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ORGANIZATION, CONSOLIDATION AND BASIS OF PRESENTATION - (Narrative) (Details) - USD ($)
12 Months Ended
Nov. 27, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Feb. 28, 2018
Feb. 21, 2018
Organization, Consolidation, And Basis of Preparation [Line Items]            
Total debt maturities   $ 44,404,000 $ 49,251,000      
Premiums Written, Net [Member] | Homeowners' Insurance Product Line [Member] | Customer Concentration Risk [Member] | ISA [Member]            
Organization, Consolidation, And Basis of Preparation [Line Items]            
Concentration risk, percentage   23.80% 23.80% 24.10%    
Premiums Written, Net [Member] | Homeowners' Insurance Product Line [Member] | Customer Concentration Risk [Member] | SageSure Insurance Managers LLC [Member]            
Organization, Consolidation, And Basis of Preparation [Line Items]            
Concentration risk, percentage   15.00% 10.20% 6.90%    
Monarch Delaware Holdings LLC [Member]            
Organization, Consolidation, And Basis of Preparation [Line Items]            
Payments to acquire equity method investments $ 12,300,000          
Ownership percentage     100.00%     42.40%
Transatlantic Reinsurance Company [Member]            
Organization, Consolidation, And Basis of Preparation [Line Items]            
Minimum quarterly consulting fee   $ 10,000,000        
Transatlantic Reinsurance Company [Member] | Monarch Delaware Holdings LLC [Member]            
Organization, Consolidation, And Basis of Preparation [Line Items]            
Capital contribution for voting interests         $ 4,400,000  
Transatlantic Reinsurance Company [Member] | Monarch Delaware Holdings LLC [Member] | Senior Debt [Member]            
Organization, Consolidation, And Basis of Preparation [Line Items]            
Total debt maturities   5,000,000        
Crosswinds [Member]            
Organization, Consolidation, And Basis of Preparation [Line Items]            
Minimum quarterly consulting fee   75,000        
Catastrophe excess of loss reinsurance program limit   $ 10,000,000.0        
Monarch National Insurance Company [Member]            
Organization, Consolidation, And Basis of Preparation [Line Items]            
Variable Interest Entity, Qualitative or Quantitative Information, Ownership Percentage   100.00%