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Summary of significant accounting policies - Product Warranties (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
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Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020
Summary of significant accounting policies |  |  |  |
Balance, beginning of the period | $ 612 | $ 757 | $ 637 | $ 848
Warranty repairs | 0 | (3) | (25) | (94)
Balance, end of the year | $ 612 | $ 754 | $ 612 | $ 754
Warranty duration |  |  | 1 year |

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- Definition The duration of a warranty. + References No definition available. + Details
Name: | rmb_WarrantyDuration
Namespace Prefix: | rmb_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- 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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- Definition Amount as of the balance sheet date of the aggregate standard product warranty liability. Does not include the balance for the extended product warranty liability. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (c)(1),(c)(5) -URI http://asc.fasb.org/extlink&oid=123368208&loc=d3e12565-110249 + Details
Name: | us-gaap_StandardProductWarrantyAccrual
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Amount of decrease in the standard product warranty accrual from payments made in cash or in kind to satisfy claims under the terms of the standard product warranty. Excludes extended product warranties. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph ((c)(2)) -URI http://asc.fasb.org/extlink&oid=123368208&loc=d3e12565-110249 + Details
Name: | us-gaap_StandardProductWarrantyAccrualPayments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration