INVESTMENTS - Carrying Value and Fair Value of HTM Debt Securities by Contractual Maturity Dates (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Carrying value	$ 53,249	$ 53,320
Held-to-maturity Securities, Debt Maturities, Fair Value;
Fair value	52,343	53,752
Mortgage-backed securities
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	129	720
After 5 but within 10 years, carrying value	101	148
After 10 years, carrying value	26,376	25,231
Carrying value	26,606	26,099
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	0	0
After 1 but within 5 years, fair value	127	720
After 5 but within 10 years, fair value	99	149
After 10 years, fair value	25,533	25,235
Fair value	25,759	26,104
State and municipal securities
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, carrying value	31	407
After 1 but within 5 years, carrying value	131	259
After 5 but within 10 years, carrying value	492	512
After 10 years, carrying value	6,745	7,719
Carrying value	7,399	8,897
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	31	425
After 1 but within 5 years, fair value	133	270
After 5 but within 10 years, fair value	495	524
After 10 years, fair value	6,679	7,983
Fair value	7,338	9,202
Foreign government
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, carrying value	114	381
After 1 but within 5 years, carrying value	1,037	359
After 5 but within 10 years, carrying value	0	0
After 10 years, carrying value	0	0
Carrying value	1,151	740
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	114	381
After 1 but within 5 years, fair value	1,023	341
After 5 but within 10 years, fair value	0	0
After 10 years, fair value	0	0
Fair value	1,137	722
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	2,244	1,669
After 10 years, carrying value	15,849	15,915
Carrying value	18,093	17,584
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	2,250	1,680
After 10 years, fair value	15,859	16,044
Fair value	$ 18,109	$ 17,724

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

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