Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5047-113901

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
-URI http://asc.fasb.org/extlink&oid=122041274&loc=d3e301413-122809

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

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

- Definition

Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

+ References

+ Details

Name:	us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of cash inflow from exercise of option under share-based payment arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=SL79508275-113901

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3255-108585

+ Details

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

- Definition

Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted average remaining contractual term for equity-based awards excluding options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- 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.

+ References

No definition available.

+ Details

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

- Definition

Number of shares authorized for issuance under share-based payment arrangement.

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Gross number of share options (or share units) granted during the period.

+ References

No definition available.

+ Details

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

- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of vesting of award under share-based payment arrangement.

+ References

+ Details

Name:	us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of share options (or share units) exercised during the current period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(2)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-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=120397183&loc=d3e187085-122770

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(28))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=123467817&loc=d3e21463-112644

+ Details

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

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=alg_QualifiedStockOptionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=alg_NonQualifiedOptionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockUnitsRSUMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardDateAxis=alg_AwardDateOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardDateAxis=alg_AwardDateTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=us-gaap_ShareBasedCompensationAwardTrancheOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=us-gaap_ShareBasedCompensationAwardTrancheTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=us-gaap_ShareBasedCompensationAwardTrancheThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=alg_SharebasedCompensationAwardTrancheFourMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_PerformanceSharesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=alg_IncentiveStockOption2015PlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=alg_SharebasedCompensationAwardTrancheFiveMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=alg_TwoZeroZeroNineEquityIncentivePlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Stock Options - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended
	May 02, 2019	May 07, 2015	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option term (years)			10 years
Qualified Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value of options granted (in dollars per share)			$ 59.01	$ 38.33	$ 39.85
Stock option expense			$ 600	$ 500	$ 500
Total unrecognized compensation cost related to non-vested share-based compensation arrangements			$ 1,200
Period cost is expected to be recognized (years)			5 years
Granted (in shares)			18,900	13,950	12,150
Exercise of stock options (in shares)			30,210	31,300	28,100
Non Qualified Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option expense			$ 0	$ 0	$ 10
Total unrecognized compensation cost related to non-vested share-based compensation arrangements			$ 0
Granted (in shares)			0	0	0
Exercise of stock options (in shares)			1,000	2,300	34,900
Proceeds from stock options exercised			$ 100	$ 100	$ 1,500
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option expense			5,400	$ 3,700	$ 2,800
Total unrecognized compensation cost related to non-vested share-based compensation arrangements			$ 6,300
Weighted average remaining contractual life (years)			1 year 7 months 9 days	1 year 11 months 12 days	2 years 5 months 23 days
Restricted Stock Units (RSUs) \| Granted prior to 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Period cost is expected to be recognized (years)			4 years
Restricted Stock Units (RSUs) \| Granted after 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (years)			3 years
Restricted Stock Units (RSUs) \| Year One after Grant Date
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual vesting percentage of award				25.00%	25.00%
Award vesting period (years)			3 years	1 year	1 year
Restricted Stock Units (RSUs) \| Year Two after Grant Date
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual vesting percentage of award				25.00%	25.00%
Restricted Stock Units (RSUs) \| Year Three after Grant Date
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual vesting percentage of award				25.00%	25.00%
Restricted Stock Units (RSUs) \| Year Four after Grant Date
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual vesting percentage of award				25.00%	25.00%
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual vesting percentage of award			50.00%
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual vesting percentage of award			50.00%
Performance period			3 years
Option term (years)			3 years
2015 ISO Plan \| Qualified Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock reserved for options to be issued (in shares)		400,000
2015 ISO Plan \| Qualified Stock Options \| Year One after Grant Date
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (years)		1 year
2015 ISO Plan \| Qualified Stock Options \| Year One after Grant Date \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual vesting percentage of award		20.00%
2015 ISO Plan \| Qualified Stock Options \| Year Two after Grant Date
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual vesting percentage of award		20.00%
2015 ISO Plan \| Qualified Stock Options \| Year Three after Grant Date
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual vesting percentage of award		20.00%
2015 ISO Plan \| Qualified Stock Options \| Year Four after Grant Date
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual vesting percentage of award		20.00%
2015 ISO Plan \| Qualified Stock Options \| Year Five after Grant Date
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual vesting percentage of award		20.00%
2009 Equity Incentive Plan \| Non Qualified Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock reserved for options to be issued (in shares)	500,000
2009 Equity Incentive Plan \| Non Qualified Options \| Year One after Grant Date
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (years)	1 year
2009 Equity Incentive Plan \| Non Qualified Options \| Year One after Grant Date \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual vesting percentage of award	20.00%
2009 Equity Incentive Plan \| Non Qualified Options \| Year Two after Grant Date
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual vesting percentage of award	20.00%
2009 Equity Incentive Plan \| Non Qualified Options \| Year Three after Grant Date
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual vesting percentage of award	20.00%
2009 Equity Incentive Plan \| Non Qualified Options \| Year Four after Grant Date
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual vesting percentage of award	20.00%
2009 Equity Incentive Plan \| Non Qualified Options \| Year Five after Grant Date
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual vesting percentage of award	20.00%