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Derivative Financial Instruments (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
--- | ---
Mar. 31, 2013 | Mar. 31, 2012
Derivative transactions entered into | $ 604 | $ 332
Derivative transactions terminated | $ 542 | $ 321

X
- Definition Derivative transactions entered into + References No definition available. + Details
Name: | cna_DerivativeTransactionsEnteredInto
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Derivative transactions terminated + References No definition available. + Details
Name: | cna_DerivativeTransactionsTerminated
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration