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Long-Term Debt and Credit Facilities - Narrative (Details)
1 Months Ended 12 Months Ended
Apr. 07, 2008
Jun. 30, 2003
USD ($)
Trust
Dec. 31, 2017
USD ($)
Dec. 31, 2012
USD ($)
Sep. 30, 2018
USD ($)
Oct. 25, 2013
USD ($)
Apr. 08, 2005
USD ($)
Trust
AGC Trust Preferred Securities [Member]              
Debt Instrument [Line Items]              
Basis points 2.50%            
AGM Trust Preferred Securities [Member]              
Debt Instrument [Line Items]              
Basis points   2.00%          
AGM [Member] | AGM CPS securities [Member]              
Debt Instrument [Line Items]              
Number of custodial trusts | Trust   4          
Maximum stock purchase obligation of each custodial trust   $ 50,000,000          
Aggregate maximum stock purchase obligation of the custodial trusts   $ 200,000,000          
Rate basis for income distributions   one-month LIBOR          
AGC [Member] | AGM CPS securities [Member]              
Debt Instrument [Line Items]              
Number of custodial trusts | Trust             4
Maximum stock purchase obligation of each custodial trust             $ 200,000,000
Aggregate maximum stock purchase obligation of the custodial trusts             $ 50,000,000
Rate basis for income distributions one-month LIBOR            
Line of Credit [Member] | Revolving Credit Facility [Member] | Wilbur L. Ross [Member]              
Debt Instrument [Line Items]              
Interest rate, as a percentage of Federal short-term or mid-term interest rate           100.00%  
Intercompany Credit Facility [Member] | Line of Credit [Member] | Revolving Credit Facility [Member] | Wilbur L. Ross [Member] | AGL [Member]              
Debt Instrument [Line Items]              
Related party, maximum borrowing capacity           $ 225,000,000  
Line of credit, outstanding amount         $ 0    
Municipal Assurance Corp [Member] | AGUS [Member]              
Debt Instrument [Line Items]              
Intercompany debt       $ 90,000,000      
Repayments of Debt     $ 10,000,000        
Notes payable, related parties         $ 60,000,000