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Loss and Loss Adjustment Expense Including Amounts for Risks Relating to Asbestos Related Illnesses and Environmental Impairment (Detail) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Commitments and Contingencies [Line Items]    
Gross reserves $ 619.8us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross $ 602.7us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross
Net reserves 461.3us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet 447.4us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet
TransRe    
Commitments and Contingencies [Line Items]    
Gross reserves 610.7us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross
/ dei_LegalEntityAxis
= y_TransReMember		 593.5us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross
/ dei_LegalEntityAxis
= y_TransReMember
Net reserves 452.3us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet
/ dei_LegalEntityAxis
= y_TransReMember		 438.3us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet
/ dei_LegalEntityAxis
= y_TransReMember
CapSpecialty Incorporated    
Commitments and Contingencies [Line Items]    
Gross reserves 9.1us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross
/ dei_LegalEntityAxis
= y_CapSpecialtyIncorporatedMember		 9.2us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross
/ dei_LegalEntityAxis
= y_CapSpecialtyIncorporatedMember
Net reserves $ 9.0us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet
/ dei_LegalEntityAxis
= y_CapSpecialtyIncorporatedMember		 $ 9.1us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet
/ dei_LegalEntityAxis
= y_CapSpecialtyIncorporatedMember