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Claim and Claim Adjustment Expense Reserves (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Claim and Claim Adjustment Expense Reserves [Abstract]
Catastrophe losses net of reinsurance	$ 29cna_CatastropheLossesNetOfReinsurance	$ 74cna_CatastropheLossesNetOfReinsurance