XML 123 R106.htm IDEA: XBRL DOCUMENT
v3.8.0.1

Reinsurance (Components of Written Premiums) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Effects of Reinsurance [Line Items] |  |  |
Direct premiums written | $ 11,141 | $ 10,946 | $ 10,345
Assumed premiums written | 377 | 297 | 319
Ceded premiums written | 4,449 | 4,255 | 3,702
Net written premiums | 7,069 | 6,988 | 6,962
Property and Casualty Insurance |  |  |
Effects of Reinsurance [Line Items] |  |  |
Direct premiums written | 10,655 | 10,451 | 9,852
Assumed premiums written | 327 | 245 | 270
Ceded premiums written | 4,449 | 4,255 | 3,702
Net written premiums | 6,533 | 6,441 | 6,420
Long term care Insurance Product Line |  |  |
Effects of Reinsurance [Line Items] |  |  |
Direct premiums written | 486 | 495 | 493
Assumed premiums written | 50 | 52 | 49
Ceded premiums written | 0 | 0 | 0
Net written premiums | $ 536 | $ 547 | $ 542
Percentage of assumed premiums written to net premiums written |  |  |
Effects of Reinsurance [Line Items] |  |  |
Percentage of assumed premium written to premium written net | 5.30% | 4.30% | 4.60%
Percentage of assumed premiums written to net premiums written | Property and Casualty Insurance |  |  |
Effects of Reinsurance [Line Items] |  |  |
Percentage of assumed premium written to premium written net | 5.00% | 3.80% | 4.20%
Percentage of assumed premiums written to net premiums written | Long term care Insurance Product Line |  |  |
Effects of Reinsurance [Line Items] |  |  |
Percentage of assumed premium written to premium written net | 9.30% | 9.50% | 9.00%

X
- Definition Percentage of Assumed Premium Written to Premium Written Net + References No definition available. + Details
Name: | cna_PercentageOfAssumedPremiumWrittenToPremiumWrittenNet
Namespace Prefix: | cna_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of written premiums assumed from other entities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b)(2),(c)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
Name: | us-gaap_AssumedPremiumsWritten
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of written premiums ceded to other entities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
Name: | us-gaap_CededPremiumsWritten
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums written. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
Name: | us-gaap_DirectPremiumsWritten
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_EffectsOfReinsuranceLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums written. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
Name: | us-gaap_PremiumsWrittenNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | us-gaap_ProductOrServiceAxis=us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ProductOrServiceAxis=cna_LongtermcareInsuranceProductLineMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cna_PercentageofassumedpremiumswrittentonetpremiumswrittenAxis=cna_PercentageOfAssumedPremiumsWrittenToNetPremiumsWrittenMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |