INVESTMENTS - Fair Value of AFS Debt Securities by Contractual Maturity Date (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Total amortized cost	$ 290,391	$ 299,892
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Total fair value	289,891	299,033
Mortgage-backed securities
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Due within 1 year, amortized cost	127	132
After 1 but within 5 years, amortized cost	633	736
After 5 but within 10 years, amortized cost	1,888	2,279
After 10 years, amortized cost	37,119	39,770
Total amortized cost	39,767	42,917
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Fair value, due within 1 year	127	132
Fair value, after 1 but within 5 years	635	738
Fair value, after 5 but within 10 years	1,877	2,265
Fair value, after 10 years	36,898	39,538
Total fair value	39,537	42,673
U.S. Treasury and federal agency securities
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Due within 1 year, amortized cost	4,182	4,945
After 1 but within 5 years, amortized cost	101,314	101,369
After 5 but within 10 years, amortized cost	11,920	17,153
After 10 years, amortized cost	37	91
Total amortized cost	117,453	123,558
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Fair value, due within 1 year	4,161	4,945
Fair value, after 1 but within 5 years	101,367	101,323
Fair value, after 5 but within 10 years	12,082	17,314
Fair value, after 10 years	37	89
Total fair value	117,647	123,671
State and municipal securities
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Due within 1 year, amortized cost	2,057	2,093
After 1 but within 5 years, amortized cost	2,675	2,668
After 5 but within 10 years, amortized cost	319	335
After 10 years, amortized cost	5,177	5,701
Total amortized cost	10,228	10,797
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Fair value, due within 1 year	2,057	2,092
Fair value, after 1 but within 5 years	2,674	2,662
Fair value, after 5 but within 10 years	316	334
Fair value, after 10 years	4,578	5,032
Total fair value	9,625	10,120
Foreign government
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Due within 1 year, amortized cost	31,111	32,540
After 1 but within 5 years, amortized cost	53,827	51,008
After 5 but within 10 years, amortized cost	12,402	12,388
After 10 years, amortized cost	2,634	2,176
Total amortized cost	99,974	98,112
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Fair value, due within 1 year	31,140	32,547
Fair value, after 1 but within 5 years	53,747	50,881
Fair value, after 5 but within 10 years	12,478	12,440
Fair value, after 10 years	2,765	2,280
Total fair value	100,130	98,148
All other
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Due within 1 year, amortized cost	3,443	2,629
After 1 but within 5 years, amortized cost	10,306	12,339
After 5 but within 10 years, amortized cost	6,250	6,566
After 10 years, amortized cost	2,970	2,974
Total amortized cost	22,969	24,508
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Fair value, due within 1 year	3,243	2,628
Fair value, after 1 but within 5 years	10,526	12,334
Fair value, after 5 but within 10 years	6,242	6,528
Fair value, after 10 years	2,941	2,931
Total fair value	$ 22,952	$ 24,421