Amount of earned premiums ceded to other entities.

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))
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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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)
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 45
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-URI http://asc.fasb.org/extlink&oid=84167274&loc=d3e27175-158546

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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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

Name:	us-gaap_CededPremiumsEarned
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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%