CAPITAL STOCK - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended	3 Months Ended		6 Months Ended
	Sep. 07, 2018	Jan. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Authorized amount of stock available for repurchase			$ 1,500.0		$ 1,500.0
Options exercised under incentive plans (in shares)				0
Preferred Stock, par value (in dollars per share)			$ 0.01		$ 0.01	$ 0.01
Preferred stock, redemption price (in dollars per share)			$ 25.00		$ 25.00
Preferred Stock, shares issued (in shares)			87,200,000		87,200,000	73,400,000
6.50% Series G Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares issued (in shares)			17,000,000	17,000,000.0	17,000,000	17,000,000
Preferred stock dividend rate, percentage				6.50%	6.50%
Proceeds from issuance of redeemable preferred stock				$ 425.0
7.625% Series C Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares issued (in shares)			7,000,000		7,000,000	7,000,000
Preferred stock dividend rate, percentage				7.625%	7.625%
Preferred stock redeemed (in shares)				5,000,000.0
Redemption of preferred stock				$ 125.0
7.625% Series E Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred stock dividend rate, percentage				7.625%
Preferred stock redeemed (in shares)				11,500,000
Redemption of preferred stock				$ 287.5
6.95% Series F Fixed-to-Floating Rate Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares issued (in shares)			28,800,000		28,800,000	28,800,000
Preferred stock dividend rate, percentage					6.95%
8.125% Series H Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred stock, redemption price (in dollars per share)			$ 25.00		$ 25.00
Preferred Stock, shares issued (in shares)			0		0	2,200,000
Preferred stock dividend rate, percentage			8.125%		8.125%
Preferred stock redeemed (in shares)			2,200,000		2,200,000
Redemption of preferred stock			$ 55.0		$ 55.0
Series I Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares issued (in shares)			16,000,000.0		16,000,000.0	0
Preferred stock dividend rate, percentage			6.75%		6.75%
Proceeds from issuance of redeemable preferred stock			$ 400.0		$ 400.0
Public Offering
Class of Stock [Line Items]
Sale of stock, shares issued (in shares)					75,000,000.0
Proceeds from sale of stock					$ 730.5
Option to purchase additional shares, period					30 days
Public Offering, Additional Share Purchase Option [Member]
Class of Stock [Line Items]
Sale of stock, shares issued (in shares)					11,300,000
Proceeds from sale of stock					$ 109.6
At-the-market Sale Program
Class of Stock [Line Items]
Sale of stock, shares issued (in shares)			8,000,000.0		56,000,000.0
Proceeds from sale of stock			$ 80.1		$ 569.1
Aggregate stock offering price		$ 1,500.0
Over-Allotment Option
Class of Stock [Line Items]
Sale of stock, shares issued (in shares)			2,400,000		2,400,000
Option to purchase additional shares, period					30 days
Mountain Merger Sub Corporation
Class of Stock [Line Items]
Shares issued as part of the consideration for asset acquisition (in shares)	43,600,000
Mountain Merger Sub Corporation | 8.125% Series H Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred Stock, par value (in dollars per share)	$ 0.01