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Insurance Company Regulatory Requirements - Narrative (Details)
£ in Millions, $ in Millions
12 Months Ended
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2018
GBP (£)
Dec. 31, 2017
GBP (£)
Jul. 17, 2013
Statutory Accounting Practices [Line Items]            
Deferred tax assets, adjusted surplus threshold, realization period 3 years          
Deferred tax assets, adjusted surplus threshold, percent 15.00%          
AGE [Member]            
Statutory Accounting Practices [Line Items]            
Own Funds | £       £ 693 £ 629  
Assured Guaranty Re [Member]            
Statutory Accounting Practices [Line Items]            
Policyholders' surplus $ 1,249 $ 1,294        
AGC [Member]            
Statutory Accounting Practices [Line Items]            
Policyholders' surplus 1,793 2,073        
MAC [Member]            
Statutory Accounting Practices [Line Items]            
Policyholders' surplus 321 270        
AGM [Member]            
Statutory Accounting Practices [Line Items]            
Policyholders' surplus 2,533 2,254        
Assured Guaranty Re Overseas [Member]            
Statutory Accounting Practices [Line Items]            
Policyholders' surplus 383 380        
Subsidiaries [Member] | Municipal Assurance Corp Holdings [Member] | AGM [Member]            
Statutory Accounting Practices [Line Items]            
Holding company's percent ownership of common stock           60.70%
Subsidiaries [Member] | Municipal Assurance Corp Holdings [Member] | MAC [Member]            
Statutory Accounting Practices [Line Items]            
Holding company's percent ownership of common stock           100.00%
Subsidiaries [Member] | Municipal Assurance Corp Holdings [Member] | AGC [Member]            
Statutory Accounting Practices [Line Items]            
Holding company's percent ownership of common stock           39.30%
Affiliated Entity [Member] | MAC [Member] | Municipal Assurance Corp Holdings [Member]            
Statutory Accounting Practices [Line Items]            
Repayment of surplus notes 0 0 $ 300      
Affiliated Entity [Member] | MAC [Member] | AGM [Member]            
Statutory Accounting Practices [Line Items]            
Repayment of surplus notes $ 0 $ 0 $ 100      
New York [Member] | Assured Guaranty Municipal Corp And Municipal Assurance Corp [Member]            
Statutory Accounting Practices [Line Items]            
Threshold for dividend payments as a percentage of policyholder surplus 10.00%          
Threshold for dividend payments, percentage of adjusted net investment income 100.00%          
New York [Member] | MAC [Member]            
Statutory Accounting Practices [Line Items]            
Amount available for distribution, current year $ 32          
Amount available for distribution, next fiscal quarter 5          
New York [Member] | AGM [Member]            
Statutory Accounting Practices [Line Items]            
Amount available for distribution, current year 172          
Amount available for distribution, next fiscal quarter $ 74          
Maryland [Member] | AGC [Member]            
Statutory Accounting Practices [Line Items]            
Threshold for dividend payments as a percentage of policyholder surplus 10.00%          
Threshold for dividend payments, percentage of adjusted net investment income 100.00%          
Amount available for distribution, next fiscal quarter $ 42          
Maryland [Member] | AGUS [Member] | AGC [Member]            
Statutory Accounting Practices [Line Items]            
Amount available for distribution, current year 123          
Bermuda [Member] | Assured Guaranty Re [Member]            
Statutory Accounting Practices [Line Items]            
Amount available for distribution, current year $ 312          
Dividend payment restrictions, percentage of statutory capital 15.00%          
Dividend restrictions on statutory capital and surplus (as a percent) 25.00%          
Capital distributions $ 128          
Statutory surplus 312          
Unencumbered assets 416          
Bermuda [Member] | Assured Guaranty Re Overseas [Member]            
Statutory Accounting Practices [Line Items]            
Amount available for distribution, current year 96          
Capital distributions 21          
Statutory surplus 96          
Unencumbered assets $ 342