Postretirement Benefits - Assumptions Are Used in Determining Components of Postretirement Benefit Obligation (Details)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Weighted average discount rates:
Used to determine benefit obligations at December 31	2.00%	3.00%	4.00%
Used to determine net periodic postretirement benefit cost for years ended December 31	3.00%	4.00%	3.50%
Assumed health care cost trend rates at December 31:
Health care cost trend rate assumed for next year	5.50%	6.00%	6.50%
Rate to which the cost trend rate is assumed to decline (the ultimate trend rate)	4.00%	4.00%	3.90%
Year that rate reaches ultimate trend rate	2075	2075	2075

No definition available.

Name:	pfc_DefinedBenefitPlanWeightedAverageDiscountRatesAbstract
Namespace Prefix:	pfc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The weighted average discount rates used to determine benefit obligations.

+ References

No definition available.

+ Details

Name:	pfc_DefinedBenefitPlanWeightedAverageDiscountRatesUsedToDetermineBenefitObligations
Namespace Prefix:	pfc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The weighted average discount rates used to determine net periodic postretirement benefit cost.

+ References

No definition available.

+ Details

Name:	pfc_DefinedBenefitPlanWeightedAverageDiscountRatesUsedToDeterminenetPeriodicPostretirementBenefitsCost
Namespace Prefix:	pfc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_DefinedBenefitPlanAssumedHealthCareCostTrendRatesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Assumed rate, for next fiscal year, based on annual change in cost of health care cost benefits used to measure expected cost of benefits covered by defined benefit postretirement plan. Factors include, but are not limited to, estimate of health care inflation, change in health care utilization or delivery pattern, technological advances, and change in health status of participant. Excludes factors for change in composition of plan population by age and dependency status.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (l)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedNextFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Ultimate trend rate for health care cost for defined benefit postretirement plan.

+ References

+ Details

Name:	us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Year ultimate health care cost trend rate is expected to be reached, in CCYY format.

+ References

+ Details

Name:	us-gaap_DefinedBenefitPlanYearHealthCareCostTrendRateReachesUltimateTrendRate
Namespace Prefix:	us-gaap_
Data Type:	us-types:gYearListItemType
Balance Type:	na
Period Type:	duration