CAPITAL STOCK - Schedule of Preferred Stock (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Preferred Stock, shares authorized (in shares)	92,150,000		92,150,000	75,950,000
Preferred Stock, shares issued (in shares)	87,200,000		87,200,000	73,400,000
Preferred Stock, shares outstanding (in shares)	87,200,000		87,200,000	73,400,000
Preferred Stock, carrying value	$ 2,110,346		$ 2,110,346	$ 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	7,000,000
Preferred Stock, shares issued (in shares)	7,000,000		7,000,000	7,000,000
Preferred Stock, shares outstanding (in shares)	7,000,000		7,000,000	7,000,000
Preferred Stock, carrying value	$ 169,466		$ 169,466	$ 169,466
Preferred Stock, contractual rate		7.625%	7.625%
Series D Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized (in shares)	18,400,000		18,400,000	18,400,000
Preferred Stock, shares issued (in shares)	18,400,000		18,400,000	18,400,000
Preferred Stock, shares outstanding (in shares)	18,400,000		18,400,000	18,400,000
Preferred Stock, carrying value	$ 445,457		$ 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)	0		0	2,200,000
Preferred Stock, shares issued (in shares)	0		0	2,200,000
Preferred Stock, shares outstanding (in shares)	0		0	2,200,000
Preferred Stock, carrying value	$ 0		$ 0	$ 55,000
Preferred Stock, contractual rate	8.125%		8.125%
Series F Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized (in shares)	28,800,000		28,800,000	28,800,000
Preferred Stock, shares issued (in shares)	28,800,000		28,800,000	28,800,000
Preferred Stock, shares outstanding (in shares)	28,800,000		28,800,000	28,800,000
Preferred Stock, carrying value	$ 696,910		$ 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	19,550,000
Preferred Stock, shares issued (in shares)	17,000,000	17,000,000.0	17,000,000	17,000,000
Preferred Stock, shares outstanding (in shares)	17,000,000		17,000,000	17,000,000
Preferred Stock, carrying value	$ 411,335		$ 411,335	$ 411,335
Preferred Stock, contractual rate		6.50%	6.50%
Preferred Stock, floating annual rate			4.172%
Series I Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized (in shares)	18,400,000		18,400,000	0
Preferred Stock, shares issued (in shares)	16,000,000.0		16,000,000.0	0
Preferred Stock, shares outstanding (in shares)	16,000,000		16,000,000	0
Preferred Stock, carrying value	$ 387,178		$ 387,178	$ 0
Preferred Stock, contractual rate	6.75%		6.75%
Preferred Stock, floating annual rate			4.989%

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