INVESTMENTS - Fair Value of AFS Debt Securities by Contractual Maturity Date (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Amoritzed cost
Amortized cost	$ 292,340	$ 292,220
Fair value
Total fair value	289,031	290,725
Mortgage-backed securities
Amoritzed cost
Amortized cost, due within 1 year	34	45
Amortized cost, after 1 but within 5 years	1,207	1,306
Amortized cost, after 5 but within 10 years	1,531	1,376
Amortized cost, after 10 years	43,186	42,504
Amortized cost	45,958	45,231
Fair value
Fair value, due within 1 year	34	45
Fair value, after 1 but within 5 years	1,202	1,304
Fair value, after 5 but within 10 years	1,503	1,369
Fair value, after 10 years	42,370	42,239
Total fair value	45,109	44,957
U.S. Treasury and federal agency securities
Amoritzed cost
Amortized cost, due within 1 year	26,550	4,913
Amortized cost, after 1 but within 5 years	91,342	111,236
Amortized cost, after 5 but within 10 years	2,190	3,008
Amortized cost, after 10 years	91	0
Amortized cost	120,173	119,157
Fair value
Fair value, due within 1 year	26,528	4,907
Fair value, after 1 but within 5 years	89,497	110,238
Fair value, after 5 but within 10 years	2,153	3,001
Fair value, after 10 years	93	0
Total fair value	118,271	118,146
State and municipal securities
Amoritzed cost
Amortized cost, due within 1 year	773	1,792
Amortized cost, after 1 but within 5 years	3,460	2,579
Amortized cost, after 5 but within 10 years	564	514
Amortized cost, after 10 years	5,088	3,985
Amortized cost	9,885	8,870
Fair value
Fair value, due within 1 year	773	1,792
Fair value, after 1 but within 5 years	3,457	2,576
Fair value, after 5 but within 10 years	584	528
Fair value, after 10 years	4,950	3,869
Total fair value	9,764	8,765
Foreign government
Amoritzed cost
Amortized cost, due within 1 year	36,246	32,130
Amortized cost, after 1 but within 5 years	47,736	53,034
Amortized cost, after 5 but within 10 years	11,805	12,949
Amortized cost, after 10 years	2,385	2,502
Amortized cost	98,172	100,615
Fair value
Fair value, due within 1 year	36,189	32,100
Fair value, after 1 but within 5 years	47,344	53,165
Fair value, after 5 but within 10 years	11,816	12,680
Fair value, after 10 years	2,476	2,588
Total fair value	97,825	100,533
All other
Amoritzed cost
Amortized cost, due within 1 year	4,881	3,998
Amortized cost, after 1 but within 5 years	10,494	9,047
Amortized cost, after 5 but within 10 years	2,004	3,415
Amortized cost, after 10 years	773	1,887
Amortized cost	18,152	18,347
Fair value
Fair value, due within 1 year	4,879	3,991
Fair value, after 1 but within 5 years	10,420	9,027
Fair value, after 5 but within 10 years	2,011	3,431
Fair value, after 10 years	752	1,875
Total fair value	$ 18,062	$ 18,324