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Investments - Schedule of Proceeds from Sales or Other Redemptions of Available-For-Sale Securities (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
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Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
Proceeds From Sale And Maturity Of Available For Sale Securities [Abstract] |  |  |  |
Proceeds from sales or other redemptions | $ 118,557 | $ 67,721 | $ 240,263 | $ 142,698

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- Definition Proceeds from sales or other redemptions of available for sale securities. + References No definition available. + Details
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Namespace Prefix: | pri_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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