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ORGANIZATION, CONSOLIDATION AND BASIS OF PRESENTATION (Narrative) (Details) - USD ($)
3 Months Ended 12 Months Ended
Feb. 21, 2018
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Document Period End Date   Mar. 31, 2018      
Monarch Delaware Holdings LLC [Member]          
Capital contribution for voting interests   $ 14,000,000      
Ownership percentage 42.40% 42.40%     100.00%
Percentage of voting interest held   50.00%      
Business Acquisition, Percentage of Voting Interests Acquired 50.00%        
Transatlantic Reinsurance Company [Member] | Monarch Delaware Holdings LLC [Member]          
Capital contribution for voting interests   $ 5,000,000      
Ownership percentage   15.20%      
Long-term debt   $ 5,000,000      
Interest rate percentage   6.00%      
Business Acquisition, Percentage of Voting Interests Acquired 15.20%        
Scenario, Forecast [Member] | Transatlantic Reinsurance Company [Member]          
Quarterly consulting fee, minimum       $ 10,000,000  
Scenario, Forecast [Member] | Crosswinds [Member]          
Quarterly consulting fee, minimum       75,000  
Purchase and sell agreement, right of first refusal, catastrophe excess of loss reinsurance program limit       $ 10,000,000.0  
Homeowners Multiperil Insurance Product Line [Member] | Customer Concentration Risk [Member] | Ivantage Select Agency Inc [Member] | Premiums Written Net [Member]          
Concentration risk, percentage   22.80% 23.80%    
Homeowners Multiperil Insurance Product Line [Member] | Customer Concentration Risk [Member] | SageSure Insurance Managers LLC [Member] | Premiums Written Net [Member]          
Concentration risk, percentage   11.80% 8.60%    
Common Class A [Member] | Monarch Delaware Holdings LLC [Member]          
Business Acquisition, Percentage of Voting Interests Acquired 42.40%        
Monarch National Insurance Company [Member]          
Percentage of voting interest held 100.00%        
Monarch National Insurance Company [Member] | TransRe [Member]          
Business Combination, Consideration Transferred $ 4,400,000        
Business Combination, Consideration Transferred, Liabilities Incurred 5,000,000        
Monarch National Insurance Company [Member] | Crosswinds [Member]          
Business Combination, Consideration Transferred $ 12,300,000