INVESTMENTS - Fair Value of AFS Debt Securities by Contractual Maturity Date (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Amoritzed cost
Amortized cost	$ 289,188	$ 292,220
Fair value
Total fair value	284,782	290,725
Mortgage-backed securities
Amoritzed cost
Amortized cost, due within 1 year	434	45
Amortized cost, after 1 but within 5 years	1,201	1,306
Amortized cost, after 5 but within 10 years	2,159	1,376
Amortized cost, after 10 years	44,735	42,504
Amortized cost	48,529	45,231
Fair value
Fair value, due within 1 year	431	45
Fair value, after 1 but within 5 years	1,194	1,304
Fair value, after 5 but within 10 years	2,119	1,369
Fair value, after 10 years	43,275	42,239
Total fair value	47,019	44,957
U.S. Treasury and federal agency securities
Amoritzed cost
Amortized cost, due within 1 year	34,543	4,913
Amortized cost, after 1 but within 5 years	81,735	111,236
Amortized cost, after 5 but within 10 years	1,893	3,008
Amortized cost, after 10 years	90	0
Amortized cost	118,261	119,157
Fair value
Fair value, due within 1 year	34,471	4,907
Fair value, after 1 but within 5 years	79,739	110,238
Fair value, after 5 but within 10 years	1,842	3,001
Fair value, after 10 years	91	0
Total fair value	116,143	118,146
State and municipal securities
Amoritzed cost
Amortized cost, due within 1 year	2,773	1,792
Amortized cost, after 1 but within 5 years	1,575	2,579
Amortized cost, after 5 but within 10 years	572	514
Amortized cost, after 10 years	4,742	3,985
Amortized cost	9,662	8,870
Fair value
Fair value, due within 1 year	2,772	1,792
Fair value, after 1 but within 5 years	1,570	2,576
Fair value, after 5 but within 10 years	590	528
Fair value, after 10 years	4,548	3,869
Total fair value	9,480	8,765
Foreign government
Amoritzed cost
Amortized cost, due within 1 year	34,686	32,130
Amortized cost, after 1 but within 5 years	47,933	53,034
Amortized cost, after 5 but within 10 years	10,371	12,949
Amortized cost, after 10 years	1,947	2,502
Amortized cost	94,937	100,615
Fair value
Fair value, due within 1 year	34,649	32,100
Fair value, after 1 but within 5 years	47,416	53,165
Fair value, after 5 but within 10 years	10,386	12,680
Fair value, after 10 years	2,010	2,588
Total fair value	94,461	100,533
All other
Amoritzed cost
Amortized cost, due within 1 year	6,439	3,998
Amortized cost, after 1 but within 5 years	9,151	9,047
Amortized cost, after 5 but within 10 years	1,614	3,415
Amortized cost, after 10 years	595	1,887
Amortized cost	17,799	18,347
Fair value
Fair value, due within 1 year	6,435	3,991
Fair value, after 1 but within 5 years	9,068	9,027
Fair value, after 5 but within 10 years	1,603	3,431
Fair value, after 10 years	573	1,875
Total fair value	$ 17,679	$ 18,324