Investment Securities - Amortized Costs and Estimated Fair Values (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Available-for-sale securities
Amortized cost	$ 198,966	$ 198,194
Gross unrealized gains	3,222	3,961
Gross unrealized losses	2,158	477
Fair value	200,030	201,678
Available-for-sale equity securities
Amortized cost		547
Gross unrealized gains		0
Gross unrealized losses		0
Fair value		547
Total available-for-sale securities
Amortized cost		198,741
Gross unrealized gains		3,961
Gross unrealized losses		477
Fair value		202,225
Held-to-maturity securities
Amortized cost	31,368	47,733
Gross unrealized gains	74	1,113
Gross unrealized losses	524	194
Fair value	30,918	48,652
Total investment securities
Amortized cost	230,334	246,474
Gross unrealized gains	3,296	5,074
Gross unrealized losses	2,682	671
Fair value	230,948	250,877
Total mortgage-backed securities
Available-for-sale securities
Amortized cost	80,415	85,886
Gross unrealized gains	689	1,141
Gross unrealized losses	1,729	355
Fair value	79,375	86,672
Held-to-maturity securities
Amortized cost	26,537	33,360
Gross unrealized gains	5	559
Gross unrealized losses	493	114
Fair value	26,049	33,805
U.S. government agencies
Available-for-sale securities
Amortized cost	64,229	69,879
Gross unrealized gains	389	736
Gross unrealized losses	1,508	335
Fair value	63,110	70,280
Held-to-maturity securities
Amortized cost	26,537	27,577
Gross unrealized gains	5	558
Gross unrealized losses	493	40
Fair value	26,049	28,095
Residential: U.S.
Available-for-sale securities
Amortized cost	6,396	8,193
Gross unrealized gains	127	185
Gross unrealized losses	36	14
Fair value	6,487	8,364
Residential: Non-U.S.
Available-for-sale securities
Amortized cost	2,639	2,882
Gross unrealized gains	94	122
Gross unrealized losses	3	1
Fair value	2,730	3,003
Commercial
Available-for-sale securities
Amortized cost	7,151	4,932
Gross unrealized gains	79	98
Gross unrealized losses	182	5
Fair value	7,048	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	27,526	22,510
Gross unrealized gains	486	266
Gross unrealized losses	196	31
Fair value	27,816	22,745
Obligations of U.S. states and municipalities
Available-for-sale securities
Amortized cost	36,659	30,490
Gross unrealized gains	1,580	1,881
Gross unrealized losses	118	33
Fair value	38,121	32,338
Held-to-maturity securities
Amortized cost	4,831	14,373
Gross unrealized gains	69	554
Gross unrealized losses	31	80
Fair value	4,869	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	24,398	26,900
Gross unrealized gains	321	426
Gross unrealized losses	45	32
Fair value	24,674	27,294
Corporate debt securities
Available-for-sale securities
Amortized cost	1,993	2,657
Gross unrealized gains	64	101
Gross unrealized losses	1	1
Fair value	2,056	2,757
Asset-backed securities: Collateralized loan obligations
Available-for-sale securities
Amortized cost	20,139	20,928
Gross unrealized gains	12	69
Gross unrealized losses	42	1
Fair value	20,109	20,996
Asset-backed securities: Other
Available-for-sale securities
Amortized cost	7,761	8,764
Gross unrealized gains	70	77
Gross unrealized losses	27	24
Fair value	7,804	8,817
U.S. government-sponsored enterprise obligations
Available-for-sale securities
Fair value	44,200	45,800
Held-to-maturity securities
Amortized cost	$ 20,600	$ 22,000

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