SECURITIES - Schedule of Investments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Equity Securities Available-for-sale, Amortized Cost	$ 18,710	$ 18,433
Equity Securities Available-for-sale, Gross Unrealized Gains	387	308
Equity Securities Available-for-sale, Gross Unrealized Losses	(56)	(85)
Equity Securities Available-for-sale, Fair Value	19,041us-gaap_AvailableForSaleSecurities	18,656us-gaap_AvailableForSaleSecurities
Securities Held-to-maturity, Amortized Cost	5,178us-gaap_HeldToMaturitySecurities	5,148us-gaap_HeldToMaturitySecurities
Securities Held-to-maturity, Gross Unrealized Gain	107	76
Securities Held-to-maturity, Gross Unrealized Losses	(4)	(31)
Securities held-to-maturity, Fair Value	5,281	5,193
Other Investment Securities, Amortized Cost	867	872
Other Investment Securities, Gross Unrealized Gains	0	0
Other Investment Securities, Gross Unrealized Losses	0	0
Other Investment Securities, Fair Value	867cfg_OtherInvestmentSecurities	872cfg_OtherInvestmentSecurities
U.S. Treasury
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Debt Securities Available-for-sale, Amortized Cost	15	15
Debt Securities Available-for-sale, Gross Unrealized Gains	0	0
Debt Securities Available-for-sale, Gross Unrealized Losses	0	0
Debt Securities Available-for-sale, Fair Value	15	15
State and political subdivisions
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Debt Securities Available-for-sale, Amortized Cost	10	10
Debt Securities Available-for-sale, Gross Unrealized Gains	0	0
Debt Securities Available-for-sale, Gross Unrealized Losses	0	0
Debt Securities Available-for-sale, Fair Value	10	10
Federal agencies and U.S. government sponsored entities
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Debt Securities Available-for-sale, Amortized Cost	17,999	17,683
Debt Securities Available-for-sale, Gross Unrealized Gains	382	301
Debt Securities Available-for-sale, Gross Unrealized Losses	(25)	(50)
Debt Securities Available-for-sale, Fair Value	18,356	17,934
Securities Held-to-maturity, Amortized Cost	3,799	3,728
Securities Held-to-maturity, Gross Unrealized Gain	52	22
Securities Held-to-maturity, Gross Unrealized Losses	(4)	(31)
Securities held-to-maturity, Fair Value	3,847	3,719
Other/non-agency
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Debt Securities Available-for-sale, Amortized Cost	667	703
Debt Securities Available-for-sale, Gross Unrealized Gains	4	4
Debt Securities Available-for-sale, Gross Unrealized Losses	(31)	(35)
Debt Securities Available-for-sale, Fair Value	640	672
Securities Held-to-maturity, Amortized Cost	1,379	1,420
Securities Held-to-maturity, Gross Unrealized Gain	55	54
Securities Held-to-maturity, Gross Unrealized Losses	0	0
Securities held-to-maturity, Fair Value	1,434	1,474
Total mortgage-backed securities
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Debt Securities Available-for-sale, Amortized Cost	18,666	18,386
Debt Securities Available-for-sale, Gross Unrealized Gains	386	305
Debt Securities Available-for-sale, Gross Unrealized Losses	(56)	(85)
Debt Securities Available-for-sale, Fair Value	18,996	18,606
Total debt securities available for sale
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Debt Securities Available-for-sale, Amortized Cost	18,691	18,411
Debt Securities Available-for-sale, Gross Unrealized Gains	386	305
Debt Securities Available-for-sale, Gross Unrealized Losses	(56)	(85)
Debt Securities Available-for-sale, Fair Value	19,021	18,631
Marketable equity securities
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Equity Securities Available-for-sale, Amortized Cost	7	10
Equity Securities Available-for-sale, Gross Unrealized Gains	1	3
Equity Securities Available-for-sale, Gross Unrealized Losses	0	0
Equity Securities Available-for-sale, Fair Value	8	13
Other equity securities
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Equity Securities Available-for-sale, Amortized Cost	12	12
Equity Securities Available-for-sale, Gross Unrealized Gains	0	0
Equity Securities Available-for-sale, Gross Unrealized Losses	0	0
Equity Securities Available-for-sale, Fair Value	12	12
Total equity securities available for sale
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Equity Securities Available-for-sale, Amortized Cost	19	22
Equity Securities Available-for-sale, Gross Unrealized Gains	1	3
Equity Securities Available-for-sale, Gross Unrealized Losses	0	0
Equity Securities Available-for-sale, Fair Value	20	25
Federal Reserve Bank stock
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Federal Reserve Bank Stock, Amortized Cost	468	477
Federal Reserve Bank Stock, Gross Unrealized Gains	0	0
Federal Reserve Bank Stock, Gross Unrealized Loss	0	0
Federal Reserve Bank Stock, Fair Value	468	477
Federal Home Loan Bank stock
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Federal Home Loan Bank Stock, Amortized Cost	393	390
Federal Home Loan Bank Stock, Gross Unrealized Gain	0	0
Federal Home Loan Bank Stock, Gross Unrealized Loss	0	0
Federal Home Loan Bank Stock, Fair Value	393	390
Total other investment securities
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Venture Capital, Amortized Cost	6	5
Venture Capital, Gross Unrealized Gain	0	0
Venture Capital, Gross Unrealized Loss	0	0
Venture Capital, Fair Value	$ 6	$ 5

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Amount of accumulated unrealized loss on Federal Home Loan Bank stock.

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Fair value of Federal Home Loan Bank (FHLB) stock.

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Amount of accumulated unrealized gain on firm holdings in private equity, mezzanine, venture capital, merchant banking, real estate and special situations investments. Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings.

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Amount of accumulated unrealized loss on firm holdings in private equity, mezzanine, venture capital, merchant banking, real estate and special situations investments. Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings.

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The amortized cost as of the balance sheet date of firm holdings in private equity, mezzanine, venture capital, merchant banking, real estate and special situations investments. Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings.

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Fair value of other investment securities.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in equity securities classified as available-for-sale.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Federal Home Loan Bank (FHLB) stock represents an equity interest in a FHLB. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).

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Federal Reserve Bank stock represents an equity interest in the Federal Reserve Bank. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).

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Fair value of investments in debt securities classified as held-to-maturity.

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