FAIR VALUE MEASUREMENT - Valuation Techniques and Inputs for Level 3 Nonrecurring Fair Value Measurements (Details) - Nonrecurring	3 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Nonrecurring fair value changes included in earnings
Nonrecurring fair value measurements included in earnings	$ 33,000,000	$ 50,000,000
Level 3
Valuation techniques and inputs
Loans held-for-sale	1,435,000,000		$ 1,794,000,000
Level 3 | Price-based
Valuation techniques and inputs
Loans held-for-sale	1,334,000,000		1,729,000,000
Other real estate owned	9,000,000		15,000,000
Loans	27,000,000
Alternative investment funds, fair value			66,000,000
Level 3 | Cash flow
Valuation techniques and inputs
Other real estate owned	3,000,000
Level 3 | Recovery analysis
Valuation techniques and inputs
Other real estate owned			2,000,000
Loans	184,000,000		251,000,000
Price | Level 3 | Price-based | Minimum
Valuation techniques and inputs
Loans held-for-sale	0.77		0.79
Loans	2.60
Non-marketable equity securities measured using the measurement alternative			45.80
Price | Level 3 | Price-based | Maximum
Valuation techniques and inputs
Loans held-for-sale	100.00		100.00
Loans	54.00
Non-marketable equity securities measured using the measurement alternative			1,514.00
Price | Level 3 | Price-based | Weighted Average
Valuation techniques and inputs
Loans held-for-sale	89.97		69.52
Loans	3.40
Non-marketable equity securities measured using the measurement alternative			570.26
Price | Level 3 | Recovery analysis | Minimum
Valuation techniques and inputs
Other real estate owned			56.30
Loans			2.60
Price | Level 3 | Recovery analysis | Maximum
Valuation techniques and inputs
Other real estate owned			83.08
Loans			85.04
Price | Level 3 | Recovery analysis | Weighted Average
Valuation techniques and inputs
Other real estate owned			58.27
Loans			28.21
Appraised value | Level 3 | Price-based | Minimum
Valuation techniques and inputs
Other real estate owned	2,993,681		8,394,102
Appraised value | Level 3 | Price-based | Maximum
Valuation techniques and inputs
Other real estate owned	8,394,102		8,394,102
Appraised value | Level 3 | Price-based | Weighted Average
Valuation techniques and inputs
Other real estate owned	$ 6,974,411		$ 8,394,102
Discount to price | Level 3 | Cash flow | Minimum
Valuation techniques and inputs
Other real estate owned	0.1300
Discount to price | Level 3 | Cash flow | Maximum
Valuation techniques and inputs
Other real estate owned	0.1300
Discount to price | Level 3 | Cash flow | Weighted Average
Valuation techniques and inputs
Other real estate owned	0.1300
Discount to price | Level 3 | Recovery analysis | Minimum
Valuation techniques and inputs
Other real estate owned			0.1300
Discount to price | Level 3 | Recovery analysis | Maximum
Valuation techniques and inputs
Other real estate owned			0.1300
Discount to price | Level 3 | Recovery analysis | Weighted Average
Valuation techniques and inputs
Other real estate owned			0.1300
Recovery rate | Level 3 | Recovery analysis | Minimum
Valuation techniques and inputs
Loans	0.3927		0.3060
Recovery rate | Level 3 | Recovery analysis | Maximum
Valuation techniques and inputs
Loans	0.9926		1.0000
Recovery rate | Level 3 | Recovery analysis | Weighted Average
Valuation techniques and inputs
Loans	0.7825		0.5051
Loans held-for-sale
Nonrecurring fair value changes included in earnings
Nonrecurring fair value measurements included in earnings	$ (2,000,000)	(35,000,000)
Other real estate owned
Nonrecurring fair value changes included in earnings
Nonrecurring fair value measurements included in earnings	1,000,000	(3,000,000)
Loans
Nonrecurring fair value changes included in earnings
Nonrecurring fair value measurements included in earnings	(27,000,000)	(32,000,000)
Non-marketable equity securities measured using the measurement alternative
Nonrecurring fair value changes included in earnings
Nonrecurring fair value measurements included in earnings	61,000,000	$ 120,000,000
Investment Security One | Level 3 | Price-based
Valuation techniques and inputs
Alternative investment funds, fair value	78,000,000
Investment Security One | Price | Level 3 | Price-based | Minimum
Valuation techniques and inputs
Non-marketable equity securities measured using the measurement alternative	33.65
Investment Security One | Price | Level 3 | Price-based | Maximum
Valuation techniques and inputs
Non-marketable equity securities measured using the measurement alternative	2,018.12
Investment Security One | Price | Level 3 | Price-based | Weighted Average
Valuation techniques and inputs
Non-marketable equity securities measured using the measurement alternative	725.12
Investment Security Two | Level 3 | Price-based
Valuation techniques and inputs
Alternative investment funds, fair value	40,000,000
Investment Security Two | Price | Level 3 | Price-based | Minimum
Valuation techniques and inputs
Non-marketable equity securities measured using the measurement alternative	2.84
Investment Security Two | Price | Level 3 | Price-based | Maximum
Valuation techniques and inputs
Non-marketable equity securities measured using the measurement alternative	2.84
Investment Security Two | Price | Level 3 | Price-based | Weighted Average
Valuation techniques and inputs
Non-marketable equity securities measured using the measurement alternative	$ 2.84