INVESTMENTS - Debt Securities in HTM in Unrecognized Loss Position (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months	$ 9,797	$ 7,391
12 months or longer	15,866	9,647
Total	25,663	17,038
Held-to-maturity Securities, Continuous Unrealized Loss Position, Gross Recognized Losses
Less than 12 months	134	76
12 months or longer	566	194
Total	700	270
Unrealized (loss), other than temporary impairment, not credit loss, recorded in AOCI	(71)	(117)
Mortgage-backed securities
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months	6,889	6,244
12 months or longer	15,108	8,678
Total	21,997	14,922
Held-to-maturity Securities, Continuous Unrealized Loss Position, Gross Recognized Losses
Less than 12 months	98	50
12 months or longer	455	107
Total	553	157
State and municipal securities
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months	1,528	353
12 months or longer	758	835
Total	2,286	1,188
Held-to-maturity Securities, Continuous Unrealized Loss Position, Gross Recognized Losses
Less than 12 months	21	5
12 months or longer	111	68
Total	132	73
Foreign government
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months	1,344	723
12 months or longer	0	0
Total	1,344	723
Held-to-maturity Securities, Continuous Unrealized Loss Position, Gross Recognized Losses
Less than 12 months	12	18
12 months or longer	0	0
Total	12	18
Asset-backed securities
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months	36	71
12 months or longer	0	134
Total	36	205
Held-to-maturity Securities, Continuous Unrealized Loss Position, Gross Recognized Losses
Less than 12 months	3	3
12 months or longer	0	19
Total	$ 3	$ 22