SECURITIES - Schedule of Investments in Continuous Loss Positions (Details) $ in Millions	Dec. 31, 2016 USD ($) Securities	Dec. 31, 2015 USD ($) Securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, number of issues | Securities	328	166
Less than 12 months, fair value	$ 15,403	$ 7,456
Less than 12 months, gross unrealized losses	$ (292)	$ (51)
12 months or longer, number of issues | Securities	45	56
12 months or longer, fair value	$ 763	$ 1,180
12 months or longer, gross unrealized losses	$ (44)	$ (64)
Total, number of issues | Securities	373	222
Total, fair value	$ 16,166	$ 8,636
Total, gross unrealized losses	$ (336)	$ (115)
State and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, number of issues | Securities	1	1
Less than 12 months, fair value	$ 8	$ 9
Less than 12 months, gross unrealized losses	$ 0	$ 0
12 months or longer, number of issues | Securities	0	0
12 months or longer, fair value	$ 0	$ 0
12 months or longer, gross unrealized losses	$ 0	$ 0
Total, number of issues | Securities	1	1
Total, fair value	$ 8	$ 9
Total, gross unrealized losses	$ 0	$ 0
U.S. Treasury and other
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, number of issues | Securities		1
Less than 12 months, fair value		$ 15
Less than 12 months, gross unrealized losses		$ 0
12 months or longer, number of issues | Securities		0
12 months or longer, fair value		$ 0
12 months or longer, gross unrealized losses		$ 0
Total, number of issues | Securities		1
Total, fair value		$ 15
Total, gross unrealized losses		$ 0
Federal agencies and U.S. government sponsored entities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, number of issues | Securities	323	162
Less than 12 months, fair value	$ 15,387	$ 7,423
Less than 12 months, gross unrealized losses	$ (292)	$ (51)
12 months or longer, number of issues | Securities	25	36
12 months or longer, fair value	$ 461	$ 819
12 months or longer, gross unrealized losses	$ (16)	$ (27)
Total, number of issues | Securities	348	198
Total, fair value	$ 15,848	$ 8,242
Total, gross unrealized losses	$ (308)	$ (78)
Other/non-agency
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, number of issues | Securities	4	2
Less than 12 months, fair value	$ 8	$ 9
Less than 12 months, gross unrealized losses	$ 0	$ 0
12 months or longer, number of issues | Securities	20	20
12 months or longer, fair value	$ 302	$ 361
12 months or longer, gross unrealized losses	$ (28)	$ (37)
Total, number of issues | Securities	24	22
Total, fair value	$ 310	$ 370
Total, gross unrealized losses	$ (28)	$ (37)
Total mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, number of issues | Securities	327	164
Less than 12 months, fair value	$ 15,395	$ 7,432
Less than 12 months, gross unrealized losses	$ (292)	$ (51)
12 months or longer, number of issues | Securities	45	56
12 months or longer, fair value	$ 763	$ 1,180
12 months or longer, gross unrealized losses	$ (44)	$ (64)
Total, number of issues | Securities	372	220
Total, fair value	$ 16,158	$ 8,612
Total, gross unrealized losses	$ (336)	$ (115)