STOCKHOLDERS' EQUITY - Other information (Details) - USD ($)	1 Months Ended			12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Aug. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2008	Jul. 31, 2017	Dec. 31, 2007
Stockholders' equity
Preferred stock, no par value	$ 0			$ 0	$ 0
Common stock, shares authorized	350,000,000			350,000,000	350,000,000
Weighted average price per share			$ 46.65
Proceeds from the issuance of common shares through public offering, net			$ 349,900,000	$ 725,315,000	$ 299,825,000
Preferred stock, shares authorized	50,000,000			50,000,000	50,000,000
Issuance of common shares through public offering			7,500,000
Common distributions declared per share				$ 1.37	$ 1.29	$ 1.24
Stock issued during period, shares, Distribution Reinvestment and Stock Purchase Plan							10,000,000.0
Shares reserved for issuance under the Stock Purchase Plan				11,000,000.0
Equity Distribution Agreement
Stockholders' equity
Common stock, shares authorized	20,000,000.0			20,000,000.0
Common Stock, Capital Shares Reserved for Future Issuance	11,700,000			11,700,000
8.00% Series E Cumulative Convertible Preferred Stock
Stockholders' equity
Preferred stock, no par value	$ 0			$ 0
Preferred stock, liquidation preference per share	$ 16.61			$ 16.61
Number of common stock shares each preferred shares can be converted to									1
Number of common stock shares to which each preferred share is convertible after special dividend				1.083
Declared preferred stock dividend				$ 1.48	$ 1.40	$ 1.33
Preferred Stock, Shares Issued	2,780,994			2,780,994	2,780,994
Common Stock
Stockholders' equity
Issuance of common shares though ATM program				6,988,000
Issuance of common shares through public offering				7,500,000	7,150,000	87,000
Share Price	$ 46.65			$ 46.65
Aggregate gross proceeds of common stock				$ 349,800,000
Series F
Stockholders' equity
Preferred stock, shares authorized	20,000,000.0			20,000,000.0
Share Price	$ 0.0001			$ 0.0001
Preferred stock, value, issued	$ 1,469			$ 1,469	$ 1,580
Preferred Stock, Shares Issued	14,691,274			14,691,274	15,802,393
Restricted Stock
Stockholders' equity
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other Than Options, Grants in Period Net of Forfeitures				100,000
Common Stock
Stockholders' equity
Issuance of common shares through public offering				7,500,000
ATM
Stockholders' equity
Common stock, shares authorized								20,000,000.0
Issuance of common shares though ATM program				7,000,000.0
Weighted average price per share				$ 45.29
Net proceeds from the issues of common shares				$ 312,300,000
Proceeds from the issuance of common shares through public offering, net				$ 316,500,000
Common Stock, Capital Shares Reserved for Future Issuance	11,700,000			11,700,000
Issuance of common shares through public offering				7,000,000.0
Forward Sales Agreement
Stockholders' equity
Common stock, shares authorized		1,300,000
Issuance of common shares though ATM program					1,300,000
Weighted average price per share	$ 47.41	$ 47.68
Proceeds from the issuance of common shares through public offering, net	$ 63,500,000				$ 63,200,000
Issuance of common shares through public offering	1,300,000
Forward price					$ 47.41