SECURITIES - Schedule of Investments in Continuous Loss Positions (Details) $ in Millions	Jun. 30, 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	11	166
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 114	$ 7,456
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Loss	$ 0	$ (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	$ 1,064	$ 1,180
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Gross Unrealized Loss	$ (42)	$ (64)
Securities Available-for-sale, Continuous Unrealized Loss Position, Number of Issues | Securities	64	222
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	$ 1,178	$ 8,636
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	$ (42)	$ (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
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	7	162
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 95	$ 7,423
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Loss	$ 0	$ (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	$ 734	$ 819
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Gross Unrealized Loss	$ (8)	$ (27)
Securities Available-for-sale, Continuous Unrealized Loss Position, Number of Issues | Securities	40	198
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	$ 829	$ 8,242
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	$ (8)	$ (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	4	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	$ 330	$ 361
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Gross Unrealized Loss	$ (34)	$ (37)
Securities Available-for-sale, Continuous Unrealized Loss Position, Number of Issues | Securities	24	22
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	$ 349	$ 370
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	$ (34)	$ (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	11	164
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 114	$ 7,432
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Loss	$ 0	$ (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	$ 1,064	$ 1,180
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Gross Unrealized Loss	$ (42)	$ (64)
Securities Available-for-sale, Continuous Unrealized Loss Position, Number of Issues | Securities	64	220
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	$ 1,178	$ 8,612
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	$ (42)	$ (115)