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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 |  |  |  |

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