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ORGANIZATION, CONSOLIDATION AND BASIS OF PRESENTATION (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Mar. 17, 2015
Premiums Written, Net [Member] | Homeowners' Insurance Product Line [Member] | Customer Concentration Risk [Member] | ISA [Member]          
Concentration risk, percentage 25.30% 24.70% 24.60% 24.10%  
Premiums Written, Net [Member] | Homeowners' Insurance Product Line [Member] | Customer Concentration Risk [Member] | SageSure [Member]          
Concentration risk, percentage 9.80% 6.30% 9.20% 6.30%  
Monarch Delaware Holdings LLC [Member]          
Capital contribution for voting interests $ 14.0   $ 14.0    
Percentage of ownership interest 42.40%   42.40%    
Percentage of voting interest held 50.00%   50.00%    
Long-term debt         $ 5.0
Interest rate percentage         6.00%
Transatlantic Reinsurance Company [Member] | Monarch Delaware Holdings LLC [Member]          
Percentage of ownership interest 15.20%   15.20%    
Capital contribution for non-voting interests $ 5.0   $ 5.0    
Interest rate percentage 6.00%   6.00%    
Transatlantic Reinsurance Company [Member] | Monarch Delaware Holdings LLC [Member] | Senior Debt [Member]          
Long-term debt $ 5.0   $ 5.0    
Maturity period of debt     6 years