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CAPITAL STOCK - Additional Information (Details) - USD ($)
$ / shares in Units, $ in Millions
 1 Months Ended 3 Months Ended
Sep. 07, 2018
Jan. 31, 2018
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Class of Stock [Line Items]          
Options exercised under incentive plans (in shares)     0 0  
Preferred Stock, par value (in dollars per share)     $ 0.01   $ 0.01
Preferred stock, redemption price (in dollars per share)     $ 25.00    
Preferred Stock, shares issued (in shares)     73,400,000   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
Preferred stock dividend rate, percentage     6.50% 6.50%  
Net proceeds from issuances of 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
Preferred stock dividend rate, percentage     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 redeemed (in shares)       11,500,000.0  
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
Preferred stock dividend rate, percentage     6.95%    
8.125% Series H Cumulative Redeemable Preferred Stock          
Class of Stock [Line Items]          
Preferred Stock, shares issued (in shares)     2,200,000   2,200,000
Preferred stock dividend rate, percentage     8.125%    
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    
Over-Allotment Option          
Class of Stock [Line Items]          
Sale of stock, shares issued (in shares)     11,300,000.0    
Proceeds from sale of stock     $ 109.6    
Option to purchase additional shares, period     30 days    
At-the-market Sale Program          
Class of Stock [Line Items]          
Sale of stock, shares issued (in shares)     48,000,000.0    
Proceeds from sale of stock     $ 489.0    
Aggregate stock offering price   $ 1,500.0      
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