Investment Securities - Fair Value and Gross Unrealized Losses by Aging Category (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Available-for-sale securities
Less than 12 months, Fair value	$ 65,250	$ 50,481
Less than 12 months, Gross unrealized losses	1,248	201
12 months or more, Fair Value	21,638	12,792
12 months or more, Gross unrealized losses	910	276
Total fair value	86,888	63,273
Total gross unrealized losses	2,158	477
Held-to-maturity securities
Less than 12 months, Fair value	22,984	8,360
Less than 12 months, Gross unrealized losses	366	88
12 months or more, Fair value	3,272	4,218
12 months or more, Gross unrealized losses	158	106
Total fair value	26,256	12,578
Total gross unrealized losses	524	194
Total investment securities with gross unrealized losses
Less than 12 months, Fair value	88,234	58,841
Less than 12 months, Gross unrealized losses	1,614	289
12 months or more, Fair value	24,910	17,010
12 months or more, Gross unrealized losses	1,068	382
Total fair value	113,144	75,851
Total gross unrealized losses	2,682	671
Total mortgage-backed securities
Available-for-sale securities
Less than 12 months, Fair value	40,001	37,677
Less than 12 months, Gross unrealized losses	1,021	148
12 months or more, Fair Value	14,550	8,908
12 months or more, Gross unrealized losses	708	207
Total fair value	54,551	46,585
Total gross unrealized losses	1,729	355
Held-to-maturity securities
Less than 12 months, Fair value	22,131	7,776
Less than 12 months, Gross unrealized losses	356	79
12 months or more, Fair value	2,595	2,087
12 months or more, Gross unrealized losses	137	35
Total fair value	24,726	9,863
Total gross unrealized losses	493	114
U.S. government agencies
Available-for-sale securities
Less than 12 months, Fair value	37,109	36,037
Less than 12 months, Gross unrealized losses	988	139
12 months or more, Fair Value	10,492	7,711
12 months or more, Gross unrealized losses	520	196
Total fair value	47,601	43,748
Total gross unrealized losses	1,508	335
Held-to-maturity securities
Less than 12 months, Fair value	22,131	4,070
Less than 12 months, Gross unrealized losses	356	38
12 months or more, Fair value	2,595	205
12 months or more, Gross unrealized losses	137	2
Total fair value	24,726	4,275
Total gross unrealized losses	493	40
Residential: U.S.
Available-for-sale securities
Less than 12 months, Fair value	1,343	1,112
Less than 12 months, Gross unrealized losses	20	5
12 months or more, Fair Value	860	596
12 months or more, Gross unrealized losses	16	9
Total fair value	2,203	1,708
Total gross unrealized losses	36	14
Residential: Non-U.S.
Available-for-sale securities
Less than 12 months, Fair value	635	0
Less than 12 months, Gross unrealized losses	2	0
12 months or more, Fair Value	180	266
12 months or more, Gross unrealized losses	1	1
Total fair value	815	266
Total gross unrealized losses	3	1
Commercial
Available-for-sale securities
Less than 12 months, Fair value	914	528
Less than 12 months, Gross unrealized losses	11	4
12 months or more, Fair Value	3,018	335
12 months or more, Gross unrealized losses	171	1
Total fair value	3,932	863
Total gross unrealized losses	182	5
Held-to-maturity securities
Less than 12 months, Fair value	0	3,706
Less than 12 months, Gross unrealized losses	0	41
12 months or more, Fair value	0	1,882
12 months or more, Gross unrealized losses	0	33
Total fair value	0	5,588
Total gross unrealized losses	0	74
U.S. Treasury and government agencies
Available-for-sale securities
Less than 12 months, Fair value	4,556	1,834
Less than 12 months, Gross unrealized losses	100	11
12 months or more, Fair Value	1,416	373
12 months or more, Gross unrealized losses	96	20
Total fair value	5,972	2,207
Total gross unrealized losses	196	31
Obligations of U.S. states and municipalities
Available-for-sale securities
Less than 12 months, Fair value	4,171	949
Less than 12 months, Gross unrealized losses	63	7
12 months or more, Fair Value	1,291	1,652
12 months or more, Gross unrealized losses	55	26
Total fair value	5,462	2,601
Total gross unrealized losses	118	33
Held-to-maturity securities
Less than 12 months, Fair value	853	584
Less than 12 months, Gross unrealized losses	10	9
12 months or more, Fair value	677	2,131
12 months or more, Gross unrealized losses	21	71
Total fair value	1,530	2,715
Total gross unrealized losses	31	80
Certificates of deposit
Available-for-sale securities
Less than 12 months, Fair value	0	0
Less than 12 months, Gross unrealized losses	0	0
12 months or more, Fair Value	0	0
12 months or more, Gross unrealized losses	0	0
Total fair value	0	0
Total gross unrealized losses	0	0
Non-U.S. government debt securities
Available-for-sale securities
Less than 12 months, Fair value	4,237	6,500
Less than 12 months, Gross unrealized losses	16	15
12 months or more, Fair Value	1,798	811
12 months or more, Gross unrealized losses	29	17
Total fair value	6,035	7,311
Total gross unrealized losses	45	32
Corporate debt securities
Available-for-sale securities
Less than 12 months, Fair value	0	0
Less than 12 months, Gross unrealized losses	0	0
12 months or more, Fair Value	38	52
12 months or more, Gross unrealized losses	1	1
Total fair value	38	52
Total gross unrealized losses	1	1
Asset-backed securities: Collateralized loan obligations
Available-for-sale securities
Less than 12 months, Fair value	10,267	0
Less than 12 months, Gross unrealized losses	42	0
12 months or more, Fair Value	0	276
12 months or more, Gross unrealized losses	0	1
Total fair value	10,267	276
Total gross unrealized losses	42	1
Asset-backed securities: Other
Available-for-sale securities
Less than 12 months, Fair value	2,018	3,521
Less than 12 months, Gross unrealized losses	6	20
12 months or more, Fair Value	2,545	720
12 months or more, Gross unrealized losses	21	4
Total fair value	4,563	4,241
Total gross unrealized losses	$ 27	$ 24