SECURITIES - Schedule of Investments in Continuous Loss Positions (Details) $ in Millions	Jun. 30, 2015 USD ($) Securities	Dec. 31, 2014 USD ($) 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	113	81
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 5,268	$ 3,362
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Loss	$ (41)	$ (26)
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Issues | Securities	56	70
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	$ 1,368	$ 2,173
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Gross Unrealized Loss	$ (57)	$ (90)
Securities Available-for-sale, Continuous Unrealized Loss Position, Number of Issues | Securities	169	151
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	$ 6,636	$ 5,535
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	$ (98)	$ (116)
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	0
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 9	$ 0
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	1
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	$ 0	$ 10
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	$ 9	$ 10
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	109	75
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 5,222	$ 3,282
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Loss	$ (40)	$ (24)
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Issues | Securities	37	52
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	$ 970	$ 1,766
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Gross Unrealized Loss	$ (28)	$ (57)
Securities Available-for-sale, Continuous Unrealized Loss Position, Number of Issues | Securities	146	127
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	$ 6,192	$ 5,048
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	$ (68)	$ (81)
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	3	6
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 37	$ 80
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Loss	$ (1)	$ (2)
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Issues | Securities	19	17
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	$ 398	$ 397
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Gross Unrealized Loss	$ (29)	$ (33)
Securities Available-for-sale, Continuous Unrealized Loss Position, Number of Issues | Securities	22	23
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	$ 435	$ 477
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	$ (30)	$ (35)
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	112	81
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 5,259	$ 3,362
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Loss	$ (41)	$ (26)
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Issues | Securities	56	69
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	$ 1,368	$ 2,163
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Gross Unrealized Loss	$ (57)	$ (90)
Securities Available-for-sale, Continuous Unrealized Loss Position, Number of Issues | Securities	168	150
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	$ 6,627	$ 5,525
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	$ (98)	$ (116)