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EMPLOYEE BENEFIT PLANS - Additional Information (Details) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2015
EMPLOYEE BENEFIT PLANS      
Period for which the average and annualized return on Barclay's short treasury index is considered for assessment of reasonableness of long term rate of return assumption 25 years    
Other Postretirement Benefits | Maximum      
EMPLOYEE BENEFIT PLANS      
Age of eligibility for limited healthcare benefits 65 years    
Retiree medical plan | Minimum      
EMPLOYEE BENEFIT PLANS      
Age of eligibility for prescription drug benefit 65 years    
Group life insurance plan      
EMPLOYEE BENEFIT PLANS      
Face amount of life insurance benefits funded $ 50,000    
Discount rate (as a percent) 4.38%    
Discount rate (as a percent) 3.74%    
Expected long-term return on plan assets (as a percent) 2.75%    
Group life insurance plan | Minimum      
EMPLOYEE BENEFIT PLANS      
Life insurance benefits $ 10,000    
Group life insurance plan | Maximum      
EMPLOYEE BENEFIT PLANS      
Life insurance benefits 75,000    
Nonqualified Plan | Qualified Pension Plan      
EMPLOYEE BENEFIT PLANS      
Amortized loss from other comprehensive income into net periodic benefit 600,000    
Employer contributions $ 8,095,000 $ 3,903,000  
Discount rate (as a percent) 3.93% 3.26%  
Discount rate (as a percent) 3.25% 3.60% 3.63%
Qualified Plan | Qualified Pension Plan      
EMPLOYEE BENEFIT PLANS      
Employer contributions $ 18,800,000 $ 43,500,000  
Target allocation (as a percent) 100.00% 100.00%  
Adjusted funding target percentage subject to benefit restrictions, maximum 80.00%    
Adjusted funding target percentage to be maintained (at least) 80.00%    
Contribution made by the company to its defined benefit pension plan year 2016 $ 18,800,000    
Discount rate (as a percent) 4.21% 3.55%  
Discount rate (as a percent) 3.55% 4.04% 4.29%
Expected long-term return on plan assets (as a percent) 7.00% 7.00% 7.25%
Qualified Plan | Qualified Pension Plan | Minimum      
EMPLOYEE BENEFIT PLANS      
Estimated contribution by employer $ 10,000,000    
Qualified Plan | Qualified Pension Plan | Maximum      
EMPLOYEE BENEFIT PLANS      
Estimated contribution by employer $ 20,000,000    
Equity securities | Qualified Plan | Qualified Pension Plan      
EMPLOYEE BENEFIT PLANS      
Target allocation (as a percent)   60.00%  
Fixed maturities | Qualified Plan | Qualified Pension Plan      
EMPLOYEE BENEFIT PLANS      
Target allocation (as a percent)   38.00%  
Cash and cash equivalents | Qualified Plan | Qualified Pension Plan      
EMPLOYEE BENEFIT PLANS      
Target allocation (as a percent)   2.00%