Fair Value (Details 3) (Fair value measurements on a recurring basis, USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2012 holding
Fair value inputs assets quantitative information
Fair value	$ 88
Maximum
Unobservable input
Securities priced via unobserveable inputs as a percentage of total portfolio	0.80%
Indemnity
Fair value inputs assets quantitative information
Fair value	5
No. of holdings	3
Indemnity \| Corporate debt securities \| Consensus pricing
Fair value inputs assets quantitative information
Fair value	1
No. of holdings	1
Unobservable input
Non-binding broker quote	$ 115.86
Indemnity \| Collateralized debt obligations (CDO) \| Present value
Fair value inputs assets quantitative information
Fair value	4
No. of holdings	2
Unobservable input
Projected LIBOR rate	0.48%
Indemnity \| Collateralized debt obligations (CDO) \| Present value \| Minimum
Unobservable input
Repayment at maturity	45.00%
Discount rate	7.50%
Indemnity \| Collateralized debt obligations (CDO) \| Present value \| Maximum
Unobservable input
Repayment at maturity	100.00%
Discount rate	15.00%
Indemnity \| Collateralized debt obligations (CDO) \| Present value \| Weighted average
Unobservable input
Repayment at maturity	90.60%
Discount rate	10.80%
Exchange
Fair value inputs assets quantitative information
Fair value	83
No. of holdings	23
Exchange \| Corporate debt securities \| Consensus pricing
Fair value inputs assets quantitative information
Fair value	15
No. of holdings	3
Exchange \| Corporate debt securities \| Consensus pricing \| Minimum
Unobservable input
Non-binding broker quote	$ 102.00
Exchange \| Corporate debt securities \| Consensus pricing \| Maximum
Unobservable input
Non-binding broker quote	$ 117.00
Exchange \| Corporate debt securities \| Comparable pricing
Fair value inputs assets quantitative information
Fair value	10
No. of holdings	2
Unobservable input
Average comparable publicly traded EBITDA multiple	7.80
Exchange \| Corporate debt securities \| Comparable pricing \| Minimum
Unobservable input
Comparable private transaction EBITDA multiples	7.5
Exchange \| Corporate debt securities \| Comparable pricing \| Maximum
Unobservable input
Comparable private transaction EBITDA multiples	17.10
Exchange \| Corporate debt securities \| Comparable pricing \| Weighted average
Unobservable input
Comparable private transaction EBITDA multiples	7.50
Average comparable publicly traded EBITDA multiple	7.50
Exchange \| Corporate debt securities \| Held at cost
Fair value inputs assets quantitative information
Fair value	1
No. of holdings	2
Unobservable input
Private securities with no observable market	100.00
Exchange \| Collateralized debt obligations (CDO) \| Consensus pricing
Fair value inputs assets quantitative information
Fair value	3
No. of holdings	4
Exchange \| Collateralized debt obligations (CDO) \| Consensus pricing \| Minimum
Unobservable input
Non-binding broker quote	$ 3.00
Exchange \| Collateralized debt obligations (CDO) \| Consensus pricing \| Maximum
Unobservable input
Non-binding broker quote	$ 85.00
Exchange \| Collateralized debt obligations (CDO) \| Consensus pricing \| Weighted average
Unobservable input
Non-binding broker quote	$ 64.70
Exchange \| Collateralized debt obligations (CDO) \| Present value
Fair value inputs assets quantitative information
Fair value	24
No. of holdings	6
Unobservable input
Projected LIBOR rate	0.48%
Exchange \| Collateralized debt obligations (CDO) \| Present value \| Minimum
Unobservable input
Repayment at maturity	45.00%
Discount rate	7.00%
Exchange \| Collateralized debt obligations (CDO) \| Present value \| Maximum
Unobservable input
Repayment at maturity	100.00%
Discount rate	15.00%
Exchange \| Collateralized debt obligations (CDO) \| Present value \| Weighted average
Unobservable input
Repayment at maturity	94.40%
Discount rate	9.20%
Exchange \| States & political subdivisions \| Comparable pricing
Fair value inputs assets quantitative information
Fair value	5
No. of holdings	1
Unobservable input
Comparable security yield	0.48%
Added yield due to lack of marketability	1.00%
Exchange \| Other debt securities \| Held at cost
Fair value inputs assets quantitative information
Fair value	5
No. of holdings	1
Unobservable input
Private securities with no observable market	100.00
Exchange \| Nonredeemable preferred stock \| Comparable pricing
Fair value inputs assets quantitative information
Fair value	6
No. of holdings	1
Unobservable input
Comparable security yield	7.68%
Exchange \| Common stock \| Comparable pricing
Fair value inputs assets quantitative information
Fair value	$ 14
No. of holdings	3
Unobservable input
Average comparable publicly traded EBITDA multiple	7.80
Exchange \| Common stock \| Comparable pricing \| Minimum
Unobservable input
Comparable private transaction EBITDA multiples	7.5
Discount for lack of marketability	5.00%
Exchange \| Common stock \| Comparable pricing \| Maximum
Unobservable input
Comparable private transaction EBITDA multiples	17.10
Discount for lack of marketability	30.00%
Exchange \| Common stock \| Comparable pricing \| Weighted average
Unobservable input
Comparable private transaction EBITDA multiples	7.50
Average comparable publicly traded EBITDA multiple	7.50
Discount for lack of marketability	30.00%

Name:	erie_FairValueInputsAbstract
Namespace Prefix:	erie_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Percentage of adjustment to the discount rate used to determine the value a security, due to the lack of a ready market where the security can be sold, used as an input to measure fair value.

+ References

No definition available.

+ Details

Name:	erie_FairValueInputsAddedForLackOfMarketability
Namespace Prefix:	erie_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	erie_FairValueInputsAssetsQuantitativeInformationLineItems
Namespace Prefix:	erie_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of adjustment to the observed price for a similar asset or liability based upon the yield of another security with similar characteristics to the security being valued, used as an input to measure fair value.

+ References

No definition available.

+ Details

Name:	erie_FairValueInputsComparableSecurityYield
Namespace Prefix:	erie_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Adjustment to the discount rate used to measure fair value for lack of ability to convert business interests into cash quickly.

+ References

No definition available.

+ Details

Name:	erie_FairValueInputsDiscountForLackOfMarketability
Namespace Prefix:	erie_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value.

+ References

No definition available.

+ Details

Name:	erie_FairValueInputsDiscountRate
Namespace Prefix:	erie_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Market valuation of entity divided by earnings before interest, taxes, depreciation, and amortization (EBITDA), used as an input to measure fair value.

+ References

No definition available.

+ Details

Name:	erie_FairValueInputsEarningsBeforeInterestTaxesDepreciationAndAmortizationMultiple
Namespace Prefix:	erie_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Valuation of a comparable, privately held entity divided by earnings before interest, taxes, depreciation, and amortization (EBITDA), used as an input to measure fair value.

+ References

No definition available.

+ Details

Name:	erie_FairValueInputsEarningsBeforeInterestTaxesDepreciationAndAmortizationMultiplePrivate
Namespace Prefix:	erie_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Price at which a dealer is willing to buy an instrument, used as an input to measure fair value.

+ References

No definition available.

+ Details

Name:	erie_FairValueInputsOfferedQuotes
Namespace Prefix:	erie_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the private securities with no observable market.

+ References

No definition available.

+ Details

Name:	erie_FairValueInputsPrivateSecuritiesWithNoObservableMarket
Namespace Prefix:	erie_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Rate at which loans or a loan portfolio are expected to repay principal balances on maturity, used as an input to measure fair value.

+ References

No definition available.

+ Details

Name:	erie_FairValueInputsRepaymentRate
Namespace Prefix:	erie_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Variable interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value.

+ References

No definition available.

+ Details

Name:	erie_FairValueInputsVariableRate
Namespace Prefix:	erie_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of securities in the entity's investment portfolio where the determination of fair value utilizes unobservable inputs.

+ References

No definition available.

+ Details

Name:	erie_NumberOfInvestmentHoldingsUnobservableInputs
Namespace Prefix:	erie_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the percentage of securities using unobserveable inputs as compared to the total portfolio.

+ References

No definition available.

+ Details

Name:	erie_PercentageOfSecuritiesUsingUnobservableInputsToTotalPortfolio
Namespace Prefix:	erie_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

This element represents an asset measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 157

-Paragraph 32

-Subparagraph c

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258

+ Details

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