STOCKHOLDERS' EQUITY - Dividends (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Class of Stock [Line Items]
Dividends declared per share of common stock (in usd per share)	$ 0.39	$ 0.39	$ 0.78	$ 0.78
Common stock dividends declared and paid	$ 168	$ 168	$ 335	$ 336
Preferred stock dividends declared and paid	$ 32	$ 28	$ 55	$ 50
Series A Preferred Stock
Class of Stock [Line Items]
Dividends declared per share of preferred stock (in usd per share)	$ 10.50	$ 13.48	$ 20.99	$ 40.98
Series B Preferred Stock
Class of Stock [Line Items]
Dividends declared per share of preferred stock (in usd per share)	30.00	30.00	30.00	30.00
Series C Preferred Stock
Class of Stock [Line Items]
Dividends declared per share of preferred stock (in usd per share)	15.94	15.94	31.88	31.88
Series D Preferred Stock
Class of Stock [Line Items]
Dividends declared per share of preferred stock (in usd per share)	15.88	15.88	31.75	31.75
Series E Preferred Stock
Class of Stock [Line Items]
Dividends declared per share of preferred stock (in usd per share)	12.50	12.50	25.00	25.00
Series F Preferred Stock
Class of Stock [Line Items]
Dividends declared per share of preferred stock (in usd per share)	$ 14.13	$ 0	$ 28.25	$ 0
Dividends Declared
Class of Stock [Line Items]
Common stock dividends declared and paid	$ 168	$ 168	$ 335	$ 336
Preferred stock dividends declared and paid	32	28	55	50
Dividends Declared | Series A Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid	2	3	5	10
Dividends Declared | Series B Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid	9	9	9	9
Dividends Declared | Series C Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid	5	5	10	10
Dividends Declared | Series D Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid	4	5	9	10
Dividends Declared | Series E Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid	6	6	11	11
Dividends Declared | Series F Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid	6	0	11	0
Dividends Paid
Class of Stock [Line Items]
Common stock dividends declared and paid	168	168	335	336
Preferred stock dividends declared and paid	23	22	55	45
Dividends Paid | Series A Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid	2	7	5	7
Dividends Paid | Series B Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid	0	0	9	9
Dividends Paid | Series C Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid	5	5	10	10
Dividends Paid | Series D Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid	4	5	9	10
Dividends Paid | Series E Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid	6	5	11	9
Dividends Paid | Series F Preferred Stock
Class of Stock [Line Items]
Preferred stock dividends declared and paid	$ 6	$ 0	$ 11	$ 0