CAPITAL STOCK - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions			1 Months Ended	3 Months Ended		9 Months Ended
	Sep. 07, 2018	Aug. 25, 2017	Jan. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
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		$ 0.01
Preferred stock, redemption price (in dollars per share)				$ 25.00		$ 25.00
Preferred Stock, shares issued (in shares)				73,400,000		73,400,000		70,700,000
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
Preferred Stock dividend rate, percentage						6.50%
Net proceeds from issuance 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		12,000,000
Preferred Stock dividend rate, percentage						7.625%
Preferred stock redeemed (in shares)						5,000,000
Redemption of preferred stock						$ 125.0
7.625% Series E Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares issued (in shares)				0		0		11,500,000
Preferred Stock dividend rate, percentage						7.625%
Preferred stock redeemed (in shares)						11,500,000
Redemption of preferred stock						$ 287.5
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		0
Preferred Stock dividend rate, percentage						8.125%
7.875% Series A Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred Stock dividend rate, percentage		7.875%
Preferred stock redeemed (in shares)		7,412,500
Redemption of preferred stock		$ 187.5
Public Offering
Class of Stock [Line Items]
Sale of stock, shares issued (in shares)				75,000,000.0	60,000,000.0	75,000,000.0	60,000,000.0
Proceeds from sale of stock				$ 753.8	$ 709.8	$ 753.8	$ 709.8
Over-Allotment Option
Class of Stock [Line Items]
Sale of stock, shares issued (in shares)						11,300,000.0	9,000,000.0
Proceeds from sale of stock						$ 113.1	$ 106.5
Option to purchase additional shares, period						30 days	30 days
At-the-market Sale Program
Class of Stock [Line Items]
Sale of stock, shares issued (in shares)						24,000,000.0
Proceeds from sale of stock						$ 251.1
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					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