Summary of Significant Accounting Policies - Schedule of Short-term and Long-term Investment Marketable Securities by Security Type (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Available For Sale Securities Fair Value To Amortized Cost Basis [Line Items]
Amortized Cost	$ 371,927
Gross Unrealized Gains	0
Gross Unrealized Losses	(838)
Estimated Fair Value	371,089
Short Term Investments
Available For Sale Securities Fair Value To Amortized Cost Basis [Line Items]
Amortized Cost	230,849	$ 42,010
Gross Unrealized Gains		1
Gross Unrealized Losses	(157)	(3)
Estimated Fair Value	230,692	42,008
Short Term Investments \| Corporate Note Securities
Available For Sale Securities Fair Value To Amortized Cost Basis [Line Items]
Amortized Cost	98,642	12,780
Gross Unrealized Losses	(95)	(3)
Estimated Fair Value	98,547	12,777
Short Term Investments \| Asset-backed Securities
Available For Sale Securities Fair Value To Amortized Cost Basis [Line Items]
Amortized Cost		4,757
Gross Unrealized Gains		1
Estimated Fair Value		4,758
Short Term Investments \| Commercial Paper
Available For Sale Securities Fair Value To Amortized Cost Basis [Line Items]
Amortized Cost	114,174	24,473
Gross Unrealized Losses	(27)
Estimated Fair Value	114,147	$ 24,473
Short Term Investments \| U.S. Government Notes
Available For Sale Securities Fair Value To Amortized Cost Basis [Line Items]
Amortized Cost	18,033
Gross Unrealized Losses	(35)
Estimated Fair Value	17,998
Long Term Investments
Available For Sale Securities Fair Value To Amortized Cost Basis [Line Items]
Amortized Cost	141,078
Gross Unrealized Losses	(681)
Estimated Fair Value	140,397
Long Term Investments \| Corporate Note Securities
Available For Sale Securities Fair Value To Amortized Cost Basis [Line Items]
Amortized Cost	117,868
Gross Unrealized Losses	(596)
Estimated Fair Value	117,272
Long Term Investments \| Asset-backed Securities
Available For Sale Securities Fair Value To Amortized Cost Basis [Line Items]
Amortized Cost	3,044
Gross Unrealized Losses	(8)
Estimated Fair Value	3,036
Long Term Investments \| U.S. Government Notes
Available For Sale Securities Fair Value To Amortized Cost Basis [Line Items]
Amortized Cost	20,166
Gross Unrealized Losses	(77)
Estimated Fair Value	$ 20,089

Available for sale securities fair value to amortized cost basis.

No definition available.

Name:	cgem_AvailableForSaleSecuritiesFairValueToAmortizedCostBasisLineItems
Namespace Prefix:	cgem_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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 http://asc.fasb.org/extlink&oid=123581744&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 http://asc.fasb.org/extlink&oid=123581744&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 http://asc.fasb.org/extlink&oid=123581744&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 http://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 http://asc.fasb.org/extlink&oid=120398452&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 http://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 http://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 http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563

+ Details

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

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=cgem_ShortTermInvestmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CorporateNoteSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CashAndCashEquivalentsAxis=us-gaap_CommercialPaperMember
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_LongtermDebtTypeAxis=cgem_LongTermInvestmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: