ALLOWANCE FOR CREDIT LOSSES, NONACCRUING LOANS AND LEASES, AND CONCENTRATIONS OF CREDIT RISK - Nonaccrual Loans and Leases by Class (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Nonaccrual loans and leases	$ 779	$ 1,019
90+ days past due and accruing	280	62
Nonaccrual with no related ACL	442	380
Commercial
Financing Receivable, Past Due [Line Items]
Nonaccrual loans and leases	266	458
90+ days past due and accruing	1	21
Nonaccrual with no related ACL	77	58
Commercial \| Commercial and industrial
Financing Receivable, Past Due [Line Items]
Nonaccrual loans and leases	163	280
90+ days past due and accruing	0	20
Nonaccrual with no related ACL	40	56
Commercial \| Commercial real estate
Financing Receivable, Past Due [Line Items]
Nonaccrual loans and leases	102	176
90+ days past due and accruing	0	0
Nonaccrual with no related ACL	37	2
Commercial \| Leases
Financing Receivable, Past Due [Line Items]
Nonaccrual loans and leases	1	2
90+ days past due and accruing	1	1
Nonaccrual with no related ACL	0	0
Retail
Financing Receivable, Past Due [Line Items]
Nonaccrual loans and leases	513	561
90+ days past due and accruing	279	41
Nonaccrual with no related ACL	365	322
Retail \| Residential mortgages (2)
Financing Receivable, Past Due [Line Items]
Nonaccrual loans and leases	174	167
90+ days past due and accruing	270	30
Nonaccrual with no related ACL	138	96
Retail \| Home equity
Financing Receivable, Past Due [Line Items]
Nonaccrual loans and leases	234	276
90+ days past due and accruing	0	0
Nonaccrual with no related ACL	189	207
Retail \| Automobile
Financing Receivable, Past Due [Line Items]
Nonaccrual loans and leases	62	72
90+ days past due and accruing	0	0
Nonaccrual with no related ACL	34	17
Retail \| Education
Financing Receivable, Past Due [Line Items]
Nonaccrual loans and leases	21	18
90+ days past due and accruing	2	2
Nonaccrual with no related ACL	2	2
Retail \| Other retail
Financing Receivable, Past Due [Line Items]
Nonaccrual loans and leases	22	28
90+ days past due and accruing	7	9
Nonaccrual with no related ACL	2	0
Retail \| FHA, VA And USDA Loans
Financing Receivable, Past Due [Line Items]
90+ days past due and accruing	$ 266	$ 21

Amount of financing receivable on nonaccrual status with no allowance for credit loss.

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

- Definition

Amount of financing receivable 90 days or more past due and still accruing.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 16
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919258-210447

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5111-111524

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 80
-URI http://asc.fasb.org/extlink&oid=121647567&loc=SL82922355-210448

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of financing receivable on nonaccrual status.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5111-111524

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 16
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919258-210447

+ Details

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

- 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

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cfg_FinanceLeasesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cfg_HomeEquityLoanAndLineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_AutomobileLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_StudentLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cfg_ConsumerOtherRetailMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cfg_FHAVAAndUSDALoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: