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Insurance - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jan. 31, 2020 | Apr. 30, 2020 | Apr. 30, 2019
Schedule of Other Liabilities [Line Items] |  |  |
Insurance policy coverage, general and automobile liability losses |  | $ 200.0 |
Insurance policy coverage, property damage |  | 75.0 |
Decrease in total reserve claims | $ 6.6 | 6.6 |
Change in reserves for insurance claims |  |  | $ 5.0
Minimum |  |  |
Schedule of Other Liabilities [Line Items] |  |  |
Self insurance retention amount per-claim |  | 1.0 |
Primary policy limit |  | 1.0 |
Maximum |  |  |
Schedule of Other Liabilities [Line Items] |  |  |
Self insurance retention amount per-claim |  | 1.5 |
Primary policy limit |  | 5.0 |
Self insurance retention amount per-claim, medical plan |  | $ 0.5 |

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- Definition Decrease In Total Reserve Claims + References No definition available. + Details
Name: | abm_DecreaseInTotalReserveClaims
Namespace Prefix: | abm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Increase in Total Reserve Claims + References No definition available. + Details
Name: | abm_IncreaseinTotalReserveClaims
Namespace Prefix: | abm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Represents commercial insurance umbrella policies that cover general liability and automobile liability losses above primary limits. + References No definition available. + Details
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Namespace Prefix: | abm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Represents the policy limit provided by the insurance program for property damage risks. + References No definition available. + Details
Name: | abm_PropertyDamagePolicyLimit
Namespace Prefix: | abm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Schedule of Other Liabilities [Line Items] + References No definition available. + Details
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Namespace Prefix: | abm_
Data Type: | xbrli:stringItemType
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- Definition Self Insured Retention Amount Per Occurrence + References No definition available. + Details
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Namespace Prefix: | abm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition Self Insured Retention Amount Per Participant Per Year For Certain Employee Medical And Dental Programs + References No definition available. + Details
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Namespace Prefix: | abm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Represents the policy limit provided by the insurance program for workers' compensation, general liability, and other insurable risks. + References No definition available. + Details
Name: | abm_WorkersCompensationGeneralLiabilityandOtherInsurableRisksInsurancePolicyLimit
Namespace Prefix: | abm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Period Type: |