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Fair Value of Financial Instruments - Carrying Value and Estimated Fair Value for Financial Instruments (Parenthetical) (Details) | Sep. 30, 2016 | Dec. 31, 2015
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Fair Value Disclosures [Abstract] |  |
Percentage of coinsurance costs | 10.00% | 10.00%

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- Definition Percentage of coinsurance covered costs. + References No definition available. + Details
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Namespace Prefix: | pri_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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