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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Accounting Policies [Abstract]      
Percentage of premium written in Iowa and contiguous states (in hundredths) 36.00%    
Percentage of net premiums written subject to policyholder dividends (in hundredths) 32.00% 26.00%  
Investment securities on deposit $ 11,600 $ 11,300  
Average annual expense for accounts receivable charge-offs over past ten years 414    
Defined benefit plan net periodic benefit cost other than service cost component 7,500 $ 5,100 $ 4,800
Property and casualty insurance [Member]      
Restructuring Reserve [Roll Forward]      
Costs incurred 967    
Property and casualty insurance [Member] | Employee severance costs [Member]      
Restructuring Cost and Reserve [Line Items]      
Total expected exit costs 1,200    
Property and casualty insurance [Member] | Software disposal costs [Member]      
Restructuring Cost and Reserve [Line Items]      
Total expected exit costs $ 729    
Property and casualty insurance [Member] | Personal Lines [Member]      
Restructuring Cost and Reserve [Line Items]      
Percentage of personal lines premiums written 6.00%