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Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Unrecognized cost of unvested share-based compensation awards.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount of cash paid related to share-based compensation liabilities.

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Net number of non-option equity instruments granted to participants.

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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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The weighted average of per share prices paid for shares purchased on the open market for issuance to employees under the plan.

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The number of shares purchased on the open market during the period for issuance to employees under the plan.

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Percentage of vesting of share-based compensation awards.

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Incentive and Deferred Compensation Plans (Details) - USD ($)	1 Months Ended			12 Months Ended
	Jun. 30, 2017	Jan. 31, 2017	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Incentive and deferred compensation plans
Number of incentive plans for executives, senior vice presidents and other selected officers				2
Number of deferred compensation plans for executives, senior vice presidents and other selected officers				2
Number of deferred compensation plans for outside directors				2
Class A
Incentive and deferred compensation plans
Total cost of shares purchased outside of publicly announced stock repurchase program				$ 7,300,000	$ 1,500,000
Class A \| Performance shares
Incentive and deferred compensation plans
Shares purchased outside of publicly announced stock repurchase program for share-based compensation awards (in shares)				60,332	15,093
Annual incentive plan (AIP)
Incentive and deferred compensation plans
Maximum percentage of employee compensation that can be deferred (as a percent)				100.00%
Long-term incentive plan (LTIP)
Incentive and deferred compensation plans
Maximum percentage of employee compensation that can be deferred (as a percent)				100.00%
Vesting period of performance and/or service condition(s) (in years and/or months)				3 years
Maximum dollar amount which may be received by any single participant during any specified period				$ 5,000,000
Compensation cost related to share-based compensation plan awards				10,300,000	$ 8,200,000	$ 13,400,000
Tax benefits recognized in income related to award expense				3,600,000	$ 2,900,000	4,700,000
Total unrecognized compensation cost related to share-based compensation plan awards pertaining to open performance periods				$ 7,600,000
Expected period of recognition related to unrecognized compensation cost (in years)				2 years
Long-term incentive plan (LTIP) \| Class A
Incentive and deferred compensation plans
Maximum number of shares authorized for grant (in shares)				1,500,000
Maximum number of shares which may be received by any single participant during any specified period (in shares)				250,000
Average price per share for shares purchased (in dollars per share)				$ 122.40	$ 98.20
Total cost of shares purchased outside of publicly announced stock repurchase program				$ 5,700,000	$ 800,000
Long-term incentive plan (LTIP) \| Class A \| Performance shares
Incentive and deferred compensation plans
Shares purchased outside of publicly announced stock repurchase program for share-based compensation awards (in shares)				46,884	7,661
Long-term incentive plan (LTIP) \| 2015-2017 LTIP performance period
Incentive and deferred compensation plans
Compensation cost related to share-based compensation plan awards				$ 8,400,000
Long-term incentive plan (LTIP) \| 2014-2016 LTIP performance period
Incentive and deferred compensation plans
Compensation cost related to share-based compensation plan awards	$ 4,700,000
Long-term incentive plan (LTIP) \| 2014-2016 LTIP performance period \| Class A
Incentive and deferred compensation plans
Compensation cost related to share-based compensation plan awards	$ 5,900,000
Average price per share for shares delivered (in dollars per share)	$ 126.21
Long-term incentive plan (LTIP) \| 2014-2016 LTIP performance period \| Class A \| Performance shares
Incentive and deferred compensation plans
Shares purchased outside of publicly announced stock repurchase program for share-based compensation awards (in shares)	46,884
Long-term incentive plan (LTIP) \| 2013-2015 LTIP performance period
Incentive and deferred compensation plans
Compensation cost related to share-based compensation plan awards			$ 12,600,000
Long-term incentive plan (LTIP) \| 2013-2015 LTIP performance period \| Class A
Incentive and deferred compensation plans
Compensation cost related to share-based compensation plan awards			$ 700,000
Average price per share for shares delivered (in dollars per share)			$ 96.64
Long-term incentive plan (LTIP) \| 2013-2015 LTIP performance period \| Class A \| Performance shares
Incentive and deferred compensation plans
Shares purchased outside of publicly announced stock repurchase program for share-based compensation awards (in shares)			7,661
Incentive compensation deferral plan
Incentive and deferred compensation plans
Vesting percent of the grant in Class A common stock (as a percent)				100.00%
Payment period for vested share credits (in years)				3 years
Stock compensation plan for outside directors
Incentive and deferred compensation plans
Compensation cost related to share-based compensation plan awards				$ 900,000	$ 500,000	$ 1,900,000
Stock compensation plan for outside directors \| Class A
Incentive and deferred compensation plans
Average price per share for shares purchased (in dollars per share)				$ 121.85	$ 99.23	$ 94.99
Total cost of shares purchased outside of publicly announced stock repurchase program				$ 1,200,000	$ 700,000	$ 9,000,000
Stock compensation plan for outside directors \| Class A \| Performance shares
Incentive and deferred compensation plans
Shares purchased outside of publicly announced stock repurchase program for share-based compensation awards (in shares)				9,663	7,432	94,938
Stock compensation plan for outside directors \| Three month vesting period \| Class A
Incentive and deferred compensation plans
Vesting period of performance and/or service condition(s) (in years and/or months)				3 months
Vesting percent of the grant in Class A common stock (as a percent)				25.00%
Equity compensation plan (ECP)
Incentive and deferred compensation plans
Compensation cost related to share-based compensation plan awards				$ 200,000	$ 800,000	$ 400,000
Total unrecognized compensation cost related to share-based compensation plan awards pertaining to open performance periods				$ 500,000
Expected period of recognition related to unrecognized compensation cost (in years)				3 years
Equity compensation plan (ECP) \| Class A
Incentive and deferred compensation plans
Maximum number of shares authorized for grant (in shares)				100,000
Maximum number of shares which may be received by any single participant during any specified period (in shares)				5,000
Average price per share for shares purchased (in dollars per share)				$ 111.55
Total cost of shares purchased outside of publicly announced stock repurchase program				$ 400,000
Equity compensation plan (ECP) \| Class A \| Performance shares
Incentive and deferred compensation plans
Shares purchased outside of publicly announced stock repurchase program for share-based compensation awards (in shares)				3,785
Equity compensation plan (ECP) \| 2017 ECP performance period \| Class A
Incentive and deferred compensation plans
Restricted stock units granted				4,000
Equity compensation plan (ECP) \| 2016 ECP performance period \| Class A
Incentive and deferred compensation plans
Restricted stock units granted					4,500
Equity compensation plan (ECP) \| 2015 ECP performance period \| Class A
Incentive and deferred compensation plans
Restricted stock units granted						5,500
Equity compensation plan (ECP) \| 2014 ECP performance period \| Class A
Incentive and deferred compensation plans
Compensation cost related to share-based compensation plan awards		$ 400,000
Average price per share for shares delivered (in dollars per share)		$ 109.41
Equity compensation plan (ECP) \| 2014 ECP performance period \| Class A \| Performance shares
Incentive and deferred compensation plans
Shares purchased outside of publicly announced stock repurchase program for share-based compensation awards (in shares)		3,785
Erie Insurance Exchange (EIE) \| Long-term incentive plan (LTIP)
Incentive and deferred compensation plans
Incentive plan compensation cost reimbursed to Indemnity from the Exchange and its subsidiaries (as a percent)				41.00%
Erie Insurance Exchange (EIE) \| Equity compensation plan (ECP)
Incentive and deferred compensation plans
Incentive plan compensation cost reimbursed to Indemnity from the Exchange and its subsidiaries (as a percent)				38.00%