Investments - Summary of Cash Equivalents and Investments Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Cash Cash Equivalents And Investments [Line Items]
Investments, estimated fair value	$ 62,344	$ 115,438
Fair Value on Recurring Basis
Cash Cash Equivalents And Investments [Line Items]
Cash equivalents and investments, amortized cost	104,982	139,910
Cash equivalents and investments, unrealized gains	7	2
Cash equivalents and investments, unrealized losses	(380)	(91)
Cash equivalents and investments, estimated fair value	104,609	139,821
Fair Value on Recurring Basis \| Money Market Funds [Member]
Cash Cash Equivalents And Investments [Line Items]
Cash equivalents, amortized cost	11,870	10,585
Cash equivalents, estimated fair value	11,870	10,585
Commercial Paper [Member] \| Fair Value on Recurring Basis
Cash Cash Equivalents And Investments [Line Items]
Investments, amortized cost	56,312	84,642
Investments, unrealized gains	7	2
Investments, unrealized losses	(43)	(28)
Investments, estimated fair value	56,276	84,616
U.S. Government Securities \| Fair Value on Recurring Basis
Cash Cash Equivalents And Investments [Line Items]
Investments, amortized cost	25,283	27,870
Investments, unrealized losses	(247)	(46)
Investments, estimated fair value	25,036	27,824
U.S. Agency Bonds \| Fair Value on Recurring Basis
Cash Cash Equivalents And Investments [Line Items]
Investments, amortized cost	9,507	8,546
Investments, unrealized losses	(72)	(15)
Investments, estimated fair value	9,435	8,531
Corporate Debt Securities \| Fair Value on Recurring Basis
Cash Cash Equivalents And Investments [Line Items]
Investments, amortized cost	2,010	8,267
Investments, unrealized losses	(18)	(2)
Investments, estimated fair value	$ 1,992	$ 8,265

Cash, cash equivalents and investments.

No definition available.

Name:	rain_CashCashEquivalentsAndInvestmentsLineItems
Namespace Prefix:	rain_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Cash equivalents and investments amortized cost.

+ References

No definition available.

+ Details

Name:	rain_CashEquivalentsAndInvestmentsAmortizedCost
Namespace Prefix:	rain_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Cash equivalents and investments fair value.

+ References

No definition available.

+ Details

Name:	rain_CashEquivalentsAndInvestmentsFairValue
Namespace Prefix:	rain_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Cash equivalents and investments unrealized gain before tax.

+ References

No definition available.

+ Details

Name:	rain_CashEquivalentsAndInvestmentsUnrealizedGainBeforeTax
Namespace Prefix:	rain_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Cash equivalents and investments unrealized loss before tax.

+ References

No definition available.

+ Details

Name:	rain_CashEquivalentsAndInvestmentsUnrealizedLossBeforeTax
Namespace Prefix:	rain_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Cash equivalents fair value disclosure.

+ References

No definition available.

+ Details

Name:	rain_CashEquivalentsFairValueDisclosure
Namespace Prefix:	rain_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI https://asc.fasb.org/extlink&oid=126897435&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=124260329&loc=d3e26610-111562

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(1))
-URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_CashEquivalentsAtCarryingValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CashAndCashEquivalentsAxis=us-gaap_MoneyMarketFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialPaperMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: