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Reinsurance and Indemnification - Catastrophe reinsurance (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Ceded Credit Risk [Line Items] |  |  |
Ceded premium earned | $ 1,407 | $ 1,306 | $ 1,274
Limit of aggregate losses under aggregate excess agreement | 3,400 |  |
Catastrophe Reinsurance Sixth Layer |  |  |
Ceded Credit Risk [Line Items] |  |  |
Limit of aggregate losses under aggregate excess agreement | $ 324 |  |
Retention percentage of reinsurance limit | 95.00% |  |
Catastrophe Reinsurance Program |  |  |
Ceded Credit Risk [Line Items] |  |  |
Ceded premium earned | $ 376 | $ 343 | $ 344
Nationwide Per Occurrence Excess Catastrophe Reinsurance Program |  |  |
Ceded Credit Risk [Line Items] |  |  |
Limit of aggregate losses under aggregate excess agreement | 4,860 |  |
Retention amount for catastrophe reinsurance agreement contracts | $ 500 |  |
Catastrophe Reinsurance First Five Layers |  |  |
Ceded Credit Risk [Line Items] |  |  |
Retention percentage of reinsurance limit | 95.00% |  |

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- Definition Represents the retention percentage of annual coverage limit provided by the insurance arrangement for catastrophe claims. + References No definition available. + Details
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- Definition The limit of aggregated losses per contract year beyond which coverage provided by an aggregate excess agreement applies. + References No definition available. + Details
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- Definition Represents the retention amount of catastrophe reinsurance agreement contracts. + References No definition available. + Details
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- 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
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- Definition Amount of earned premiums ceded to other entities. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17(Column C)) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574992-122915 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(1)) -URI http://asc.fasb.org/extlink&oid=116637232&loc=SL114874131-224263 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -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 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=84167274&loc=d3e27175-158546 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 Reference 6: http://www.xbrl.org/2003/role/disclosureRef -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
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