Fair Value (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Fair Value, Assets, Level 1 to Level 2, Transfers	$ 0	$ 0	$ 0	$ 0
Fair Value, Assets, Level 2 to Level 1, Transfers	0	$ 10	0	$ 10
Other Invested Assets Overseas Deposit	$ 30		$ 30		$ 27

Overseas deposits are excluded from the fair value hierarchy because their fair value is recorded using the net asset value per share (or equivalent) practical expedient, which can be redeemed at net asset value in 90 days or less

No definition available.

Name:	cna_OtherInvestedAssetsOverseasDeposit
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant