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Reinsurance and Indemnification - Catastrophe reinsurance (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2018 USD ($) contract | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Ceded Credit Risk [Line Items] |  |  |
Ceded premium earned | $ 1,306 | $ 1,274 | $ 1,296
Retention amount for catastrophe reinsurance agreement contracts | 500 |  |
Catastrophe Reinsurance Program |  |  |
Ceded Credit Risk [Line Items] |  |  |
Ceded premium earned | 343 | $ 344 | $ 381
Nationwide Per Occurrence Excess Catastrophe Reinsurance Program |  |  |
Ceded Credit Risk [Line Items] |  |  |
Limit of aggregate losses under aggregate excess agreement | 4,500 |  |
Retention amount for catastrophe reinsurance agreement contracts | $ 500 |  |
Catastrophe reinsurance, number of agreements | contract | 4 |  |

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