SECURITIES - Schedule of Investments in Continuous Loss Positions (Details) $ in Millions	Sep. 30, 2017 USD ($) Securities	Dec. 31, 2016 USD ($) Securities
Number of Issues
Less than 12 Months | Securities	298	328
12 Months or Longer | Securities	41	45
Total | Securities	339	373
Fair Value
Less than 12 Months	$ 13,113	$ 15,403
12 Months or Longer	666	763
Total	13,779	16,166
Gross Unrealized Losses
Less than 12 Months	(193)	(292)
12 Months or Longer	(26)	(44)
Total	$ (219)	$ (336)
State and political subdivisions
Number of Issues
Less than 12 Months | Securities	0	1
12 Months or Longer | Securities	0	0
Total | Securities	0	1
Fair Value
Less than 12 Months	$ 0	$ 8
12 Months or Longer	0	0
Total	0	8
Gross Unrealized Losses
Less than 12 Months	0	0
12 Months or Longer	0	0
Total	$ 0	$ 0
Federal agencies and U.S. government sponsored entities
Number of Issues
Less than 12 Months | Securities	298	323
12 Months or Longer | Securities	30	25
Total | Securities	328	348
Fair Value
Less than 12 Months	$ 13,113	$ 15,387
12 Months or Longer	545	461
Total	13,658	15,848
Gross Unrealized Losses
Less than 12 Months	(193)	(292)
12 Months or Longer	(18)	(16)
Total	$ (211)	$ (308)
Other/non-agency
Number of Issues
Less than 12 Months | Securities	0	4
12 Months or Longer | Securities	11	20
Total | Securities	11	24
Fair Value
Less than 12 Months	$ 0	$ 8
12 Months or Longer	121	302
Total	121	310
Gross Unrealized Losses
Less than 12 Months	0	0
12 Months or Longer	(8)	(28)
Total	$ (8)	$ (28)
Total mortgage-backed securities
Number of Issues
Less than 12 Months | Securities	298	327
12 Months or Longer | Securities	41	45
Total | Securities	339	372
Fair Value
Less than 12 Months	$ 13,113	$ 15,395
12 Months or Longer	666	763
Total	13,779	16,158
Gross Unrealized Losses
Less than 12 Months	(193)	(292)
12 Months or Longer	(26)	(44)
Total	$ (219)	$ (336)