SECURITIES - Schedule of Investments in Continuous Loss Positions (Details) $ in Millions	Mar. 31, 2017 USD ($) Securities	Dec. 31, 2016 USD ($) Securities
Schedule of Available-for-sale Securities [Line Items]
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Issues | Securities	331	328
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 15,344	$ 15,403
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Loss	$ (306)	$ (292)
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Issues | Securities	41	45
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	$ 678	$ 763
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Gross Unrealized Loss	$ (31)	$ (44)
Securities Available-for-sale, Continuous Unrealized Loss Position, Number of Issues | Securities	372	373
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	$ 16,022	$ 16,166
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	$ (337)	$ (336)
State and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Issues | Securities	1	1
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 7	$ 8
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Loss	$ 0	$ 0
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Issues | Securities	0	0
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	$ 0	$ 0
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Gross Unrealized Loss	$ 0	$ 0
Securities Available-for-sale, Continuous Unrealized Loss Position, Number of Issues | Securities	1	1
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	$ 7	$ 8
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	$ 0	$ 0
Federal agencies and U.S. government sponsored entities
Schedule of Available-for-sale Securities [Line Items]
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Issues | Securities	323	323
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 15,287	$ 15,387
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Loss	$ (305)	$ (292)
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Issues | Securities	24	25
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	$ 449	$ 461
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Gross Unrealized Loss	$ (13)	$ (16)
Securities Available-for-sale, Continuous Unrealized Loss Position, Number of Issues | Securities	347	348
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	$ 15,736	$ 15,848
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	$ (318)	$ (308)
Other/non-agency
Schedule of Available-for-sale Securities [Line Items]
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Issues | Securities	7	4
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 50	$ 8
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Loss	$ (1)	$ 0
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Issues | Securities	17	20
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	$ 229	$ 302
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Gross Unrealized Loss	$ (18)	$ (28)
Securities Available-for-sale, Continuous Unrealized Loss Position, Number of Issues | Securities	24	24
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	$ 279	$ 310
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	$ (19)	$ (28)
Total mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Issues | Securities	330	327
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 15,337	$ 15,395
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Loss	$ (306)	$ (292)
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Issues | Securities	41	45
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	$ 678	$ 763
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Gross Unrealized Loss	$ (31)	$ (44)
Securities Available-for-sale, Continuous Unrealized Loss Position, Number of Issues | Securities	371	372
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	$ 16,015	$ 16,158
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	$ (337)	$ (336)