Name:	agii_StockRepurchasedDuringPeriodOnOpenMarketValue
Namespace Prefix:	agii_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_ClassOfStockLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Aggregate dividends declared during the period for each share of common stock outstanding.

+ References

+ Details

Name:	us-gaap_CommonStockDividendsPerShareDeclared
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of shares of common stock issued as dividends during the period. Excludes stock splits.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644

+ Details

Name:	us-gaap_CommonStockDividendsShares
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Date the declared dividend will be paid, in CCYY-MM-DD format.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586

+ Details

Name:	us-gaap_DividendPayableDateToBePaidDayMonthAndYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of paid and unpaid common stock dividends declared with the form of settlement in cash.

+ References

+ Details

Name:	us-gaap_DividendsCommonStockCash
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Date the dividend to be paid was declared, in CCYY-MM-DD format.

+ References

+ Details

Name:	us-gaap_DividendsPayableDateDeclaredDayMonthAndYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Date the holder must own the stock to be entitled to the dividend, in CCYY-MM-DD format.

+ References

No definition available.

+ Details

Name:	us-gaap_DividendsPayableDateOfRecordDayMonthAndYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Equity impact of the value of stock that has been repurchased during the period and has not been retired and is not held in treasury. Some state laws may mandate the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock.

+ References

+ Details

Name:	us-gaap_StockRepurchasedDuringPeriodValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Number of shares that have been repurchased during the period and are being held in treasury.

+ References

+ Details

Name:	us-gaap_TreasuryStockSharesAcquired
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Equity impact of the cost of common and preferred stock that were repurchased during the period. Recorded using the cost method.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.3-04)

-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 30

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6405813&loc=d3e23239-112655

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_DividendsAxis=agii_StockDividendMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
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Period Type:

- Details

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Shareholders Equity - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	0 Months Ended				3 Months Ended		0 Months Ended
	Mar. 19, 2015	Feb. 17, 2015	Mar. 17, 2014	Feb. 18, 2014	Mar. 31, 2015	Mar. 31, 2014	Nov. 05, 2013	May 05, 2015
Class Of Stock [Line Items]
Dividend declared per common share		$ 0.20		$ 0.15	$ 0.20	$ 0.14
Dividends payable date		Mar. 16, 2015		Mar. 17, 2014
Dividend payable, date of record		Mar. 02, 2015		Mar. 03, 2014
Stock dividend declared		10.00%agii_CommonStockDividendRatePercentage		10.00%agii_CommonStockDividendRatePercentage	10.00%agii_CommonStockDividendRatePercentage
Dividends declaration date		Feb. 17, 2015		Feb. 18, 2014
Additional stock issued as dividend		2,554,506us-gaap_CommonStockDividendsShares
Cash dividends paid - common shares, total	$ 5.7us-gaap_DividendsCommonStockCash		$ 4.0us-gaap_DividendsCommonStockCash
2013 Repurchase Authorization [Member]
Class Of Stock [Line Items]
Total number of shares authorized for purchase, Value					74.7		150.0
Common shares repurchased on open market					353,054
Common shares repurchased on open market, total cost					18.1
Common shares purchased					196,864
Common shares purchased, total cost					$ 9.9
10% Stock Dividend [Member]
Class Of Stock [Line Items]
Dividend declared per common share				$ 0.14
Subsequent Event [Member]
Class Of Stock [Line Items]
Dividend declared per common share								$ 0.20
Dividends payable date								Jun. 15, 2015
Dividend payable, date of record								Jun. 01, 2015