Securities - Details of Securities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 13,365	$ 12,446
Gross Unrealized Gains	132	183
Gross Unrealized Losses	137	283
Fair Value	13,360us-gaap_AvailableForSaleSecurities	12,346us-gaap_AvailableForSaleSecurities
Held to maturities, Amortized Cost	5,015us-gaap_HeldToMaturitySecurities	4,756us-gaap_HeldToMaturitySecurities
Held to maturities, Gross Unrealized Gains	16	6
Held to maturities, Gross Unrealized Losses	57	145
Held to maturities, Fair Value	4,974	4,617
Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11,310	11,120
Gross Unrealized Gains	96	152
Gross Unrealized Losses	136	272
Fair Value	11,270	11,000
Held to maturities, Amortized Cost	4,755	4,736
Held to maturities, Gross Unrealized Gains	15	6
Held to maturities, Gross Unrealized Losses	57	145
Held to maturities, Fair Value	4,713	4,597
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	29	17
Gross Unrealized Gains	3	3
Gross Unrealized Losses
Fair Value	32	20
Held to maturities, Amortized Cost	20	20
Held to maturities, Gross Unrealized Gains
Held to maturities, Gross Unrealized Losses
Held to maturities, Fair Value	20	20
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22	39
Gross Unrealized Gains	1	1
Gross Unrealized Losses
Fair Value	23	40
Other Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,004	1,270
Gross Unrealized Gains	32	27
Gross Unrealized Losses	1	11
Fair Value	2,035	1,286
Held to maturities, Amortized Cost	240
Held to maturities, Gross Unrealized Gains	1
Held to maturities, Gross Unrealized Losses
Held to maturities, Fair Value	$ 241

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

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

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.

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

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