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Income Taxes - Narrative (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2015
Mar. 31, 2014
Income Taxes [Line Items]        
Corporate tax rate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate      
Realization assessment period 3 years      
Interest and penalties related to uncertain tax positions $ 0.5us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense $ 1.0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense    
Accrued interest and penalties, uncertain tax positions 5.0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued 4.5us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued 5.0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued  
Unrecognized tax benefits that would impact effective tax rate $ 29us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate $ 28us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate 29us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate  
United Kingdom [Member]        
Income Taxes [Line Items]        
Corporate tax rate     21.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
/ us-gaap_IncomeTaxAuthorityAxis
= country_GB
23.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
/ us-gaap_IncomeTaxAuthorityAxis
= country_GB
Value added tax rate 20.00%ago_EffectiveIncomeTaxRateReconciliationValueAddedTaxRatePercent
/ us-gaap_IncomeTaxAuthorityAxis
= country_GB
     
UNITED KINGDOM        
Income Taxes [Line Items]        
Corporate tax rate 20.25%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
/ us-gaap_StatutoryAccountingPracticesByJurisdictionAxis
= country_GB
21.50%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
/ us-gaap_StatutoryAccountingPracticesByJurisdictionAxis
= country_GB
   
Subsequent to April 1, 2015 | United Kingdom [Member]        
Income Taxes [Line Items]        
Corporate tax rate 20.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
/ us-gaap_IncomeTaxAuthorityAxis
= country_GB
/ us-gaap_TaxPeriodAxis
= ago_SubsequenttoApril12015Member