INVESTMENTS - Debt Securities in HTM in Unrecognized Loss Position (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months	$ 18,353	$ 17,814
12 months or longer	19,104	19,328
Total	37,457	37,142
Held-to-maturity Securities, Continuous Unrealized Loss Position, Gross Recognized Losses
Less than 12 months	114	190
12 months or longer	351	663
Total	465	853
Unrealized loss, other than temporary impairment, not credit loss, recorded in AOCI	(683)	(653)
Mortgage-backed securities
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months	4,577	2,822
12 months or longer	18,150	18,086
Total	22,727	20,908
Held-to-maturity Securities, Continuous Unrealized Loss Position, Gross Recognized Losses
Less than 12 months	11	20
12 months or longer	287	559
Total	298	579
State and municipal securities
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months	206	981
12 months or longer	954	1,242
Total	1,160	2,223
Held-to-maturity Securities, Continuous Unrealized Loss Position, Gross Recognized Losses
Less than 12 months	6	34
12 months or longer	64	104
Total	70	138
Foreign government
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months	989	1,003
12 months or longer	0	0
Total	989	1,003
Held-to-maturity Securities, Continuous Unrealized Loss Position, Gross Recognized Losses
Less than 12 months	11	24
12 months or longer	0	0
Total	11	24
Asset-backed securities
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months	12,581	13,008
12 months or longer	0	0
Total	12,581	13,008
Held-to-maturity Securities, Continuous Unrealized Loss Position, Gross Recognized Losses
Less than 12 months	86	112
12 months or longer	0	0
Total	$ 86	$ 112