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Transactions with Affiliates - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Ceded Credit Risk [Line Items]        
Premiums and losses related to certain products 63.50% 63.90% 64.30%  
Coverage under catastrophic occurrence       $ 1,500,000
Charges for services   $ 134,143,158 $ 126,153,511 124,999,770
Maximum [Member] | Lease Office Equipment And Automobile [Member]        
Ceded Credit Risk [Line Items]        
Lease office equipment and automobiles range   10 years    
Minimum [Member] | Lease Office Equipment And Automobile [Member]        
Ceded Credit Risk [Line Items]        
Lease office equipment and automobiles range   3 years    
Atlantic States [Member]        
Ceded Credit Risk [Line Items]        
Percentage of share in results of pooled business subsidiary 80.00% 80.00%    
Coverage under catastrophic occurrence   $ 2,500,000 2,000,000 2,000,000
Donegal Mutual [Member]        
Ceded Credit Risk [Line Items]        
Percentage of share in results of pooled business owned by third party 20.00% 20.00%    
Premiums and losses related to certain products 100.00%      
Premiums and losses related to business of MICO   25.00%    
Coverage under catastrophic occurrence $ 2,000,000      
Excess of loss reinsured under an agreement   $ 500,000    
Period of Donegal Mutual under lease agreement   10 years    
Le Mars [Member]        
Ceded Credit Risk [Line Items]        
Coverage under catastrophic occurrence   $ 1,000,000 750,000 $ 750,000
Southern [Member]        
Ceded Credit Risk [Line Items]        
Premiums and losses related to certain products   100.00%    
Coverage under catastrophic occurrence   $ 2,000,000 $ 1,500,000  
Excess of loss reinsured under an agreement   500,000    
Insurance Subsidiaries [Member]        
Ceded Credit Risk [Line Items]        
Coverage under catastrophic occurrence $ 5,000,000 5,000,000    
Excess of loss reinsured under an agreement   $ 5,000,000