FAIR VALUE MEASUREMENT - Valuation Techniques and Inputs for Level 3 Nonrecurring Fair Value Measurements (Details) - Nonrecurring	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Nonrecurring fair value changes included in earnings
Nonrecurring fair value measurements included in earnings	$ (36,000,000)	$ (17,000,000)	$ 31,000,000	$ 66,000,000
Level 3
Valuation techniques and inputs
Loans held-for-sale	1,850,000,000		1,850,000,000		$ 1,794,000,000
Level 3 | Price-based
Valuation techniques and inputs
Loans held-for-sale	1,854,000,000		1,854,000,000		1,729,000,000
Other real estate owned	13,000,000		13,000,000		15,000,000
Non-marketable equity securities measured using the measurement alternative					66,000,000
Level 3 | Recovery analysis
Valuation techniques and inputs
Other real estate owned	3,000,000		3,000,000
Level 3 | Recovery analysis
Valuation techniques and inputs
Other real estate owned					2,000,000
Loans					251,000,000
Price | Level 3 | Price-based
Valuation techniques and inputs
Loans	128,000,000		128,000,000
Price | Level 3 | Price-based | Minimum
Valuation techniques and inputs
Loans held-for-sale	86.00		86.00		80.90
Loans	2.40		2.40
Non-marketable equity securities measured using the measurement alternative	13.36		13.36		45.80
Price | Level 3 | Price-based | Maximum
Valuation techniques and inputs
Loans held-for-sale	100.00		100.00		100.00
Loans	57.00		57.00
Non-marketable equity securities measured using the measurement alternative	13.36		13.36		1,514.00
Price | Level 3 | Price-based | Weighted Average
Valuation techniques and inputs
Loans held-for-sale	98.89		98.89		98.47
Loans	28.91		28.91
Non-marketable equity securities measured using the measurement alternative	13.36		13.36		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	2,196,044		2,196,044		8,394,102
Appraised value | Level 3 | Price-based | Maximum
Valuation techniques and inputs
Other real estate owned	8,394,102		8,394,102		8,394,102
Appraised value | Level 3 | Price-based | Weighted Average
Valuation techniques and inputs
Other real estate owned	5,627,285		5,627,285		$ 8,394,102
Appraised value | Level 3 | Cash flow
Valuation techniques and inputs
Loans	$ 35,000,000		$ 35,000,000
Appraised value | Level 3 | Cash flow | Minimum
Valuation techniques and inputs
Loans, Measurement Input	17,521,218		17,521,218
Appraised value | Level 3 | Cash flow | Maximum
Valuation techniques and inputs
Loans, Measurement Input	43,646,426		43,646,426
Appraised value | Level 3 | Cash flow | Weighted Average
Valuation techniques and inputs
Loans, Measurement Input	30,583,822		30,583,822
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
Valuation techniques and inputs
Loans	$ 123,000,000		$ 123,000,000
Recovery rate | Level 3 | Recovery analysis | Minimum
Valuation techniques and inputs
Loans, Measurement Input	0.0009		0.0009
Recovery rate | Level 3 | Recovery analysis | Maximum
Valuation techniques and inputs
Loans, Measurement Input	0.8860		0.8860
Recovery rate | Level 3 | Recovery analysis | Weighted Average
Valuation techniques and inputs
Loans, Measurement Input	0.4758		0.4758
Recovery rate | Level 3 | Recovery analysis | Minimum
Valuation techniques and inputs
Loans, Measurement Input					0.3060
Recovery rate | Level 3 | Recovery analysis | Maximum
Valuation techniques and inputs
Loans, Measurement Input					1.0000
Recovery rate | Level 3 | Recovery analysis | Weighted Average
Valuation techniques and inputs
Loans, Measurement Input					0.5051
Loans held-for-sale
Nonrecurring fair value changes included in earnings
Nonrecurring fair value measurements included in earnings	$ (12,000,000)	(1,000,000)	$ 0	8,000,000
Other real estate owned
Nonrecurring fair value changes included in earnings
Nonrecurring fair value measurements included in earnings	(1,000,000)	(1,000,000)	(2,000,000)	(2,000,000)
Loans
Nonrecurring fair value changes included in earnings
Nonrecurring fair value measurements included in earnings	(41,000,000)	(22,000,000)	(50,000,000)	(51,000,000)
Non-marketable equity securities measured using the measurement alternative
Nonrecurring fair value changes included in earnings
Nonrecurring fair value measurements included in earnings	$ 18,000,000	$ 7,000,000	$ 83,000,000	$ 111,000,000