Investment Securities (Contractual Maturity of Debt Securities) (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, amortized cost	$ 57,212,000,000	$ 59,906,000,000
Available-for-sale Securities, Fair value, Total	57,618,000,000	60,104,000,000
Held to Maturity Securities, Amortized Cost, Total	18,513,000,000	15,843,000,000
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available for Sale Securities, Amortized Cost, 1 year or less	624,000,000
Available for Sale Securities, Amortized Cost, After 1 year through 5 years	12,962,000,000
Available for Sale Securities, Amortized Cost, After 5 years through 10 years	8,652,000,000
Available for Sale Securities, Amortized Cost, After 10 years	34,387,000,000
Securities available for sale, amortized cost	56,625,000,000	59,303,000,000
Available-for-sale Securities, Fair value, 1 year or less	626,000,000
Available-for-sale Securities, Fair value, After 1 year through 5 years	12,958,000,000
Available-for-sale Securities, Fair value, After 5 years through 10 years	8,695,000,000
Available-for-sale Securities, Fair value, After 10 years	34,754,000,000
Available-for-sale Securities, Fair value, Total	$ 57,033,000,000	59,503,000,000
Weighted-average yield, GAAP basis, available for sale securities	2.67%
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount	$ 221,000,000
Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Net Carrying Amount	638,000,000
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	1,695,000,000
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	15,959,000,000
Held to Maturity Securities, Amortized Cost, Total	18,513,000,000	15,843,000,000
Held-to-maturity Securities, Fair Value, 1 year or less	221,000,000
Held-to-maturity Securities, Fair Value, After 1 year through 5 years	654,000,000
Held-to-maturity Securities, Fair Value, After 5 years through 10 years	1,751,000,000
Held-to-maturity Securities, Fair Value, After 10 years	15,939,000,000
Held-to-maturity Securities, Debt Maturities, Fair Value, Total	$ 18,565,000,000	$ 15,866,000,000
Weighted-average yield, GAAP basis, held to maturity securities	3.26%
One Year or Less | Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Weighted-average yield, GAAP basis, available for sale securities	2.73%
Weighted-average yield, GAAP basis, held to maturity securities	3.58%
After One Year Through Five Years | Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Weighted-average yield, GAAP basis, available for sale securities	2.12%
Weighted-average yield, GAAP basis, held to maturity securities	3.92%
After Five Years Through Ten Years | Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Weighted-average yield, GAAP basis, available for sale securities	2.29%
Weighted-average yield, GAAP basis, held to maturity securities	3.44%
After Ten Years | Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Weighted-average yield, GAAP basis, available for sale securities	2.97%
Weighted-average yield, GAAP basis, held to maturity securities	3.21%
US Treasury and Government Agencies Securities | Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available for Sale Securities, Amortized Cost, 1 year or less	$ 85,000,000
Available for Sale Securities, Amortized Cost, After 1 year through 5 years	8,780,000,000
Available for Sale Securities, Amortized Cost, After 5 years through 10 years	4,449,000,000
Available for Sale Securities, Amortized Cost, After 10 years	1,118,000,000
Securities available for sale, amortized cost	14,432,000,000
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	377,000,000
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	364,000,000
Held to Maturity Securities, Amortized Cost, Total	741,000,000
Residential mortgage-backed Securities | Mortgage-backed Securities Agency | Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available for Sale Securities, Amortized Cost, 1 year or less	3,000,000
Available for Sale Securities, Amortized Cost, After 1 year through 5 years	45,000,000
Available for Sale Securities, Amortized Cost, After 5 years through 10 years	561,000,000
Available for Sale Securities, Amortized Cost, After 10 years	24,925,000,000
Securities available for sale, amortized cost	25,534,000,000
Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Net Carrying Amount	67,000,000
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	346,000,000
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	14,090,000,000
Held to Maturity Securities, Amortized Cost, Total	14,503,000,000
Residential mortgage-backed Securities | Mortgage-backed Securities Non-agency | Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available for Sale Securities, Amortized Cost, After 10 years	2,443,000,000
Securities available for sale, amortized cost	2,443,000,000
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	167,000,000
Held to Maturity Securities, Amortized Cost, Total	167,000,000
Commercial Mortgage Backed Securities | Mortgage-backed Securities Agency | Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available for Sale Securities, Amortized Cost, 1 year or less	2,000,000
Available for Sale Securities, Amortized Cost, After 1 year through 5 years	259,000,000
Available for Sale Securities, Amortized Cost, After 5 years through 10 years	624,000,000
Available for Sale Securities, Amortized Cost, After 10 years	1,075,000,000
Securities available for sale, amortized cost	1,960,000,000
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount	172,000,000
Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Net Carrying Amount	177,000,000
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	4,000,000
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	54,000,000
Held to Maturity Securities, Amortized Cost, Total	407,000,000
Commercial Mortgage Backed Securities | Mortgage-backed Securities Non-agency | Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available for Sale Securities, Amortized Cost, After 5 years through 10 years	451,000,000
Available for Sale Securities, Amortized Cost, After 10 years	2,152,000,000
Securities available for sale, amortized cost	2,603,000,000
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	538,000,000
Held to Maturity Securities, Amortized Cost, Total	538,000,000
Asset backed | Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available for Sale Securities, Amortized Cost, 1 year or less	13,000,000
Available for Sale Securities, Amortized Cost, After 1 year through 5 years	1,846,000,000
Available for Sale Securities, Amortized Cost, After 5 years through 10 years	1,897,000,000
Available for Sale Securities, Amortized Cost, After 10 years	1,575,000,000
Securities available for sale, amortized cost	5,331,000,000
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	114,000,000
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	86,000,000
Held to Maturity Securities, Amortized Cost, Total	200,000,000
Other debt | Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available for Sale Securities, Amortized Cost, 1 year or less	521,000,000
Available for Sale Securities, Amortized Cost, After 1 year through 5 years	2,032,000,000
Available for Sale Securities, Amortized Cost, After 5 years through 10 years	670,000,000
Available for Sale Securities, Amortized Cost, After 10 years	1,099,000,000
Securities available for sale, amortized cost	4,322,000,000
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount	49,000,000
Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Net Carrying Amount	394,000,000
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	854,000,000
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	660,000,000
Held to Maturity Securities, Amortized Cost, Total	$ 1,957,000,000