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Common Stock (Details) (USD $)	3 Months Ended								12 Months Ended			0 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 30, 2015	Jan. 02, 2015	Sep. 25, 2014	Sep. 24, 2014
Class of Stock [Line Items]
Authorized shares to be repurchased	1,000,000us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased								1,000,000us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased
Shares previously purchased									42,600us-gaap_TreasuryStockSharesAcquired
Dividends declared (in dollars per share)	$ 0.07us-gaap_CommonStockDividendsPerShareDeclared	$ 0.07us-gaap_CommonStockDividendsPerShareDeclared	$ 0.07us-gaap_CommonStockDividendsPerShareDeclared	$ 0.07us-gaap_CommonStockDividendsPerShareDeclared	$ 0.07us-gaap_CommonStockDividendsPerShareDeclared	$ 0.07us-gaap_CommonStockDividendsPerShareDeclared	$ 0.07us-gaap_CommonStockDividendsPerShareDeclared	$ 0.07us-gaap_CommonStockDividendsPerShareDeclared	$ 0.28us-gaap_CommonStockDividendsPerShareDeclared	$ 0.28us-gaap_CommonStockDividendsPerShareDeclared	$ 0.24us-gaap_CommonStockDividendsPerShareDeclared
Subsequent Event
Class of Stock [Line Items]
Dividends declared (in dollars per share)													$ 0.08us-gaap_CommonStockDividendsPerShareDeclared / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Dividends paid (in dollars per share)												$ 0.08us-gaap_CommonStockDividendsPerShareCashPaid / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Share Repurchase Agreement with Capital Southwest Corporation and Capital Southwest Venture Corporation
Class of Stock [Line Items]
Authorized shares to be repurchased															849,690us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased / us-gaap_ShareRepurchaseProgramAxis = alg_ShareRepurchaseAgreementwithCapitalSouthwestCorporationandCapitalSouthwestVentureCorporationMember
Purchase price per share															$ 40.255us-gaap_TreasuryStockAcquiredAverageCostPerShare / us-gaap_ShareRepurchaseProgramAxis = alg_ShareRepurchaseAgreementwithCapitalSouthwestCorporationandCapitalSouthwestVentureCorporationMember
Share Price (in dollars per share)															$ 41.50us-gaap_SharePrice / us-gaap_ShareRepurchaseProgramAxis = alg_ShareRepurchaseAgreementwithCapitalSouthwestCorporationandCapitalSouthwestVentureCorporationMember
Retirement of shares (shares)														849,690us-gaap_TreasuryStockSharesRetired / us-gaap_ShareRepurchaseProgramAxis = alg_ShareRepurchaseAgreementwithCapitalSouthwestCorporationandCapitalSouthwestVentureCorporationMember

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- Definition

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

+ 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 S99

-Paragraph 1

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

+ 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 S99

-Paragraph 1

-Subparagraph (SX 210.3-04)

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Price of a single share of a number of saleable stocks of a company.

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- Definition

The number of shares authorized to be repurchased by an entity's Board of Directors under a stock repurchase plan.

+ References

No definition available.

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- Definition

Total cost of shares repurchased divided by the total number of shares repurchased.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 30

-Section 45

-Paragraph 1

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

+ 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

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-Number 210

-Section 02

-Paragraph 29, 30

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Number of shares of common and preferred stock retired from treasury during the period.

+ 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

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