General (Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Cumulative Effect of New Accounting Principle in Period of Adoption on Other Assets		$ (2)
Cumulative Effect of New Accounting Principle in Period of Adoption on Long Term Debt		(2)
Other Invested Assets Overseas Deposit	$ 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