INVESTMENTS - Debt Securities Held-to-Maturity (Details) - USD ($)	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Debt Securities Held-to-maturity
Carrying value, net	$ 83,332,000,000		$ 80,775,000,000
Gross unrealized gains	3,176,000,000		1,557,000,000
Gross unrealized losses	500,000,000		109,000,000
Fair value, net	86,008,000,000		82,223,000,000
Allowance for credit losses on HTM debt securities	107,000,000	$ 76,000,000	0
Mortgage-backed securities - U.S. government-sponsored agency guaranteed
Debt Securities Held-to-maturity
Carrying value, net	49,649,000,000		46,637,000,000
Gross unrealized gains	2,442,000,000		1,047,000,000
Gross unrealized losses	6,000,000		21,000,000
Fair value, net	52,085,000,000		47,663,000,000
Mortgage-backed securities - Non-U.S. residential
Debt Securities Held-to-maturity
Carrying value, net	1,083,000,000		1,039,000,000
Gross unrealized gains	0		5,000,000
Gross unrealized losses	6,000,000		0
Fair value, net	1,077,000,000		1,044,000,000
Commercial
Debt Securities Held-to-maturity
Carrying value, net	673,000,000		582,000,000
Gross unrealized gains	1,000,000		1,000,000
Gross unrealized losses	1,000,000		0
Fair value, net	673,000,000		583,000,000
Mortgage-backed securities
Debt Securities Held-to-maturity
Carrying value, net	51,405,000,000		48,258,000,000
Gross unrealized gains	2,443,000,000		1,053,000,000
Gross unrealized losses	13,000,000		21,000,000
Fair value, net	53,835,000,000		49,290,000,000
State and municipal
Debt Securities Held-to-maturity
Carrying value, net	9,152,000,000		9,104,000,000
Gross unrealized gains	650,000,000		455,000,000
Gross unrealized losses	16,000,000		28,000,000
Fair value, net	9,786,000,000		9,531,000,000
Foreign government
Debt Securities Held-to-maturity
Carrying value, net	1,237,000,000		1,934,000,000
Gross unrealized gains	78,000,000		37,000,000
Gross unrealized losses	0		1,000,000
Fair value, net	1,315,000,000		1,970,000,000
Asset-backed securities
Debt Securities Held-to-maturity
Carrying value, net	21,538,000,000		21,479,000,000
Gross unrealized gains	5,000,000		12,000,000
Gross unrealized losses	471,000,000		59,000,000
Fair value, net	$ 21,072,000,000		$ 21,432,000,000