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3) Investments (Details) | Mar. 31, 2021 | Dec. 31, 2020
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Text Block [Abstract] |  |
Average market value over amortized cost | 97.30% | 94.70%

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- Definition Represents the Average market value over amortized cost, as of the indicated date. + References No definition available. + Details
Name: | snfc_AverageMarketValueOverAmortizedCost
Namespace Prefix: | snfc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_TextBlockAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration