SECURITIES - Schedule of Investments in Continuous Loss Positions (Details) $ in Millions	Dec. 31, 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	166	81
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 7,456	$ 3,362
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Loss	$ (51)	$ (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	$ 1,180	$ 2,173
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Gross Unrealized Loss	$ (64)	$ (90)
Securities Available-for-sale, Continuous Unrealized Loss Position, Number of Issues | Securities	222	151
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	$ 8,636	$ 5,535
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	$ (115)	$ (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
U.S. Treasury and other
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
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 15
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Loss	$ 0
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Issues | Securities	0
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	$ 0
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Gross Unrealized Loss	$ 0
Securities Available-for-sale, Continuous Unrealized Loss Position, Number of Issues | Securities	1
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	$ 15
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	$ 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	162	75
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 7,423	$ 3,282
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Loss	$ (51)	$ (24)
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Issues | Securities	36	52
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	$ 819	$ 1,766
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Gross Unrealized Loss	$ (27)	$ (57)
Securities Available-for-sale, Continuous Unrealized Loss Position, Number of Issues | Securities	198	127
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	$ 8,242	$ 5,048
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	$ (78)	$ (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	2	6
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 9	$ 80
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Loss	$ 0	$ (2)
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Issues | Securities	20	17
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	$ 361	$ 397
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Gross Unrealized Loss	$ (37)	$ (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	$ 370	$ 477
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	$ (37)	$ (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	164	81
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 7,432	$ 3,362
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Loss	$ (51)	$ (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	$ 1,180	$ 2,163
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Gross Unrealized Loss	$ (64)	$ (90)
Securities Available-for-sale, Continuous Unrealized Loss Position, Number of Issues | Securities	220	150
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	$ 8,612	$ 5,525
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	$ (115)	$ (116)