FAIR VALUE MEASUREMENT - Level 3 Fair Value Rollforward Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Long-term debt
Fair value, assets and liabilities measured on recurring basis, level 3 fair-value category disclosures
Transfers into Level 3, liabilities	$ 181	$ 947	$ 702	$ 2,168
Transfers out of Level 3, assets		1,600	1,500
Transfers out of Level 3, liabilities	490	1,550	1,457	3,393
Trading non-derivative assets
Fair value, assets and liabilities measured on recurring basis, level 3 fair-value category disclosures
Transfers out of Level 3, assets	826	2,373	2,048	5,190
Transfers into Level 3	204	586	1,185	3,296
Trading non-derivative assets | Other
Fair value, assets and liabilities measured on recurring basis, level 3 fair-value category disclosures
Transfers out of Level 3, assets	607	897	1,400	2,379
Transfers into Level 3	38	296	470	1,754
Trading non-derivative assets | Mortgage-backed securities
Fair value, assets and liabilities measured on recurring basis, level 3 fair-value category disclosures
Transfers out of Level 3, assets	47	560	190	1,331
Transfers into Level 3	138	73	308	799
Trading non-derivative assets | State and municipal securities
Fair value, assets and liabilities measured on recurring basis, level 3 fair-value category disclosures
Transfers out of Level 3, assets	0	37	48	253
Transfers into Level 3	0	118	24	136
Investments
Fair value, assets and liabilities measured on recurring basis, level 3 fair-value category disclosures
Transfers out of Level 3, assets	35	418	206	1,334
Transfers into Level 3	36	129	175	631
Investments | Other
Fair value, assets and liabilities measured on recurring basis, level 3 fair-value category disclosures
Transfers out of Level 3, assets	0	0	0	0
Transfers into Level 3	0	10	0	10
Investments | Mortgage-backed securities
Fair value, assets and liabilities measured on recurring basis, level 3 fair-value category disclosures
Transfers out of Level 3, assets	5	14	107	79
Transfers into Level 3	3	52	4	70
Investments | State and municipal securities
Fair value, assets and liabilities measured on recurring basis, level 3 fair-value category disclosures
Transfers out of Level 3, assets	3	338	36	1,121
Transfers into Level 3	$ 21	$ 5	$ 70	$ 396