Investment Securities - Schedule of Contractual Maturities of Debt Securities (Details) $ in Millions	Sep. 30, 2015 USD ($)
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, Under 1 Year	$ 9,983
Available for sale, 1 to 5 Years	29,816
Available for sale, 6 to 10 Years	17,360
Available for sale, Over 10 Years	22,592
Held to maturity, Under 1 Year	1,198
Held to maturity, 1 to 5 Years	5,008
Held to maturity, 6 to 10 Years	7,751
Held to maturity, Over 10 Years	3,506
Total asset-backed securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, Under 1 Year	514
Available for sale, 1 to 5 Years	4,417
Available for sale, 6 to 10 Years	4,131
Available for sale, Over 10 Years	2,499
Held to maturity, Under 1 Year	60
Held to maturity, 1 to 5 Years	1,055
Held to maturity, 6 to 10 Years	689
Held to maturity, Over 10 Years	1,139
US Treasury and federal agencies, direct obligations
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, Under 1 Year	1,403
Available for sale, 1 to 5 Years	9,228
Available for sale, 6 to 10 Years	2,245
Available for sale, Over 10 Years	478
Held to maturity, Under 1 Year	0
Held to maturity, 1 to 5 Years	1,625
Held to maturity, 6 to 10 Years	6,197
Held to maturity, Over 10 Years	109
US Treasury and federal agencies, mortgage-backed securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, Under 1 Year	135
Available for sale, 1 to 5 Years	2,268
Available for sale, 6 to 10 Years	3,784
Available for sale, Over 10 Years	12,642
Held to maturity, Under 1 Year	2
Held to maturity, 1 to 5 Years	6
Held to maturity, 6 to 10 Years	7
Held to maturity, Over 10 Years	31
Asset-backed securities, student loans
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, Under 1 Year	335
Available for sale, 1 to 5 Years	3,858
Available for sale, 6 to 10 Years	2,271
Available for sale, Over 10 Years	1,180
Held to maturity, Under 1 Year	0
Held to maturity, 1 to 5 Years	193
Held to maturity, 6 to 10 Years	313
Held to maturity, Over 10 Years	1,136
Asset-backed securities, credit cards
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, Under 1 Year	144
Available for sale, 1 to 5 Years	262
Available for sale, 6 to 10 Years	1,098
Available for sale, Over 10 Years	0
Held to maturity, Under 1 Year	0
Held to maturity, 1 to 5 Years	630
Held to maturity, 6 to 10 Years	267
Held to maturity, Over 10 Years	0
Asset-backed securities, sub-prime
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, Under 1 Year	6
Available for sale, 1 to 5 Years	5
Available for sale, 6 to 10 Years	4
Available for sale, Over 10 Years	443
Asset-backed securities, other
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, Under 1 Year	29
Available for sale, 1 to 5 Years	292
Available for sale, 6 to 10 Years	758
Available for sale, Over 10 Years	876
Held to maturity, Under 1 Year	60
Held to maturity, 1 to 5 Years	232
Held to maturity, 6 to 10 Years	109
Held to maturity, Over 10 Years	3
Total non-U.S. debt securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, Under 1 Year	5,999
Available for sale, 1 to 5 Years	9,600
Available for sale, 6 to 10 Years	1,461
Available for sale, Over 10 Years	2,767
Held to maturity, Under 1 Year	784
Held to maturity, 1 to 5 Years	2,179
Held to maturity, 6 to 10 Years	364
Held to maturity, Over 10 Years	1,428
Non-U.S. debt securities, mortgage-backed securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, Under 1 Year	1,554
Available for sale, 1 to 5 Years	3,134
Available for sale, 6 to 10 Years	657
Available for sale, Over 10 Years	2,571
Held to maturity, Under 1 Year	362
Held to maturity, 1 to 5 Years	815
Held to maturity, 6 to 10 Years	100
Held to maturity, Over 10 Years	1,428
Non-U.S. debt securities, asset-backed securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, Under 1 Year	391
Available for sale, 1 to 5 Years	2,475
Available for sale, 6 to 10 Years	136
Available for sale, Over 10 Years	196
Held to maturity, Under 1 Year	276
Held to maturity, 1 to 5 Years	1,320
Held to maturity, 6 to 10 Years	151
Held to maturity, Over 10 Years	0
Government securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, Under 1 Year	2,647
Available for sale, 1 to 5 Years	1,064
Available for sale, 6 to 10 Years	0
Available for sale, Over 10 Years	0
Held to maturity, Under 1 Year	123
Held to maturity, 1 to 5 Years	0
Held to maturity, 6 to 10 Years	113
Held to maturity, Over 10 Years	0
Non-U.S. debt securities, other
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, Under 1 Year	1,407
Available for sale, 1 to 5 Years	2,927
Available for sale, 6 to 10 Years	668
Available for sale, Over 10 Years	0
Held to maturity, Under 1 Year	23
Held to maturity, 1 to 5 Years	44
Held to maturity, 6 to 10 Years	0
Held to maturity, Over 10 Years	0
State and political subdivisions
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, Under 1 Year	616
Available for sale, 1 to 5 Years	2,372
Available for sale, 6 to 10 Years	5,075
Available for sale, Over 10 Years	1,911
Held to maturity, Under 1 Year	2
Held to maturity, 1 to 5 Years	0
Held to maturity, 6 to 10 Years	0
Held to maturity, Over 10 Years	0
Collateralized mortgage obligations
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, Under 1 Year	333
Available for sale, 1 to 5 Years	215
Available for sale, 6 to 10 Years	490
Available for sale, Over 10 Years	2,261
Held to maturity, Under 1 Year	350
Held to maturity, 1 to 5 Years	143
Held to maturity, 6 to 10 Years	494
Held to maturity, Over 10 Years	799
Other U.S. debt securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, Under 1 Year	983
Available for sale, 1 to 5 Years	1,716
Available for sale, 6 to 10 Years	174
Available for sale, Over 10 Years	$ 34