INVESTMENTS - Carrying Value and Fair Value of HTM Debt Securities by Contractual Maturity Dates (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Carrying value	$ 52,897	$ 53,320
Held-to-maturity Securities, Debt Maturities, Fair Value;
Fair value	52,345	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	130	720
After 5 but within 10 years, carrying value	180	148
After 10 years, carrying value	26,249	25,231
Carrying value	26,559	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	128	720
After 5 but within 10 years, fair value	179	149
After 10 years, fair value	25,622	25,235
Fair value	25,929	26,104
State and municipal securities
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, carrying value	67	407
After 1 but within 5 years, carrying value	187	259
After 5 but within 10 years, carrying value	464	512
After 10 years, carrying value	6,762	7,719
Carrying value	7,480	8,897
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	67	425
After 1 but within 5 years, fair value	194	270
After 5 but within 10 years, fair value	468	524
After 10 years, fair value	6,799	7,983
Fair value	7,528	9,202
Foreign government
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, carrying value	362	381
After 1 but within 5 years, carrying value	986	359
After 5 but within 10 years, carrying value	0	0
After 10 years, carrying value	0	0
Carrying value	1,348	740
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	362	381
After 1 but within 5 years, fair value	971	341
After 5 but within 10 years, fair value	0	0
After 10 years, fair value	0	0
Fair value	1,333	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	1,441	1,669
After 10 years, carrying value	16,069	15,915
Carrying value	17,510	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	1,445	1,680
After 10 years, fair value	16,110	16,044
Fair value	$ 17,555	$ 17,724

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

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