Securities - Details of Securities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 13,037	$ 13,365	$ 12,412
Gross Unrealized Gains	160	132	165
Gross Unrealized Losses	77	137	218
Fair Value	13,120us-gaap_AvailableForSaleSecurities	13,360us-gaap_AvailableForSaleSecurities	12,359us-gaap_AvailableForSaleSecurities
Held to maturities, Amortized Cost	5,005us-gaap_HeldToMaturitySecurities	5,015us-gaap_HeldToMaturitySecurities	4,826us-gaap_HeldToMaturitySecurities
Held to maturities, Gross Unrealized Gains	28	16	9
Held to maturities, Gross Unrealized Losses	30	57	102
Held to maturities, Fair Value	5,003	4,974	4,733
Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11,116	11,310	10,541
Gross Unrealized Gains	124	96	136
Gross Unrealized Losses	77	136	208
Fair Value	11,163	11,270	10,469
Held to maturities, Amortized Cost	4,749	4,755	4,806
Held to maturities, Gross Unrealized Gains	26	15	9
Held to maturities, Gross Unrealized Losses	30	57	102
Held to maturities, Fair Value	4,745	4,713	4,713
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	30	29	17
Gross Unrealized Gains	3	3	4
Fair Value	33	32	21
Held to maturities, Amortized Cost	22	20	20
Held to maturities, Fair Value	22	20	20
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21	22	37
Gross Unrealized Gains	1	1
Fair Value	22	23	37
Other Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,870	2,004	1,817
Gross Unrealized Gains	32	32	25
Gross Unrealized Losses		1	10
Fair Value	1,902	2,035	1,832
Held to maturities, Amortized Cost	234	240
Held to maturities, Gross Unrealized Gains	2	1
Held to maturities, Fair Value	$ 236	$ 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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