CAPITAL STOCK - Schedule of Preferred Stock (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Class of Stock [Line Items]
Shares Authorized (in shares)	85,150,000		75,950,000
Shares Issued (in shares)	81,900,000		73,400,000
Shares Outstanding (in shares)	81,900,000		73,400,000
Carrying Value	$ 1,982,026		$ 1,778,168	[1]
Series C
Class of Stock [Line Items]
Shares Authorized (in shares)	0		7,000,000
Shares Issued (in shares)	0		7,000,000
Shares Outstanding (in shares)	0		7,000,000
Carrying Value	$ 0		$ 169,466
Contractual Rate	7.625%
Series D
Class of Stock [Line Items]
Shares Authorized (in shares)	18,400,000		18,400,000
Shares Issued (in shares)	18,400,000		18,400,000
Shares Outstanding (in shares)	18,400,000		18,400,000
Carrying Value	$ 445,457		$ 445,457
Contractual Rate	7.50%
Series H
Class of Stock [Line Items]
Shares Authorized (in shares)	0		2,200,000
Shares Issued (in shares)	0	2,200,000	2,200,000
Shares Outstanding (in shares)	0		2,200,000
Carrying Value	$ 0		$ 55,000
Contractual Rate	8.125%
Series F
Class of Stock [Line Items]
Shares Authorized (in shares)	28,800,000		28,800,000
Shares Issued (in shares)	28,800,000		28,800,000
Shares Outstanding (in shares)	28,800,000		28,800,000
Carrying Value	$ 696,910		$ 696,910
Contractual Rate	6.95%
Floating annual rate	4.993%
Series G
Class of Stock [Line Items]
Shares Authorized (in shares)	19,550,000		19,550,000
Shares Issued (in shares)	17,000,000	17,000,000.0	17,000,000
Shares Outstanding (in shares)	17,000,000		17,000,000
Carrying Value	$ 411,335		$ 411,335
Contractual Rate	6.50%	6.50%
Floating annual rate	4.172%
Series I
Class of Stock [Line Items]
Shares Authorized (in shares)	18,400,000		0
Shares Issued (in shares)	17,700,000		0
Shares Outstanding (in shares)	17,700,000		0
Carrying Value	$ 428,324		$ 0
Contractual Rate	6.75%
Floating annual rate	4.989%

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