INVESTMENTS - Carrying Value and Fair Value of HTM Debt Securities by Contractual Maturity Dates (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Carrying value	$ 80,775	$ 63,357
Held-to-maturity Securities, Debt Maturities, Fair Value;
Fair value	82,223	62,890
Mortgage-backed securities
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, carrying value	17	3
After 1 but within 5 years, carrying value	458	539
After 5 but within 10 years, carrying value	1,662	997
After 10 years, carrying value	46,121	34,407
Carrying value	48,258	35,946
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	17	3
After 1 but within 5 years, fair value	463	540
After 5 but within 10 years, fair value	1,729	1,011
After 10 years, fair value	47,081	34,024
Fair value	49,290	35,578
State and municipal securities
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, carrying value	2	37
After 1 but within 5 years, carrying value	123	168
After 5 but within 10 years, carrying value	597	540
After 10 years, carrying value	8,382	6,883
Carrying value	9,104	7,628
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	26	37
After 1 but within 5 years, fair value	160	174
After 5 but within 10 years, fair value	590	544
After 10 years, fair value	8,755	6,902
Fair value	9,531	7,657
Foreign government
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, carrying value	650	60
After 1 but within 5 years, carrying value	1,284	967
After 5 but within 10 years, carrying value	0	0
After 10 years, carrying value	0	0
Carrying value	1,934	1,027
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	652	36
After 1 but within 5 years, fair value	1,318	967
After 5 but within 10 years, fair value	0	0
After 10 years, fair value	0	0
Fair value	1,970	1,003
All other
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, carrying value	0	0
After 1 but within 5 years, carrying value	0	0
After 5 but within 10 years, carrying value	8,545	2,535
After 10 years, carrying value	12,934	16,221
Carrying value	21,479	18,756
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	0	0
After 1 but within 5 years, fair value	0	0
After 5 but within 10 years, fair value	8,543	2,539
After 10 years, fair value	12,889	16,113
Fair value	$ 21,432	$ 18,652

Amount of investment in debt security measured at amortized cost (held-to-maturity).

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