Represent the amount of aggregate payment upon achievement of development and commercial milestones.

No definition available.

Name:	rytm_AggregatePaymentUponAchievementOfDevelopmentAndCommercialMilestones
Namespace Prefix:	rytm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the amount of milestone fee.

+ References

No definition available.

+ Details

Name:	rytm_MilestoneFee
Namespace Prefix:	rytm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the amount of milestone prepayment.

+ References

No definition available.

+ Details

Name:	rytm_MilestonePrepayment
Namespace Prefix:	rytm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of non-cash research and development license expense recognized.

+ References

No definition available.

+ Details

Name:	rytm_NonCashResearchAndDevelopmentLicenseExpense
Namespace Prefix:	rytm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the amount of non-refundable and non-creditable signing fee.

+ References

No definition available.

+ Details

Name:	rytm_NonRefundableAndNonCreditableSigningFee
Namespace Prefix:	rytm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the number of programs.

+ References

No definition available.

+ Details

Name:	rytm_NumberOfPrograms
Namespace Prefix:	rytm_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the amount of one-time non-refundable non-creditable sales milestone payment upon achievement of certain sales levels.

+ References

No definition available.

+ Details

Name:	rytm_OnetimeNonRefundableNonCreditableSalesMilestonePaymentUponAchievementOfCertainSalesLevels
Namespace Prefix:	rytm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the amount of one-time non-refundable development milestone payment upon achievement of certain regulatory successes.

+ References

No definition available.

+ Details

Name:	rytm_OnetimeNonrefundableDevelopmentMilestonePaymentUponAchievementOfCertainRegulatorySuccesses
Namespace Prefix:	rytm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the percentage payment based on revenue received.

+ References

No definition available.

+ Details

Name:	rytm_PaymentPercentageBasedOnRevenuesReceived
Namespace Prefix:	rytm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The period after the date of the first commercial sale.

+ References

No definition available.

+ Details

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Namespace Prefix:	rytm_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the period for royalty payment after first commercial sale.

+ References

No definition available.

+ Details

Name:	rytm_RoyaltyPaymentPeriodAfterFirstCommercialSale
Namespace Prefix:	rytm_
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

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

- Definition

Number of new stock issued during the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementScenarioAxis=us-gaap_PredecessorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

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

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	srt_CounterpartyNameAxis=rytm_CamurusAbMember
Namespace Prefix:
Data Type:	na
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- Details

Name:	srt_CounterpartyNameAxis=rytm_TakedaMember
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Significant Agreements (Details) $ in Thousands			1 Months Ended					9 Months Ended
Significant Agreements (Details) $ in Thousands	Apr. 03, 2018 USD ($) shares	Feb. 26, 2010 USD ($) item	Mar. 31, 2018	Dec. 31, 2017 USD ($)	Jul. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	Jan. 31, 2016 USD ($)	Sep. 30, 2018 USD ($)
Significant Agreements
Non-cash research and development license expense								$ 4,448
Ipsen \| Predecessor \| License agreement
Significant Agreements
Number of programs \| item		2
Aggregate payment upon achievement of development and commercial milestones		$ 40,000
Milestone prepayment					$ 1,000
Ipsen \| Predecessor \| License agreement \| Minimum
Significant Agreements
Payment based on revenue received, as percentage		10.00%
Ipsen \| Predecessor \| License agreement \| Maximum
Significant Agreements
Payment based on revenue received, as percentage		20.00%
Camurus \| Predecessor \| License agreement
Significant Agreements
Non-refundable and non-creditable signing fee							$ 500
One-time non-refundable development milestone payment							$ 7,750
Royalty payment period							10 years
One-time non-refundable non-creditable sales milestone payment							$ 57
Milestone fee				$ 1,000		$ 250
Takeda \| License agreement
Significant Agreements
Issuance of stock (in shares) \| shares	223,544
One-time non-refundable development milestone payment	$ 70,000
One-time non-refundable non-creditable sales milestone payment	70,000
Non-cash research and development license expense	$ 4,448
Period after date of first commercial sale (in years)			10 years