COMMON STOCK AND PREFERRED STOCK - Additional Information (Details)				1 Months Ended	12 Months Ended
	Aug. 25, 2017 USD ($) $ / shares shares	Jul. 27, 2017 shares	Jul. 12, 2016 $ / shares	Mar. 31, 2012 agreement shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2012 shares	Aug. 31, 2015 USD ($)
Class of Stock [Line Items]
Common stock, par value (in dollars per share) | $ / shares					$ 0.01	$ 0.01
Common stock, shares authorized (in shares)					1,929,300,000	1,945,437,500
Conversion of Stock, Shares Issued								4,000,000
Common stock, shares issued (in shares)					1,159,585,078	1,018,913,249
Common stock, shares outstanding (in shares)					1,159,585,078	1,018,913,249
Options exercised under incentive plans, shares (in shares)					0	0	0
Gross proceeds from issuance of stock | $					$ 1,700,000,000
Direct purchase and dividend reinvestment program - value raised | $					$ 2,542,000	$ 2,362,000	$ 2,246,000
Direct purchase and dividend reinvestment program - issued shares (in shares)					219,000	228,000	221,000
Common stock repurchase program, authorized amount | $									$ 1,000,000,000
Common stock repurchase program, repurchased share (in shares)					0	11,132,226
Common stock repurchase program, repurchased value | $						$ 102,700,000
Number of distribution agreements (agreement) | agreement				6
Number of common shares authorized for issuance under the Distribution Agency Agreement (in shares)				125,000,000		125,000,000	125,000,000
Redemption of Series A Cumulative Preferred Stock | $					$ 185,312,000
Common Stock
Class of Stock [Line Items]
Shares issued during period (in shares)					140,450,700
Direct purchase and dividend reinvestment program - value raised | $					$ 2,000	$ 2,000	$ 2,000
7.625% Series C Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)					12,000,000	12,650,000
Preferred stock dividend rate, percentage					7.625%	7.625%	7.625%
Preferred Stock redeemable price, per share (in dollars per share) | $ / shares					$ 25
Preferred Stock, shares issued (in shares)					12,000,000	12,000,000
Preferred Stock, par value (in dollars per share) | $ / shares					$ 0.01
Preferred Stock liquidation preference, per share (in dollars per share) | $ / shares					$ 25
Preferred Stock, shares outstanding (in shares)					12,000,000	12,000,000
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
Preferred stock dividend rate, percentage					7.50%	7.50%	7.50%
Preferred Stock redeemable price, per share (in dollars per share) | $ / shares					$ 25
Preferred Stock, shares issued (in shares)					18,400,000	18,400,000
Preferred Stock, par value (in dollars per share) | $ / shares					$ 0.01
Preferred Stock liquidation preference, per share (in dollars per share) | $ / shares					$ 25.00
Preferred Stock, shares outstanding (in shares)					18,400,000	18,400,000
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
Preferred stock dividend rate, percentage			7.625%		7.625%	7.625%	7.625%
Preferred Stock redeemable price, per share (in dollars per share) | $ / shares					$ 25
Preferred Stock, shares issued (in shares)					11,500,000	11,500,000
Preferred Stock, par value (in dollars per share) | $ / shares			$ 0.01		$ 0.01
Preferred Stock liquidation preference, per share (in dollars per share) | $ / shares					$ 25
Preferred Stock, shares outstanding (in shares)					11,500,000	11,500,000
6.95% Series F Fixed-to-Floating Rate Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)					28,800,000	0
Preferred stock dividend rate, percentage					6.95%	6.95%	6.95%
Shares issued during period (in shares)					28,800,000
Preferred Stock redeemable price, per share (in dollars per share) | $ / shares					$ 25.00
Preferred Stock, shares issued (in shares)					28,800,000	0
Preferred Stock liquidation preference, per share (in dollars per share) | $ / shares					$ 25.00
Preferred Stock, shares outstanding (in shares)					28,800,000	0
Net proceeds from issuance of preferred stock | $					$ 720,000,000
Preferred stock, basis spread					4.993%
6.00% Series B Cumulative Convertible Preferred Stock
Class of Stock [Line Items]
Preferred stock dividend rate, percentage								6.00%
Conversion of Stock, Shares Converted		4,600,000						1,300,000
Preferred Stock, shares issued (in shares)					0	0	0
Preferred Stock, shares outstanding (in shares)					0	0	0
7.875% Series A Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)					0	7,412,500
Preferred stock dividend rate, percentage					7.875%	7.875%	7.875%
Preferred stock redeemed (in shares)	7,412,500
Redemption of Series A Cumulative Preferred Stock | $	$ 187,500,000
Preferred Stock redeemable price, per share (in dollars per share) | $ / shares	$ 25.00					$ 25
Preferred Stock, shares issued (in shares)					0	7,412,500
Preferred Stock, par value (in dollars per share) | $ / shares						$ 0.01
Preferred Stock liquidation preference, per share (in dollars per share) | $ / shares						$ 25
Preferred Stock, shares outstanding (in shares)					0	7,412,500