CAPITAL STOCK - Schedule of Preferred Stock (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Class of Stock [Line Items]
Preferred Stock, shares authorized (in shares)	75,950,000		75,950,000
Preferred Stock, shares issued (in shares)	73,400,000		73,400,000
Preferred Stock, shares outstanding (in shares)	73,400,000		73,400,000
Preferred Stock, carrying value	$ 1,778,168		$ 1,778,168	[1]
Series C Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized (in shares)	7,000,000		7,000,000
Preferred Stock, shares issued (in shares)	7,000,000		7,000,000
Preferred Stock, shares outstanding (in shares)	7,000,000		7,000,000
Preferred Stock, carrying value	$ 169,466		$ 169,466
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 H Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized (in shares)	2,200,000		2,200,000
Preferred Stock, shares issued (in shares)	2,200,000		2,200,000
Preferred Stock, shares outstanding (in shares)	2,200,000		2,200,000
Preferred Stock, carrying value	$ 55,000		$ 55,000
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
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%
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		19,550,000
Preferred Stock, shares issued (in shares)	17,000,000	17,000,000.0	17,000,000
Preferred Stock, shares outstanding (in shares)	17,000,000		17,000,000
Preferred Stock, carrying value	$ 411,335		$ 411,335
Preferred Stock,, contractual rate	6.50%	6.50%
Preferred Stock, floating annual rate	4.172%

[1] (1) Derived from the audited consolidated financial statements at December 31, 2018.