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Derivative Financial Instruments (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2013 | Jun. 30, 2012 | Jun. 30, 2013 | Jun. 30, 2012
Derivative transactions entered into | $ 1,443 | $ 447 | $ 2,047 | $ 779
Derivative transactions terminated | $ 1,523 | $ 391 | $ 2,065 | $ 712

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