CAPITAL STOCK - Schedule of Preferred Stock (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Preferred Stock, shares authorized (in shares)	75,950,000	70,700,000
Preferred Stock, shares issued (in shares)	73,400,000	70,700,000
Preferred Stock, shares outstanding (in shares)	73,400,000	70,700,000
Preferred Stock, carrying value	$ 1,778,168	$ 1,720,381
Series C Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized (in shares)	7,000,000	12,000,000
Preferred Stock, shares issued (in shares)	7,000,000	12,000,000
Preferred Stock, shares outstanding (in shares)	7,000,000	12,000,000
Preferred Stock, carrying value	$ 169,466	$ 290,514
Preferred Stock,, contractual rate	7.625%
Series D Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized (in shares)	18,400,000	18,400,000
Preferred Stock, shares issued (in shares)	18,400,000	18,400,000
Preferred Stock, shares outstanding (in shares)	18,400,000	18,400,000
Preferred Stock, carrying value	$ 445,457	$ 445,457
Preferred Stock,, contractual rate	7.50%
Series E Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized (in shares)	0	11,500,000
Preferred Stock, shares issued (in shares)	0	11,500,000
Preferred Stock, shares outstanding (in shares)	0	11,500,000
Preferred Stock, carrying value	$ 0	$ 287,500
Preferred Stock,, contractual rate	7.625%
Series H Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized (in shares)	2,200,000	0
Preferred Stock, shares issued (in shares)	2,200,000.0	0
Preferred Stock, shares outstanding (in shares)	2,200,000	0
Preferred Stock, carrying value	$ 55,000	$ 0
Preferred Stock,, contractual rate	8.125%
Series F Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized (in shares)	28,800,000	28,800,000
Preferred Stock, shares issued (in shares)	28,800,000	28,800,000.0
Preferred Stock, shares outstanding (in shares)	28,800,000	28,800,000
Preferred Stock, carrying value	$ 696,910	$ 696,910
Preferred Stock,, contractual rate	6.95%	6.95%
Preferred Stock, floating annual rate	4.993%
Series G Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized (in shares)	19,550,000	0
Preferred Stock, shares issued (in shares)	17,000,000.0	0
Preferred Stock, shares outstanding (in shares)	17,000,000	0
Preferred Stock, carrying value	$ 411,335	$ 0
Preferred Stock,, contractual rate	6.50%
Preferred Stock, floating annual rate	4.172%