INVESTMENT SECURITIES (Schedule of present information regarding securities in a continuous unrealized loss position by duration of time in a loss position) (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Schedule Of Available For Sale and Held To Maturity Securities [Line Items]
Available for sale, less than 12 months, fair value	$ 225,558	$ 145,936
Available for sale, less than 12 months, unrealized losses	(20,232)	(1,474)
Available for sale, 12 months or more, fair value	18,906	6,571
Available for sale, 12 months or more, unrealized losses	(2,502)	(188)
Available for sale, total fair value	244,464	152,507
Available for sale, total unrealized losses	(22,734)	(1,662)
Held to maturity, less than 12 months, fair value	11,546	0
Held to Maturity, Less than 12 months, unrecognized losses	(668)	0
Held to Maturity, 12 months or more, fair value	73	324
Held to Maturity, 12 months or more, unrecognized losses	(4)	(1)
Held to maturity, total fair value	11,619	324
Held to maturity, total unrecognized losses	672	1
US Government Agencies Debt Securities [Member]
Schedule Of Available For Sale and Held To Maturity Securities [Line Items]
Available for sale, less than 12 months, fair value	111,918	84,123
Available for sale, less than 12 months, unrealized losses	(11,311)	(636)
Available for sale, 12 months or more, fair value	3,809	0
Available for sale, 12 months or more, unrealized losses	(584)	0
Available for sale, total fair value	115,727	84,123
Available for sale, total unrealized losses	(11,895)	(636)
Held to maturity, less than 12 months, fair value	10,750	0
Held to Maturity, Less than 12 months, unrecognized losses	(624)	0
Held to Maturity, 12 months or more, fair value	73	324
Held to Maturity, 12 months or more, unrecognized losses	(4)	(1)
Held to maturity, total fair value	10,823	324
Held to maturity, total unrecognized losses	628	1
US States and Political Subdivisions Debt Securities [Member]
Schedule Of Available For Sale and Held To Maturity Securities [Line Items]
Available for sale, less than 12 months, fair value	23,728	14,472
Available for sale, less than 12 months, unrealized losses	(2,716)	(252)
Available for sale, 12 months or more, fair value	4,700	0
Available for sale, 12 months or more, unrealized losses	(1,018)	0
Available for sale, total fair value	28,428	14,472
Available for sale, total unrealized losses	(3,734)	(252)
Held to maturity, less than 12 months, fair value	401
Held to Maturity, Less than 12 months, unrecognized losses	(33)
Held to Maturity, 12 months or more, fair value	0
Held to Maturity, 12 months or more, unrecognized losses	0
Held to maturity, total fair value	401
Held to maturity, total unrecognized losses	33
Corporate Debt Securities [Member]
Schedule Of Available For Sale and Held To Maturity Securities [Line Items]
Available for sale, less than 12 months, fair value	9,648	0
Available for sale, less than 12 months, unrealized losses	(352)	0
Available for sale, 12 months or more, fair value	0	0
Available for sale, 12 months or more, unrealized losses	0	0
Available for sale, total fair value	9,648	0
Available for sale, total unrealized losses	(352)	0
Residential Mortgage Backed Securities [Member]
Schedule Of Available For Sale and Held To Maturity Securities [Line Items]
Available for sale, less than 12 months, fair value	14,578	5,589
Available for sale, less than 12 months, unrealized losses	(632)	(161)
Available for sale, 12 months or more, fair value	4,465	0
Available for sale, 12 months or more, unrealized losses	(508)	0
Available for sale, total fair value	19,043	5,589
Available for sale, total unrealized losses	(1,140)	(161)
Held to maturity, less than 12 months, fair value	395
Held to Maturity, Less than 12 months, unrecognized losses	(11)
Held to Maturity, 12 months or more, fair value	0
Held to Maturity, 12 months or more, unrecognized losses	0
Held to maturity, total fair value	395
Held to maturity, total unrecognized losses	11
Government-Sponsored Agency Securities
Schedule Of Available For Sale and Held To Maturity Securities [Line Items]
Available for sale, less than 12 months, fair value	15,506	15,956
Available for sale, less than 12 months, unrealized losses	(2,189)	(215)
Available for sale, 12 months or more, fair value	0	0
Available for sale, 12 months or more, unrealized losses	0	0
Available for sale, total fair value	15,506	15,956
Available for sale, total unrealized losses	(2,189)	(215)
Collateralized loan obligations
Schedule Of Available For Sale and Held To Maturity Securities [Line Items]
Available for sale, less than 12 months, fair value	4,823	5,010
Available for sale, less than 12 months, unrealized losses	(202)	(16)
Available for sale, 12 months or more, fair value	0	0
Available for sale, 12 months or more, unrealized losses	0	0
Available for sale, total fair value	4,823	5,010
Available for sale, total unrealized losses	(202)	(16)
Commercial Mortgage Backed Securities [Member]
Schedule Of Available For Sale and Held To Maturity Securities [Line Items]
Available for sale, less than 12 months, fair value	44,618	20,786
Available for sale, less than 12 months, unrealized losses	(2,811)	(194)
Available for sale, 12 months or more, fair value	2,154	2,027
Available for sale, 12 months or more, unrealized losses	(336)	(104)
Available for sale, total fair value	46,772	22,813
Available for sale, total unrealized losses	(3,147)	(298)
SBA Pool Securities [Member]
Schedule Of Available For Sale and Held To Maturity Securities [Line Items]
Available for sale, less than 12 months, fair value	739	0
Available for sale, less than 12 months, unrealized losses	(19)	0
Available for sale, 12 months or more, fair value	3,778	4,544
Available for sale, 12 months or more, unrealized losses	(56)	(84)
Available for sale, total fair value	4,517	4,544
Available for sale, total unrealized losses	$ (75)	$ (84)

Debt securities, held to maturity, continuous unrealized loss position, 12 months or longer.

No definition available.

Name:	frst_DebtSecuritiesHeldToMaturityContinuousUnrealizedLossPosition12MonthsOrLonger
Namespace Prefix:	frst_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Debt securities, held to maturity, continuous unrealized loss position, 12 months or longer, accumulated loss.

+ References

No definition available.

+ Details

Name:	frst_DebtSecuritiesHeldToMaturityContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	frst_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Debt securities, held to maturity, continuous unrealized loss position, less than 12 months,

+ References

No definition available.

+ Details

Name:	frst_DebtSecuritiesHeldToMaturityContinuousUnrealizedLossPositionLessThan12Months
Namespace Prefix:	frst_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Debt securities, held to maturity, continuous unrealized loss position, less than 12 months, accumulated loss.

+ References

No definition available.

+ Details

Name:	frst_DebtSecuritiesHeldToMaturityContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	frst_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Debt securities, held to maturity, unrealized loss position, accumulated loss, total.

+ References

No definition available.

+ Details

Name:	frst_DebtSecuritiesHeldToMaturityUnrealizedLossPositionAccumulatedLossTotal
Namespace Prefix:	frst_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Debt securities, held to maturity, unrealized loss position, total.

+ References

No definition available.

+ Details

Name:	frst_DebtSecuritiesHeldToMaturityUnrealizedLossPositionTotal
Namespace Prefix:	frst_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	frst_ScheduleOfAvailableForSaleAndHeldToMaturitySecuritiesLineItems
Namespace Prefix:	frst_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 5
-URI https://asc.fasb.org/extlink&oid=124269663&loc=SL82922890-210455

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27337-111563

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 55
-Paragraph 8
-URI https://asc.fasb.org/extlink&oid=121590138&loc=SL82922954-210456

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 5
-URI https://asc.fasb.org/extlink&oid=124269663&loc=SL82922890-210455

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 55
-Paragraph 8
-URI https://asc.fasb.org/extlink&oid=121590138&loc=SL82922954-210456

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 4
-Subparagraph (a)(1)
-URI https://asc.fasb.org/extlink&oid=124269663&loc=SL82922888-210455

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 55
-Paragraph 8
-URI https://asc.fasb.org/extlink&oid=121590138&loc=SL82922954-210456

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=frst_SbaPoolSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: