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Leases - Additional Information (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Leases [Abstract] |  |  |
Operating lease expense | $ 6.1 | $ 5.5 | $ 5.1
Additional operating lease not yet commenced | $ 0.9 |  |

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- Definition Lessee, Operating Lease, Lease Not Yet Commenced, Amount + References No definition available. + Details
Name: | alg_LesseeOperatingLeaseLeaseNotYetCommencedAmount
Namespace Prefix: | alg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_LeasesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of operating lease expense. Excludes sublease income. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 45 -Paragraph 4 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121603541&loc=SL77918638-209977 + Details
Name: | us-gaap_OperatingLeaseExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration