FAIR VALUE MEASUREMENT - Valuation Techniques and Inputs for Level 3 Nonrecurring Fair Value Measurements (Details) - Nonrecurring	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Nonrecurring fair value changes included in earnings
Nonrecurring fair value measurements included in earnings	$ (55,000,000)	$ (42,000,000)	$ 2,000,000	$ 62,000,000
Level 3
Valuation techniques and inputs
Loans held-for-sale	1,243,000,000		1,243,000,000		$ 1,794,000,000
Level 3 | Price-based
Valuation techniques and inputs
Loans held-for-sale	839,000,000		839,000,000		1,729,000,000
Other real estate owned	9,000,000		9,000,000		15,000,000
Loans	59,000,000		59,000,000
Alternative investment funds, fair value					66,000,000
Level 3 | Recovery analysis
Valuation techniques and inputs
Other real estate owned					2,000,000
Loans	62,000,000		62,000,000		251,000,000
Price | Level 3 | Price-based | Minimum
Valuation techniques and inputs
Loans held-for-sale	82.90		82.90		0.79
Loans	2.65		2.65
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		100.00
Loans	63.00		63.00
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	96.67		96.67		69.52
Loans	30.48		30.48
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	2,953,240		2,953,240		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	$ 6,978,072		$ 6,978,072		$ 8,394,102
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
Price | Level 3 | Recovery analysis | Minimum
Valuation techniques and inputs
Loans	0.8525		0.8525		0.3060
Price | Level 3 | Recovery analysis | Maximum
Valuation techniques and inputs
Loans	0.9950		0.9950		1.0000
Price | Level 3 | Recovery analysis | Weighted Average
Valuation techniques and inputs
Loans	0.9560		0.9560		0.5051
Loans held-for-sale
Nonrecurring fair value changes included in earnings
Nonrecurring fair value measurements included in earnings	$ (14,000,000)	(7,000,000)	$ (1,000,000)	(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)	0	(1,000,000)
Loans
Nonrecurring fair value changes included in earnings
Nonrecurring fair value measurements included in earnings	(44,000,000)	(33,000,000)	(62,000,000)	(33,000,000)
Non-marketable equity securities measured using the measurement alternative
Nonrecurring fair value changes included in earnings
Nonrecurring fair value measurements included in earnings	4,000,000	$ (1,000,000)	65,000,000	$ 104,000,000
Investment Security One | Level 3 | Price-based
Valuation techniques and inputs
Alternative investment funds, fair value	17,000,000		17,000,000
Investment Security One | Price | Level 3 | Price-based | Minimum
Valuation techniques and inputs
Non-marketable equity securities measured using the measurement alternative	13.78		13.78
Investment Security One | Price | Level 3 | Price-based | Maximum
Valuation techniques and inputs
Non-marketable equity securities measured using the measurement alternative	13.78		13.78
Investment Security One | Price | Level 3 | Price-based | Weighted Average
Valuation techniques and inputs
Non-marketable equity securities measured using the measurement alternative	$ 13.78		$ 13.78