INVESTMENTS - Debt Securities in HTM in Unrecognized Loss Position (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months	$ 19,599	$ 9,716
12 months or longer	7,163	7,322
Total	26,762	17,038
Held-to-maturity Securities, Continuous Unrealized Loss Position, Gross Recognized Losses
Less than 12 months	449	76
12 months or longer	361	194
Total	810	270
Unrealized loss, other than temporary impairment, not credit loss, recorded in AOCI	69	117
Mortgage-backed securities
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months	16,731	8,569
12 months or longer	5,805	6,353
Total	22,536	14,922
Held-to-maturity Securities, Continuous Unrealized Loss Position, Gross Recognized Losses
Less than 12 months	410	50
12 months or longer	251	107
Total	661	157
State and municipal securities
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months	1,518	353
12 months or longer	747	835
Total	2,265	1,188
Held-to-maturity Securities, Continuous Unrealized Loss Position, Gross Recognized Losses
Less than 12 months	24	5
12 months or longer	108	68
Total	132	73
Foreign government
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months	1,334	723
12 months or longer	0	0
Total	1,334	723
Held-to-maturity Securities, Continuous Unrealized Loss Position, Gross Recognized Losses
Less than 12 months	15	18
12 months or longer	0	0
Total	15	18
Asset-backed securities
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months	16	71
12 months or longer	611	134
Total	627	205
Held-to-maturity Securities, Continuous Unrealized Loss Position, Gross Recognized Losses
Less than 12 months	0	3
12 months or longer	2	19
Total	$ 2	$ 22