Securities - Available for Sale Securities (Unrealized Loss Position) (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Gross unrealized losses	$ 9	$ 23	$ 54
Fair value less than 12 months, total temporarily impaired securities	5,693	1,520	9,204
Gross unrealized losses less than 12 months, total temporarily impaired securities	18	5	78
Fair value 12 months or longer, total temporarily impaired securities	2,105	4,706	4,529
Gross unrealized losses 12 months or longer, total temporarily impaired securities	18	81	130
Fair value, total temporarily impaired securities	7,798	6,226	13,733
Gross unrealized losses, total temporarily impaired securities	36	86	208
U.S. Treasury, Agencies and Corporations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	149
Gross unrealized losses, less than 12 months	0
Fair value, 12 months or longer	0
Gross unrealized losses, 12 months or longer	0
Fair Value	149
Gross unrealized losses, Total	0
Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	3,929	770	5,190
Gross unrealized losses, less than 12 months	15	3	43
Fair value, 12 months or longer	1,537	3,348	3,206
Gross unrealized losses, 12 months or longer	11	59	95
Fair Value	5,466	4,118	8,396
Gross unrealized losses, Total	26	62	138
Held-to-maturity Securities, Fair Value less than 12 months	1,412	214	1,793
Held-to-maturity Securities, Gross unrealized losses, less than 12 months	3	1	16
Held-to-maturity Securities, Fair Value 12 months or longer	565	1,355	1,320
Held-to-maturity Securities, Gross unrealized losses, 12 months or longer	6	22	34
Held-to-maturity Securities, Fair Value	1,977	1,569	3,113
Held-to-maturity Securities, Gross unrealized losses	9	23	50
Agency Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	122
Gross unrealized losses, less than 12 months	0
Fair value, 12 months or longer	0
Gross unrealized losses, 12 months or longer	0
Fair Value	122
Gross unrealized losses, Total	0
Held-to-maturity Securities, Gross unrealized losses	0
Agency Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	81	420	1,670
Gross unrealized losses, less than 12 months	0	1	15
Fair value, 12 months or longer	0	0	0
Gross unrealized losses, 12 months or longer	0	0	0
Fair Value	81	420	1,670
Gross unrealized losses, Total	0	1	15
Held-to-maturity Securities, Fair Value less than 12 months		110	547
Held-to-maturity Securities, Gross unrealized losses, less than 12 months		0	4
Held-to-maturity Securities, Fair Value 12 months or longer		0	0
Held-to-maturity Securities, Gross unrealized losses, 12 months or longer		0	0
Held-to-maturity Securities, Fair Value		110	547
Held-to-maturity Securities, Gross unrealized losses	0	0	4
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	0	2	0
Gross unrealized losses, less than 12 months	0	0	0
Fair value, 12 months or longer	3	3	3
Gross unrealized losses, 12 months or longer	1	0	1
Fair Value	3	5	3
Gross unrealized losses, Total	1	0	1
Held-to-maturity Securities, Fair Value less than 12 months		4	4
Held-to-maturity Securities, Gross unrealized losses, less than 12 months		0	0
Held-to-maturity Securities, Fair Value 12 months or longer		0	0
Held-to-maturity Securities, Gross unrealized losses, 12 months or longer		0	0
Held-to-maturity Securities, Fair Value		4	4
Held-to-maturity Securities, Gross unrealized losses	0	0	0
Gross unrealized losses, available for sale securities (less than)		1
Gross unrealized losses, held-to-maturity securities (less than)		$ 1	$ 1
U.S. Treasury, Agencies, Corporations, Agency Residential Mortgage-backed Securities, and Agency Commercial Mortgage-backed Securities Available for Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses, available for sale securities (less than)	$ 1

Available for sale and held to maturity securities continuous unrealized loss position aggregate losses.

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

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