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FINANCIAL INSTRUMENTS Financial Instruments Additional Information (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]      
Transfers out of Level 3 $ 1,768 $ 0  
Realized gains 87,600 (113,800) $ 22,200
Gross realized gains 267,900 188,700 350,500
Gross realized losses 180,300 302,500 328,300
Fair value of cash and investments that have been pledged as security against letters of credit issued 200,400 310,900  
Collateral Held in Trust 377,900 86,400  
Net investment income (loss)      
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]      
Change in unrealized gains and losses on trading securities (41,400) 210,000 $ (265,400)
Commodities      
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]      
Fair value of investments in excess of 10% of stockholders' equity 121,500 137,300  
Bayer AG [Domain]      
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]      
Fair value of investments in excess of 10% of stockholders' equity 103,600    
Mylan NV [Domain]      
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]      
Fair value of investments in excess of 10% of stockholders' equity 84,800    
General Motors [Domain]      
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]      
Fair value of investments in excess of 10% of stockholders' equity 205,500 $ 92,900  
Brighthouse Financial Inc [Domain] [Domain]      
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]      
Fair value of investments in excess of 10% of stockholders' equity $ 132,400    
Percent of Stockholders' Equity, Total | Percent Investment Concentration Risk | Commodities      
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]      
Concentration risk, percentage 14.40% 15.50%  
Percent of Stockholders' Equity, Total | Percent Investment Concentration Risk | Bayer AG [Domain]      
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]      
Concentration risk, percentage 12.30%    
Percent of Stockholders' Equity, Total | Percent Investment Concentration Risk | Mylan NV [Domain]      
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]      
Concentration risk, percentage 10.00%    
Percent of Stockholders' Equity, Total | Percent Investment Concentration Risk | General Motors [Domain]      
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]      
Concentration risk, percentage 24.30% 10.50%  
Percent of Stockholders' Equity, Total | Percent Investment Concentration Risk | Brighthouse Financial Inc [Domain] [Domain]      
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]      
Concentration risk, percentage 15.70%