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Long-Term Debt and Credit Facilities - Principal and Carrying Amounts of Debt (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Jun. 20, 2014
Dec. 20, 2006
Nov. 22, 2006
May 18, 2004
Jul. 31, 2003
Nov. 26, 2002
Dec. 19, 2001
Debt Instrument [Line Items]                  
Principal $ 1,558,000,000 $ 1,589,000,000              
Carrying Value 1,292,000,000 1,306,000,000              
AGUS [Member]                  
Debt Instrument [Line Items]                  
Principal 850,000,000 850,000,000              
Carrying Value 843,000,000 843,000,000              
AGMH [Member]                  
Debt Instrument [Line Items]                  
Principal 730,000,000 730,000,000              
Carrying Value 461,000,000 453,000,000              
AGM [Member]                  
Debt Instrument [Line Items]                  
Principal 6,000,000 9,000,000              
Carrying Value 6,000,000 10,000,000              
Senior Notes [Member] | AGUS [Member] | 7% Senior Notes [Member]                  
Debt Instrument [Line Items]                  
Principal 200,000,000 200,000,000       $ 200,000,000.0      
Carrying Value $ 197,000,000 197,000,000              
Senior Notes [Member] | AGUS [Member] | 5% Senior Notes [Member]                  
Debt Instrument [Line Items]                  
Interest rate of debt (as a percent) 5.00%                
Principal $ 500,000,000 500,000,000 $ 500,000,000            
Carrying Value 496,000,000 496,000,000              
Enhanced Junior Subordinated Debentures [Member] | AGUS [Member]                  
Debt Instrument [Line Items]                  
Principal 150,000,000 150,000,000              
Carrying Value 150,000,000 150,000,000              
Corporate securities [Member] | AGMH [Member] | 6.875% QUIBS [Member]                  
Debt Instrument [Line Items]                  
Principal 100,000,000 100,000,000             $ 100,000,000.0
Carrying Value $ 70,000,000 69,000,000              
Notes Payable, Other Payables [Member] | AGMH [Member] | 6.25% Notes [Member]                  
Debt Instrument [Line Items]                  
Interest rate of debt (as a percent) 6.25%                
Principal $ 230,000,000 230,000,000           $ 230,000,000.0  
Carrying Value $ 142,000,000 141,000,000              
Notes Payable, Other Payables [Member] | AGMH [Member] | 5.6% Notes [Member]                  
Debt Instrument [Line Items]                  
Interest rate of debt (as a percent) 5.60%                
Principal $ 100,000,000 100,000,000         $ 100,000,000.0    
Carrying Value 57,000,000 56,000,000              
Notes Payable, Other Payables [Member] | AGM [Member]                  
Debt Instrument [Line Items]                  
Principal 6,000,000 9,000,000              
Carrying Value 6,000,000 10,000,000              
Junior Subordinated Debt [Member] | AGMH [Member]                  
Debt Instrument [Line Items]                  
Interest rate of debt (as a percent)       6.40%          
Principal 300,000,000 300,000,000     $ 300,000,000        
Carrying Value 192,000,000 187,000,000              
Consolidation, Eliminations [Member]                  
Debt Instrument [Line Items]                  
Principal (28,000,000) 0              
Carrying Value (18,000,000) $ 0              
AGMH [Member] | Consolidation, Eliminations [Member] | Junior Subordinated Debt [Member] | AGUS [Member]                  
Debt Instrument [Line Items]                  
Principal 28,000,000                
Loss on extinguishment of debt $ 9,000,000