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Related Party Transactions Narrative (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Due to Related Parties $ 153us-gaap_DueToRelatedPartiesCurrentAndNoncurrent $ 197us-gaap_DueToRelatedPartiesCurrentAndNoncurrent  
Fees and expenses of investment facilities and services [Member]      
Amounts reimbursed to Loews for services provided to the Company 39us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= cna_FeesandexpensesofinvestmentfacilitiesandservicesMember
37us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= cna_FeesandexpensesofinvestmentfacilitiesandservicesMember
36us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= cna_FeesandexpensesofinvestmentfacilitiesandservicesMember
Due to Related Parties 21us-gaap_DueToRelatedPartiesCurrentAndNoncurrent
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= cna_FeesandexpensesofinvestmentfacilitiesandservicesMember
19us-gaap_DueToRelatedPartiesCurrentAndNoncurrent
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= cna_FeesandexpensesofinvestmentfacilitiesandservicesMember
 
Loews [Member]      
Amounts earned from Loews for insurance premiums 2us-gaap_RevenueFromRelatedParties
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_MajorityShareholderMember
2us-gaap_RevenueFromRelatedParties
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_MajorityShareholderMember
2us-gaap_RevenueFromRelatedParties
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_MajorityShareholderMember
Corporate Services and Related Travel Expenses [Member]      
Amounts reimbursed to Loews for services provided to the Company   4us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= cna_CorporateServicesandRelatedTravelExpensesMember
4us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= cna_CorporateServicesandRelatedTravelExpensesMember
Taxes paid [Member]      
Due to Related Parties $ 132us-gaap_DueToRelatedPartiesCurrentAndNoncurrent
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= cna_TaxespaidMember
$ 178us-gaap_DueToRelatedPartiesCurrentAndNoncurrent
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= cna_TaxespaidMember