Investment Securities - Amortized Costs and Estimated Fair Values (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Available-for-sale securities
Amortized cost	$ 199,854	$ 198,194
Gross unrealized gains	3,872	3,961
Gross unrealized losses	1,717	477
Fair value	202,009	201,678
Available-for-sale equity securities
Amortized cost	0	547
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair value	0	547
Total available-for-sale securities
Amortized cost	199,854	198,741
Gross unrealized gains	3,872	3,961
Gross unrealized losses	1,717	477
Fair value	202,009	202,225
Held-to-maturity securities
Amortized cost	31,006	47,733
Gross unrealized gains	156	1,113
Gross unrealized losses	289	194
Fair value	30,873	48,652
Total investment securities
Amortized cost	230,860	246,474
Gross unrealized gains	4,028	5,074
Gross unrealized losses	2,006	671
Fair value	232,882	250,877
Total mortgage-backed securities
Available-for-sale securities
Amortized cost	80,009	85,886
Gross unrealized gains	855	1,141
Gross unrealized losses	1,435	355
Fair value	79,429	86,672
Held-to-maturity securities
Amortized cost	26,168	33,360
Gross unrealized gains	60	559
Gross unrealized losses	269	114
Fair value	25,959	33,805
U.S. government agencies
Available-for-sale securities
Amortized cost	62,595	69,879
Gross unrealized gains	521	736
Gross unrealized losses	1,194	335
Fair value	61,922	70,280
Held-to-maturity securities
Amortized cost	26,168	27,577
Gross unrealized gains	60	558
Gross unrealized losses	269	40
Fair value	25,959	28,095
Residential: U.S. \| Residential mortgage-backed securities
Available-for-sale securities
Amortized cost	6,996	8,193
Gross unrealized gains	152	185
Gross unrealized losses	30	14
Fair value	7,118	8,364
Residential: Non-U.S. \| Residential mortgage-backed securities
Available-for-sale securities
Amortized cost	2,457	2,882
Gross unrealized gains	107	122
Gross unrealized losses	2	1
Fair value	2,562	3,003
Commercial
Available-for-sale securities
Amortized cost	7,961	4,932
Gross unrealized gains	75	98
Gross unrealized losses	209	5
Fair value	7,827	5,025
Held-to-maturity securities
Amortized cost	0	5,783
Gross unrealized gains	0	1
Gross unrealized losses	0	74
Fair value	0	5,710
U.S. Treasury and government agencies
Available-for-sale securities
Amortized cost	24,962	22,510
Gross unrealized gains	524	266
Gross unrealized losses	142	31
Fair value	25,344	22,745
Obligations of U.S. states and municipalities
Available-for-sale securities
Amortized cost	37,459	30,490
Gross unrealized gains	1,926	1,881
Gross unrealized losses	55	33
Fair value	39,330	32,338
Held-to-maturity securities
Amortized cost	4,838	14,373
Gross unrealized gains	96	554
Gross unrealized losses	20	80
Fair value	4,914	14,847
Certificates of deposit
Available-for-sale securities
Amortized cost	75	59
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair value	75	59
Non-U.S. government debt securities
Available-for-sale securities
Amortized cost	25,307	26,900
Gross unrealized gains	413	426
Gross unrealized losses	34	32
Fair value	25,686	27,294
Corporate debt securities
Available-for-sale securities
Amortized cost	2,078	2,657
Gross unrealized gains	58	101
Gross unrealized losses	3	1
Fair value	2,133	2,757
Asset-backed securities: Collateralized loan obligations
Available-for-sale securities
Amortized cost	21,145	20,928
Gross unrealized gains	22	69
Gross unrealized losses	21	1
Fair value	21,146	20,996
Asset-backed securities: Other
Available-for-sale securities
Amortized cost	8,819	8,764
Gross unrealized gains	74	77
Gross unrealized losses	27	24
Fair value	8,866	8,817
U.S. government-sponsored enterprise obligations
Total available-for-sale securities
Fair value	39,100	45,800
Held-to-maturity securities
Amortized cost	$ 20,100	$ 22,000

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