INVESTMENTS - Debt Securities in HTM in Unrecognized Loss Position (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months	$ 20,906	$ 9,716
12 months or longer	10,616	7,322
Total	31,522	17,038
Held-to-maturity Securities, Continuous Unrealized Loss Position, Gross Recognized Losses
Less than 12 months	475	76
12 months or longer	604	194
Total	1,079	270
Unrealized loss, other than temporary impairment, not credit loss, recorded in AOCI	65	117
Mortgage-backed securities
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months	13,815	8,569
12 months or longer	9,815	6,353
Total	23,630	14,922
Held-to-maturity Securities, Continuous Unrealized Loss Position, Gross Recognized Losses
Less than 12 months	392	50
12 months or longer	477	107
Total	869	157
State and municipal securities
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months	2,283	353
12 months or longer	799	835
Total	3,082	1,188
Held-to-maturity Securities, Continuous Unrealized Loss Position, Gross Recognized Losses
Less than 12 months	58	5
12 months or longer	127	68
Total	185	73
Foreign government
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months	1,138	723
12 months or longer	0	0
Total	1,138	723
Held-to-maturity Securities, Continuous Unrealized Loss Position, Gross Recognized Losses
Less than 12 months	14	18
12 months or longer	0	0
Total	14	18
Asset-backed securities
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months	3,670	71
12 months or longer	2	134
Total	3,672	205
Held-to-maturity Securities, Continuous Unrealized Loss Position, Gross Recognized Losses
Less than 12 months	11	3
12 months or longer	0	19
Total	$ 11	$ 22