SECURITIES - Schedule of Investments in Continuous Loss Positions (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015 Securities	Dec. 31, 2014 Securities
Schedule of Available-for-sale Securities [Line Items]
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Issues	21	81
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 646	$ 3,362
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Loss	(4)	(26)
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Issues	57	70
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	1,677	2,173
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Gross Unrealized Loss	(56)	(90)
Securities Available-for-sale, Continuous Unrealized Loss Position, Number of Issues	78	151
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	2,323	5,535
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	(60)	(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	1	0
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	10	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	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	1	1
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	10	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	14	75
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	561	3,282
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Loss	(2)	(24)
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Issues	40	52
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	1,292	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	54	127
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	1,853	5,048
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	(29)	(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	6	6
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	75	80
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Loss	(2)	(2)
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Issues	17	17
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	385	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	23	23
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	460	477
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	(31)	(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	20	81
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	636	3,362
Securities Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Loss	(4)	(26)
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Issues	57	69
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	1,677	2,163
Securities Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Gross Unrealized Loss	(56)	(90)
Securities Available-for-sale, Continuous Unrealized Loss Position, Number of Issues	77	150
Securities Available-for-sale, Continuous Unrealized Loss Position, Fair Value	2,313	5,525
Securities Available-for-sale, Continuous Unrealized Loss Position, Gross Unrealized Loss	$ (60)	$ (116)