SECURITIES - Schedule of Investments in Continuous Loss Positions (Details) $ in Millions	Dec. 31, 2017 USD ($) Securities	Dec. 31, 2016 USD ($) Securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, number of issues \| Securities	300	328
Less than 12 months, fair value	$ 10,218	$ 15,403
Less than 12 months, gross unrealized losses	$ (98)	$ (292)
12 months or longer, number of issues \| Securities	162	45
12 months or longer, fair value	$ 8,145	$ 763
12 months or longer, gross unrealized losses	$ (232)	$ (44)
Total, number of issues \| Securities	462	373
Total, fair value	$ 18,363	$ 16,166
Total, gross unrealized losses	$ (330)	$ (336)
State and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, number of issues \| Securities		1
Less than 12 months, fair value		$ 8
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		$ 8
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	294	323
Less than 12 months, fair value	$ 10,163	$ 15,387
Less than 12 months, gross unrealized losses	$ (97)	$ (292)
12 months or longer, number of issues \| Securities	152	25
12 months or longer, fair value	$ 8,061	$ 461
12 months or longer, gross unrealized losses	$ (226)	$ (16)
Total, number of issues \| Securities	446	348
Total, fair value	$ 18,224	$ 15,848
Total, gross unrealized losses	$ (323)	$ (308)
Other/non-agency
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, number of issues \| Securities	6	4
Less than 12 months, fair value	$ 55	$ 8
Less than 12 months, gross unrealized losses	$ (1)	$ 0
12 months or longer, number of issues \| Securities	10	20
12 months or longer, fair value	$ 84	$ 302
12 months or longer, gross unrealized losses	$ (6)	$ (28)
Total, number of issues \| Securities	16	24
Total, fair value	$ 139	$ 310
Total, gross unrealized losses	$ (7)	$ (28)
Total mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, number of issues \| Securities	300	327
Less than 12 months, fair value	$ 10,218	$ 15,395
Less than 12 months, gross unrealized losses	$ (98)	$ (292)
12 months or longer, number of issues \| Securities	162	45
12 months or longer, fair value	$ 8,145	$ 763
12 months or longer, gross unrealized losses	$ (232)	$ (44)
Total, number of issues \| Securities	462	372
Total, fair value	$ 18,363	$ 16,158
Total, gross unrealized losses	$ (330)	$ (336)

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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Number of investment positions in available-for-sale investments in a continuous unrealized loss position for less than one year or normal operating cycle, if longer, for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

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