STOCKHOLDERS' EQUITY - Authorized Preferred Shares (Details) - USD ($) $ / shares in Units, $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Preferred Stock
Preferred stock, authorized (in shares)	100,000,000	100,000,000
Preferred stock, par value (in dollars per share)	$ 25.00	$ 25.00
Preferred stock, issued (in shares)	1,150,000	850,000
Preferred stock, value, issued	$ 1,133	$ 840
Series A Preferred Stock
Preferred Stock
Preferred stock, liquidation preference per share (in dollars per share)	$ 1,000
Preferred stock, issued (in shares)	250,000	250,000
Preferred stock, outstanding (in shares)	250,000	250,000
Preferred stock, value, issued	$ 247	$ 247
Preferred stock, value, outstanding	$ 247	$ 247
Series B Preferred Stock
Preferred Stock
Preferred stock, liquidation preference per share (in dollars per share)	$ 1,000
Preferred stock, issued (in shares)	300,000	300,000
Preferred stock, outstanding (in shares)	300,000	300,000
Preferred stock, value, issued	$ 296	$ 296
Preferred stock, value, outstanding	$ 296	$ 296
Series C Preferred Stock
Preferred Stock
Preferred stock, liquidation preference per share (in dollars per share)	$ 1,000
Preferred stock, issued (in shares)	300,000	300,000
Preferred stock, outstanding (in shares)	300,000	300,000
Preferred stock, value, issued	$ 297	$ 297
Preferred stock, value, outstanding	$ 297	$ 297
Series D Preferred Stock
Preferred Stock
Preferred stock, liquidation preference per share (in dollars per share)	$ 1,000
Preferred stock, issued (in shares)	300,000	0
Preferred stock, outstanding (in shares)	300,000	0
Preferred stock, value, issued	$ 293	$ 0
Preferred stock, value, outstanding	$ 293	$ 0
Liquidation preference per depository share (usd per share)	$ 25
Depositary shares issued (in shares)	12,000,000