SECURITIES - Income Recognized from Investment Securities (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Interest Income, Securities, Operating, by Taxable Status [Abstract]
Taxable	$ 619us-gaap_InterestIncomeSecuritiesTaxable	$ 477us-gaap_InterestIncomeSecuritiesTaxable	$ 618us-gaap_InterestIncomeSecuritiesTaxable
Non-taxable	0	0	2
Interest-bearing deposits in banks	5	11	4
Total interest income from investment securities	624us-gaap_InterestIncomeOperating	488us-gaap_InterestIncomeOperating	624us-gaap_InterestIncomeOperating
Gain (Loss) on Sale of Investments [Abstract]
Gains on sale of debt securities	33cfg_GainonSaleofDebtInvestments	144cfg_GainonSaleofDebtInvestments	93cfg_GainonSaleofDebtInvestments
Losses on sale of debt securities	(5)cfg_LossonSaleofDebtInvestments	0cfg_LossonSaleofDebtInvestments	0cfg_LossonSaleofDebtInvestments
Gains on sale of marketable equity securities	0cfg_MarketableSecuritiesRealizedGain	0cfg_MarketableSecuritiesRealizedGain	2cfg_MarketableSecuritiesRealizedGain
Total	$ 28us-gaap_GainLossOnSaleOfInvestments	$ 144us-gaap_GainLossOnSaleOfInvestments	$ 95us-gaap_GainLossOnSaleOfInvestments

Gain on sale of debt investment securities.

No definition available.

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- Definition

Loss on sale of debt investment securities.

+ References

No definition available.

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- Definition

Realized gains on sale of marketable equity securities.

+ References

No definition available.

+ Details

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- Definition

The net realized gain (loss) on investments sold during the period, not including gains (losses) on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity, which, for cash flow reporting, is a component of proceeds from investing activities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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- Definition

Interest income derived from funds deposited with both domestic and foreign financial institutions including funds in money market and other accounts.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.4)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 4

-Article 9

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- Definition

Amount of operating interest income, including, but not limited to, amortization and accretion of premiums and discounts on securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1(e))

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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- Definition

Amount of operating interest income, including amortization and accretion of premiums and discounts, on securities subject to state, federal and other income tax.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.2)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 2

-Article 9

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- Definition

Amount of operating interest income, including amortization and accretion of premiums and discounts, on securities exempt from state, federal and other income tax.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.2)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 2

-Article 9

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