Fair Value (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Fair Value, Assets, Level 2 to Level 1, Transfers	$ 0	$ 63
Fair Value, Assets, Level 1 to Level 2, Transfers	0	52
Other Invested Assets Overseas Deposit	$ 31	$ 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 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