COMMON STOCK AND PREFERRED STOCK - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended		3 Months Ended	6 Months Ended		12 Months Ended
	Aug. 25, 2017	Jul. 31, 2018	Jan. 31, 2018	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Class of Stock [Line Items]
Common Stock, par value (in dollars per share)				$ 0.01	$ 0.01		$ 0.01
Common Stock, shares authorized (in shares)				1,909,750,000	1,909,750,000		1,929,300,000
Common Stock, shares issued (in shares)				1,164,333,831	1,164,333,831		1,159,585,078
Common Stock, shares outstanding (in shares)				1,164,333,831	1,164,333,831		1,159,585,078
Options exercised under incentive plans (In shares)					0	0
Direct purchase and dividend reinvestment program - value raised					$ 1,546	$ 1,270
Direct purchase and dividend reinvestment program - issued shares of common stock (in shares)					147,000	113,000
Net proceeds from issuance of stock					$ 461,044	$ 1,270
Redemption of preferred stock					412,500
Common Stock
Class of Stock [Line Items]
Direct purchase and dividend reinvestment program - value raised					1	$ 1
At-the-market Sale Program
Class of Stock [Line Items]
Aggregate stock offering price			$ 1,500,000
Net proceeds from issuance of stock					$ 48,200
At-the-market Sale Program | Subsequent Event
Class of Stock [Line Items]
Net proceeds from issuance of stock		$ 23,700
At-the-market Sale Program | Common Stock
Class of Stock [Line Items]
Number of shares issued during period (in shares)					4,588,496
At-the-market Sale Program | Common Stock | Subsequent Event
Class of Stock [Line Items]
Number of shares issued during period (in shares)		2,283,700
7.625% Series C Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized (in shares)				12,000,000	12,000,000		12,000,000
Preferred Stock dividend rate, percentage					7.625%	7.625%	7.625%
Preferred stock redeemed (in shares)					5,000,000
Redemption of preferred stock					$ 125,000
Preferred stock, redemption price (in dollars per share)				$ 25	$ 25
Preferred Stock, shares issued (in shares)				7,000,000	7,000,000		12,000,000
Preferred Stock, shares outstanding (in shares)				7,000,000	7,000,000		12,000,000
Preferred Stock, par value (in dollars per share)				$ 0.01	$ 0.01		$ 0.01
Preferred Stock liquidation preference, per share (in dollars per share)				$ 25	$ 25		$ 25.00
Preferred stock dividends paid					$ 1,000
Preferred stock, dividends paid (in dollars per share)					$ 0.196
7.50% Series D Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized (in shares)				18,400,000	18,400,000		18,400,000
Preferred Stock dividend rate, percentage					7.50%	7.50%	7.50%
Preferred stock, redemption price (in dollars per share)				$ 25	$ 25
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, par value (in dollars per share)				$ 0.01	$ 0.01		$ 0.01
Preferred Stock liquidation preference, per share (in dollars per share)				$ 25.00	$ 25.00		$ 25.00
6.95% Series F Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized (in shares)				28,800,000	28,800,000		28,800,000
Preferred Stock dividend rate, percentage					6.95%	6.95%	6.95%
Number of shares issued during period (in shares)				28,800,000			28,800,000
Preferred stock, redemption price (in dollars per share)				$ 25.00	$ 25.00
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, par value (in dollars per share)				$ 0.01	$ 0.01		$ 0.01
Preferred Stock liquidation preference, per share (in dollars per share)				$ 25.00	$ 25.00		$ 25.00
Preferred stock, basis spread					4.993%
6.50% Series G Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized (in shares)				19,550,000	19,550,000		0
Preferred Stock dividend rate, percentage					6.50%	6.50%	6.50%
Number of shares issued during period (in shares)					17,000,000
Preferred stock, redemption price (in dollars per share)				$ 25.00	$ 25.00
Preferred Stock, shares issued (in shares)				17,000,000	17,000,000		0
Preferred Stock, shares outstanding (in shares)				17,000,000	17,000,000		0
Preferred Stock liquidation preference, per share (in dollars per share)				$ 25.00	$ 25.00
Preferred stock, basis spread					4.172%
Proceeds from stock issuance, before discounts and offering expenses					$ 425,000
7.875% Series A Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred Stock dividend rate, percentage	7.875%				7.875%	7.875%
Preferred stock redeemed (in shares)	7,412,500
Redemption of preferred stock	$ 187,500
Preferred stock, redemption price (in dollars per share)	$ 25
7.625% Series E Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized (in shares)				11,500,000	11,500,000		11,500,000
Preferred Stock dividend rate, percentage					7.625%	7.625%	7.625%
Preferred stock redeemed (in shares)					11,500,000
Redemption of preferred stock					$ 287,500
Preferred Stock, shares issued (in shares)				0	0		11,500,000
Preferred Stock, shares outstanding (in shares)				0	0		11,500,000
Preferred Stock, par value (in dollars per share)							$ 0.01
Preferred Stock liquidation preference, per share (in dollars per share)							$ 25