SECURITIES - Schedule of Investments in Continuous Loss Positions (Details) $ in Millions	Mar. 31, 2016 USD ($) Securities	Dec. 31, 2015 USD ($) 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	22	166
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 642	$ 7,456
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Loss	$ (1)	$ (51)
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Issues | Securities	53	56
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	$ 1,071	$ 1,180
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Gross Unrealized Loss	$ (52)	$ (64)
Securities Available-for-sale, Continuous Unrealized Loss Position, Number of Issues | Securities	75	222
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	$ 1,713	$ 8,636
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	$ (53)	$ (115)
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
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value		$ 9
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		$ 9
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss		$ 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	1
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 9	$ 15
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	$ 9	$ 15
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	15	162
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 614	$ 7,423
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Loss	$ (1)	$ (51)
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Issues | Securities	33	36
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	$ 727	$ 819
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Gross Unrealized Loss	$ (12)	$ (27)
Securities Available-for-sale, Continuous Unrealized Loss Position, Number of Issues | Securities	48	198
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	$ 1,341	$ 8,242
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	$ (13)	$ (78)
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	6	2
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 19	$ 9
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	20	20
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	$ 344	$ 361
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Gross Unrealized Loss	$ (40)	$ (37)
Securities Available-for-sale, Continuous Unrealized Loss Position, Number of Issues | Securities	26	22
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	$ 363	$ 370
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	$ (40)	$ (37)
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	21	164
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 633	$ 7,432
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Loss	$ (1)	$ (51)
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Issues | Securities	53	56
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	$ 1,071	$ 1,180
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Gross Unrealized Loss	$ (52)	$ (64)
Securities Available-for-sale, Continuous Unrealized Loss Position, Number of Issues | Securities	74	220
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	$ 1,704	$ 8,612
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	$ (53)	$ (115)