Investment Securities - Schedule of Gross Pre-Tax Unrealized Losses on Investment Securities (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	$ 27,086	$ 18,094
Available for sale, gross unrealized losses less than 12 months	357	131
Available for sale, fair value 12 months or longer	5,221	6,391
Available for sale, gross unrealized losses 12 months or longer	251	137
Available for sale, fair value total	32,307	24,485
Available for sale, gross unrealized losses total	608	268
Held to maturity, fair value less than 12 months	20,917	21,664
Held-to-maturity, gross unrealized losses, less than 12 months	610	150
Held to maturity, fair value 12 months or longer	13,064	9,373
Held to maturity, gross unrealized losses 12 months or longer	537	239
Held to maturity, fair value total	33,981	31,037
Held to maturity, gross unrealized losses total	1,147	389
US Treasury and federal agencies, direct obligations
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months		0
Available for sale, gross unrealized losses less than 12 months		0
Available for sale, fair value 12 months or longer		67
Available for sale, gross unrealized losses 12 months or longer		1
Available for sale, fair value total		67
Available for sale, gross unrealized losses total		1
Held to maturity, fair value less than 12 months	8,963	14,439
Held-to-maturity, gross unrealized losses, less than 12 months	222	109
Held to maturity, fair value 12 months or longer	5,879	2,447
Held to maturity, gross unrealized losses 12 months or longer	103	34
Held to maturity, fair value total	14,842	16,886
Held to maturity, gross unrealized losses total	325	143
US Treasury and federal agencies, mortgage-backed securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	10,651	5,161
Available for sale, gross unrealized losses less than 12 months	175	31
Available for sale, fair value 12 months or longer	3,708	3,341
Available for sale, gross unrealized losses 12 months or longer	201	98
Available for sale, fair value total	14,359	8,502
Available for sale, gross unrealized losses total	376	129
Held to maturity, fair value less than 12 months	11,692	6,785
Held-to-maturity, gross unrealized losses, less than 12 months	386	38
Held to maturity, fair value 12 months or longer	6,303	5,988
Held to maturity, gross unrealized losses 12 months or longer	416	187
Held to maturity, fair value total	17,995	12,773
Held to maturity, gross unrealized losses total	802	225
Total U.S. Treasury and federal agencies
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	10,651	5,161
Available for sale, gross unrealized losses less than 12 months	175	31
Available for sale, fair value 12 months or longer	3,708	3,408
Available for sale, gross unrealized losses 12 months or longer	201	99
Available for sale, fair value total	14,359	8,569
Available for sale, gross unrealized losses total	376	130
Held to maturity, fair value less than 12 months	20,655	21,224
Held-to-maturity, gross unrealized losses, less than 12 months	608	147
Held to maturity, fair value 12 months or longer	12,182	8,435
Held to maturity, gross unrealized losses 12 months or longer	519	221
Held to maturity, fair value total	32,837	29,659
Held to maturity, gross unrealized losses total	1,127	368
Asset-backed securities, student loans
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	262	0
Available for sale, gross unrealized losses less than 12 months	1	0
Available for sale, fair value 12 months or longer	142	769
Available for sale, gross unrealized losses 12 months or longer	0	4
Available for sale, fair value total	404	769
Available for sale, gross unrealized losses total	1	4
Held to maturity, fair value less than 12 months	160	440
Held-to-maturity, gross unrealized losses, less than 12 months	1	3
Held to maturity, fair value 12 months or longer	533	423
Held to maturity, gross unrealized losses 12 months or longer	6	6
Held to maturity, fair value total	693	863
Held to maturity, gross unrealized losses total	7	9
Asset-backed securities, credit cards
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	0	1,289
Available for sale, gross unrealized losses less than 12 months	0	25
Available for sale, fair value 12 months or longer	491	0
Available for sale, gross unrealized losses 12 months or longer	24	0
Available for sale, fair value total	491	1,289
Available for sale, gross unrealized losses total	24	25
Asset-backed securities, other
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	357
Available for sale, gross unrealized losses less than 12 months	1
Available for sale, fair value 12 months or longer	0
Available for sale, gross unrealized losses 12 months or longer	0
Available for sale, fair value total	357
Available for sale, gross unrealized losses total	1
Asset-backed securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	619	1,289
Available for sale, gross unrealized losses less than 12 months	2	25
Available for sale, fair value 12 months or longer	633	769
Available for sale, gross unrealized losses 12 months or longer	24	4
Available for sale, fair value total	1,252	2,058
Available for sale, gross unrealized losses total	26	29
Held to maturity, fair value less than 12 months	160	440
Held-to-maturity, gross unrealized losses, less than 12 months	1	3
Held to maturity, fair value 12 months or longer	533	423
Held to maturity, gross unrealized losses 12 months or longer	6	6
Held to maturity, fair value total	693	863
Held to maturity, gross unrealized losses total	7	9
Non-U.S. debt securities, mortgage-backed securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	1,063	1,059
Available for sale, gross unrealized losses less than 12 months	4	4
Available for sale, fair value 12 months or longer	57	469
Available for sale, gross unrealized losses 12 months or longer	0	1
Available for sale, fair value total	1,120	1,528
Available for sale, gross unrealized losses total	4	5
Held to maturity, fair value less than 12 months	97	0
Held-to-maturity, gross unrealized losses, less than 12 months	1	0
Held to maturity, fair value 12 months or longer	125	239
Held to maturity, gross unrealized losses 12 months or longer	5	6
Held to maturity, fair value total	222	239
Held to maturity, gross unrealized losses total	6	6
Non-U.S. debt securities, asset-backed securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	769
Available for sale, gross unrealized losses less than 12 months	3
Available for sale, fair value 12 months or longer	0
Available for sale, gross unrealized losses 12 months or longer	0
Available for sale, fair value total	769
Available for sale, gross unrealized losses total	3
Non-U.S. debt securities, government securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	10,408	7,629
Available for sale, gross unrealized losses less than 12 months	115	48
Available for sale, fair value 12 months or longer	0	68
Available for sale, gross unrealized losses 12 months or longer	0	1
Available for sale, fair value total	10,408	7,697
Available for sale, gross unrealized losses total	115	49
Non-U.S. debt securities, other
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	1,468	816
Available for sale, gross unrealized losses less than 12 months	14	4
Available for sale, fair value 12 months or longer	164	289
Available for sale, gross unrealized losses 12 months or longer	3	2
Available for sale, fair value total	1,632	1,105
Available for sale, gross unrealized losses total	17	6
Total non-U.S. debt securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	13,708	9,504
Available for sale, gross unrealized losses less than 12 months	136	56
Available for sale, fair value 12 months or longer	221	826
Available for sale, gross unrealized losses 12 months or longer	3	4
Available for sale, fair value total	13,929	10,330
Available for sale, gross unrealized losses total	139	60
Held to maturity, fair value less than 12 months	97	0
Held-to-maturity, gross unrealized losses, less than 12 months	1	0
Held to maturity, fair value 12 months or longer	125	239
Held to maturity, gross unrealized losses 12 months or longer	5	6
Held to maturity, fair value total	222	239
Held to maturity, gross unrealized losses total	6	6
State and political subdivisions
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	662	734
Available for sale, gross unrealized losses less than 12 months	10	6
Available for sale, fair value 12 months or longer	266	901
Available for sale, gross unrealized losses 12 months or longer	7	17
Available for sale, fair value total	928	1,635
Available for sale, gross unrealized losses total	17	23
Collateralized mortgage obligations
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	217	399
Available for sale, gross unrealized losses less than 12 months	5	5
Available for sale, fair value 12 months or longer	77	136
Available for sale, gross unrealized losses 12 months or longer	2	4
Available for sale, fair value total	294	535
Available for sale, gross unrealized losses total	7	9
Held to maturity, fair value less than 12 months	5	0
Held-to-maturity, gross unrealized losses, less than 12 months	0	0
Held to maturity, fair value 12 months or longer	224	276
Held to maturity, gross unrealized losses 12 months or longer	7	6
Held to maturity, fair value total	229	276
Held to maturity, gross unrealized losses total	7	6
Other U.S. debt securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	1,229	1,007
Available for sale, gross unrealized losses less than 12 months	29	8
Available for sale, fair value 12 months or longer	316	345
Available for sale, gross unrealized losses 12 months or longer	14	7
Available for sale, fair value total	1,545	1,352
Available for sale, gross unrealized losses total	$ 43	15
U.S. equity securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months		0
Available for sale, gross unrealized losses less than 12 months		0
Available for sale, fair value 12 months or longer		6
Available for sale, gross unrealized losses 12 months or longer		2
Available for sale, fair value total		6
Available for sale, gross unrealized losses total		$ 2