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Share-based Compensation - Summary of RSU Activity (Parenthetical) (Detail) | 9 Months Ended
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Sep. 30, 2021
Restricted Stock Units (RSUs) [Member] | Long Term Incentive Plan [Member] |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |
The target percentage of RSUs earned and vested | 105.60%

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- Definition The performance percentage of share-based awards earned during the period for share based compensation other than options. + References No definition available. + Details
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Namespace Prefix: | rm_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

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

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

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- Details
Name: | us-gaap_PlanNameAxis=rm_LongTermIncentivePlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |