INVESTMENTS - Fair Value of AFS Debt Securities by Contractual Maturity Date (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Amoritzed cost
Investments	$ 447,524	$ 368,563
Fair value
Total fair value	343,690	280,265
Mortgage-backed securities
Amoritzed cost
Amortized cost, due within 1 year	40	20
Amortized cost, after 1 but within 5 years	572	573
Amortized cost, after 5 but within 10 years	702	594
Amortized cost, after 10 years	40,946	34,640
Amortized cost	42,260	35,827
Fair value
Fair value, due within 1 year	40	20
Fair value, after 1 but within 5 years	575	574
Fair value, after 5 but within 10 years	776	626
Fair value, after 10 years	41,969	34,877
Total fair value	43,360	36,097
U.S. Treasury and federal agency securities
Amoritzed cost
Amortized cost, due within 1 year	51,342	40,757
Amortized cost, after 1 but within 5 years	104,573	70,128
Amortized cost, after 5 but within 10 years	1,742	854
Amortized cost, after 10 years	0	26
Amortized cost	157,657	111,765
Fair value
Fair value, due within 1 year	51,490	40,688
Fair value, after 1 but within 5 years	106,840	69,850
Fair value, after 5 but within 10 years	1,787	851
Fair value, after 10 years	0	29
Total fair value	160,117	111,418
State and municipal
Amoritzed cost
Amortized cost, due within 1 year	396	932
Amortized cost, after 1 but within 5 years	535	714
Amortized cost, after 5 but within 10 years	251	195
Amortized cost, after 10 years	3,022	3,183
Amortized cost	4,204	5,024
Fair value
Fair value, due within 1 year	397	932
Fair value, after 1 but within 5 years	544	723
Fair value, after 5 but within 10 years	276	215
Fair value, after 10 years	2,899	3,108
Total fair value	4,116	4,978
Foreign government
Amoritzed cost
Amortized cost, due within 1 year	47,862	42,611
Amortized cost, after 1 but within 5 years	63,739	58,820
Amortized cost, after 5 but within 10 years	5,766	8,192
Amortized cost, after 10 years	1,894	1,335
Amortized cost	119,261	110,958
Fair value
Fair value, due within 1 year	47,993	42,666
Fair value, after 1 but within 5 years	64,797	59,071
Fair value, after 5 but within 10 years	5,845	8,198
Fair value, after 10 years	1,928	1,368
Total fair value	120,563	111,303
All other
Amoritzed cost
Amortized cost, due within 1 year	5,696	7,306
Amortized cost, after 1 but within 5 years	8,533	8,279
Amortized cost, after 5 but within 10 years	1,120	818
Amortized cost, after 10 years	127	116
Amortized cost	15,476	16,519
Fair value
Fair value, due within 1 year	5,720	7,311
Fair value, after 1 but within 5 years	8,615	8,275
Fair value, after 5 but within 10 years	1,108	797
Fair value, after 10 years	91	86
Total fair value	$ 15,534	$ 16,469