INVESTMENTS - Fair Value of AFS Debt Securities by Contractual Maturity Date (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Amoritzed cost
Investments	$ 472,959	$ 447,359
Fair value
Total fair value	304,036	335,084
Mortgage-backed securities
Amoritzed cost
Amortized cost, due within 1 year	70	27
Amortized cost, after 1 but within 5 years	387	567
Amortized cost, after 5 but within 10 years	819	688
Amortized cost, after 10 years	41,261	42,171
Amortized cost	42,537	43,453
Fair value
Fair value, due within 1 year	70	27
Fair value, after 1 but within 5 years	389	571
Fair value, after 5 but within 10 years	876	757
Fair value, after 10 years	41,850	43,184
Total fair value	43,185	44,539
U.S. Treasury and federal agency securities
Amoritzed cost
Amortized cost, due within 1 year	29,917	34,834
Amortized cost, after 1 but within 5 years	90,482	108,160
Amortized cost, after 5 but within 10 years	1,222	1,150
Amortized cost, after 10 years	2	0
Amortized cost	121,623	144,144
Fair value
Fair value, due within 1 year	30,048	34,951
Fair value, after 1 but within 5 years	91,455	110,091
Fair value, after 5 but within 10 years	1,195	1,162
Fair value, after 10 years	2	0
Total fair value	122,700	146,204
State and municipal
Amoritzed cost
Amortized cost, due within 1 year	408	427
Amortized cost, after 1 but within 5 years	117	189
Amortized cost, after 5 but within 10 years	240	276
Amortized cost, after 10 years	2,518	2,861
Amortized cost	3,283	3,753
Fair value
Fair value, due within 1 year	408	428
Fair value, after 1 but within 5 years	118	198
Fair value, after 5 but within 10 years	239	267
Fair value, after 10 years	2,486	2,826
Total fair value	3,251	3,719
Foreign government
Amoritzed cost
Amortized cost, due within 1 year	48,334	48,133
Amortized cost, after 1 but within 5 years	63,516	67,365
Amortized cost, after 5 but within 10 years	5,562	5,908
Amortized cost, after 10 years	1,714	2,061
Amortized cost	119,126	123,467
Fair value
Fair value, due within 1 year	48,426	48,258
Fair value, after 1 but within 5 years	63,789	68,586
Fair value, after 5 but within 10 years	5,599	6,011
Fair value, after 10 years	1,800	2,113
Total fair value	119,614	124,968
All other
Amoritzed cost
Amortized cost, due within 1 year	6,332	6,661
Amortized cost, after 1 but within 5 years	7,886	7,814
Amortized cost, after 5 but within 10 years	992	1,018
Amortized cost, after 10 years	94	99
Amortized cost	15,304	15,592
Fair value
Fair value, due within 1 year	6,338	6,665
Fair value, after 1 but within 5 years	7,898	7,891
Fair value, after 5 but within 10 years	979	1,034
Fair value, after 10 years	71	64
Total fair value	$ 15,286	$ 15,654