Fair Value (Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Other invested assets overseas deposit	$ 45	$ 39

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 experiment, 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