STOCKHOLDERS' EQUITY - Preferred Stock Outstanding Information (Details) - $ / shares	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Redemption notice period	90 days
Redemption price per share (usd per share)	$ 1,000
Series A Preferred Stock
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)	250,000	250,000
Preferred stock, dividend rate	5.50%
Series B Preferred Stock
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)	300,000	300,000
Preferred stock, dividend rate	6.00%
Series C Preferred Stock
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)	300,000	300,000
Preferred stock, dividend rate	6.375%
Series D Preferred Stock
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)	300,000	0
Preferred stock, dividend rate	6.35%
Liquidation preference per depository share (usd per share)	$ 25
Depository share interest percentage	2.50%
Depositary shares issued (in shares)	12,000,000
LIBOR | Series A Preferred Stock
Class of Stock [Line Items]
Preferred stock, dividend payment rate, basis spread on variable rate	3.96%
LIBOR | Series B Preferred Stock
Class of Stock [Line Items]
Preferred stock, dividend payment rate, basis spread on variable rate	3.003%
LIBOR | Series C Preferred Stock
Class of Stock [Line Items]
Preferred stock, dividend payment rate, basis spread on variable rate	3.157%
LIBOR | Series D Preferred Stock
Class of Stock [Line Items]
Preferred stock, dividend payment rate, basis spread on variable rate	3.642%