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Incentive and Deferred Compensation Plans - Narrative (Details) $ / shares in Units, $ in Millions | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2024 USD ($) | Dec. 31, 2024 USD ($) plan $ / shares shares | Dec. 31, 2023 USD ($) $ / shares shares | Dec. 31, 2022 USD ($) $ / shares shares | Dec. 31, 2021 USD ($) | Apr. 23, 2024 shares | Apr. 22, 2024 shares
Schedule of Incentive Plans and Deferred Compensation [Line Items] |  |  |  |  |  |  |
Number of incentive plans for executives, senior vice presidents and other selected officers | plan |  | 2 |  |  |  |  |
Number of deferred compensation plans for executives, senior vice presidents and other selected officers | plan |  | 2 |  |  |  |  |
Number of deferred compensation plans for outside directors | plan |  | 2 |  |  |  |  |
Annual Incentive Plan |  |  |  |  |  |  |
Schedule of Incentive Plans and Deferred Compensation [Line Items] |  |  |  |  |  |  |
Maximum percentage of employee compensation that can be deferred (as a percent) |  | 100.00% |  |  |  |  |
Long-Term Incentive Plan |  |  |  |  |  |  |
Schedule of Incentive Plans and Deferred Compensation [Line Items] |  |  |  |  |  |  |
Maximum percentage of employee compensation that can be deferred (as a percent) |  | 100.00% |  |  |  |  |
Vesting period of performance and/or service condition(s) |  | 3 years |  |  |  |  |
Compensation cost related to share-based compensation plan awards |  | $ 1.5 | $ 7.3 | $ 3.4 |  |  |
Tax benefits recognized in income related to award expense |  | 0.3 | 1.5 | 0.7 |  |  |
Total unrecognized compensation cost related to share-based compensation plan awards pertaining to open performance periods |  | $ 9.4 |  |  |  |  |
Expected period of recognition related to unrecognized compensation cost (in years) |  | 2 years |  |  |  |  |
Long-Term Incentive Plan | Phantom performance share |  |  |  |  |  |  |
Schedule of Incentive Plans and Deferred Compensation [Line Items] |  |  |  |  |  |  |
Vesting percent of the grant in Class A common stock (as a percent) |  | 75.00% |  |  |  |  |
Long-Term Incentive Plan | Time-vesting phantom share |  |  |  |  |  |  |
Schedule of Incentive Plans and Deferred Compensation [Line Items] |  |  |  |  |  |  |
Vesting percent of the grant in Class A common stock (as a percent) |  | 25.00% |  |  |  |  |
Long-Term Incentive Plan | Erie Insurance Exchange (EIE) |  |  |  |  |  |  |
Schedule of Incentive Plans and Deferred Compensation [Line Items] |  |  |  |  |  |  |
Incentive plan compensation cost reimbursed to Indemnity from the Exchange and its subsidiaries (as a percent) |  | 39.00% |  |  |  |  |
Long-Term Incentive Plan | Performance Period Long-Term Incentive Plan 2021-2023 |  |  |  |  |  |  |
Schedule of Incentive Plans and Deferred Compensation [Line Items] |  |  |  |  |  |  |
Estimated plan award for non-vested awards | $ 5.5 |  |  |  |  |  |
Long-Term Incentive Plan | Performance Period Long-Term Incentive Plan 2020-2022 |  |  |  |  |  |  |
Schedule of Incentive Plans and Deferred Compensation [Line Items] |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award, share-based liabilities paid |  |  | 0.2 |  |  |  |
Long-Term Incentive Plan | Performance Period Long-Term Incentive Plan 2019-2021 |  |  |  |  |  |  |
Schedule of Incentive Plans and Deferred Compensation [Line Items] |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award, share-based liabilities paid |  |  |  | 3.8 |  |  |
Long-Term Incentive Plan | Performance Period Long-Term Incentive Plan 2018-2020 |  |  |  |  |  |  |
Schedule of Incentive Plans and Deferred Compensation [Line Items] |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award, share-based liabilities paid |  |  |  |  | $ 3.8 |  |
Long-Term Incentive Plan | Class A common stock |  |  |  |  |  |  |
Schedule of Incentive Plans and Deferred Compensation [Line Items] |  |  |  |  |  |  |
Maximum number of shares authorized for grant (in shares) | shares |  | 1,500,000 |  |  |  |  |
Stock Compensation Plan for Outside Directors |  |  |  |  |  |  |
Schedule of Incentive Plans and Deferred Compensation [Line Items] |  |  |  |  |  |  |
Compensation cost related to share-based compensation plan awards |  | $ 1.1 | 0.9 | 0.8 |  |  |
Maximum period shares may be credited |  | 10 years |  |  |  |  |
Stock Compensation Plan for Outside Directors | Class A common stock |  |  |  |  |  |  |
Schedule of Incentive Plans and Deferred Compensation [Line Items] |  |  |  |  |  |  |
Maximum number of shares authorized for grant (in shares) | shares |  | 250,000 |  |  |  |  |
Share-based compensation arrangement by share-based payment award, share-based liabilities paid |  | $ 1.6 | $ 1.4 | $ 1.2 |  |  |
Average price per share for shares purchased (in dollars per share) | $ / shares |  | $ 405.60 | $ 238.73 | $ 201.93 |  |  |
Stock Compensation Plan for Outside Directors | Class A common stock | Three Month Vesting Period |  |  |  |  |  |  |
Schedule of Incentive Plans and Deferred Compensation [Line Items] |  |  |  |  |  |  |
Vesting period of performance and/or service condition(s) |  | 3 months |  |  |  |  |
Vesting percent of the grant in Class A common stock (as a percent) |  | 25.00% |  |  |  |  |
Stock Compensation Plan for Outside Directors | Class A common stock | Performance Shares |  |  |  |  |  |  |
Schedule of Incentive Plans and Deferred Compensation [Line Items] |  |  |  |  |  |  |
Shares purchased for share-based compensation awards (in shares) | shares |  | 4,044 | 5,894 | 6,048 |  |  |
Incentive Compensation Deferral Plan | Class A common stock |  |  |  |  |  |  |
Schedule of Incentive Plans and Deferred Compensation [Line Items] |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award, share-based liabilities paid |  |  | $ 0.4 | $ 1.2 |  |  |
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 |  |  |  |  |
Average price per share for shares purchased (in dollars per share) | $ / shares |  |  | $ 230.71 | $ 176.44 |  |  |
Incentive Compensation Deferral Plan | Class A common stock | Performance Shares |  |  |  |  |  |  |
Schedule of Incentive Plans and Deferred Compensation [Line Items] |  |  |  |  |  |  |
Shares purchased for share-based compensation awards (in shares) | shares |  |  | 1,608 | 7,046 |  |  |
Equity Compensation Plan |  |  |  |  |  |  |
Schedule of Incentive Plans and Deferred Compensation [Line Items] |  |  |  |  |  |  |
Compensation cost related to share-based compensation plan awards |  | $ 6.9 | $ 4.0 | $ 0.8 |  |  |
Total unrecognized compensation cost related to share-based compensation plan awards pertaining to open performance periods |  | $ 2.6 |  |  |  |  |
Expected period of recognition related to unrecognized compensation cost (in years) |  | 2 years |  |  |  |  |
Equity Compensation Plan | Erie Insurance Exchange (EIE) |  |  |  |  |  |  |
Schedule of Incentive Plans and Deferred Compensation [Line Items] |  |  |  |  |  |  |
Incentive plan compensation cost reimbursed to Indemnity from the Exchange and its subsidiaries (as a percent) |  | 35.00% | 35.00% | 3.00% |  |  |
Equity Compensation Plan | Class A common stock |  |  |  |  |  |  |
Schedule of Incentive Plans and Deferred Compensation [Line Items] |  |  |  |  |  |  |
Maximum number of shares authorized for grant (in shares) | shares |  |  |  |  |  | 250,000 | 100,000
Maximum number of shares which may be received by any single participant during any specified period (in shares) | shares |  | 10,000 |  |  |  |  |
Equity Compensation Plan | Class A common stock | Performance Period Equity Compensation Plan 2019 |  |  |  |  |  |  |
Schedule of Incentive Plans and Deferred Compensation [Line Items] |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award, share-based liabilities paid |  |  | $ 0.4 |  |  |  |
Average price per share for shares purchased (in dollars per share) | $ / shares |  |  | $ 252.32 |  |  |  |
Equity Compensation Plan | Class A common stock | Performance Period Equity Compensation Plan 2019 | Performance Shares |  |  |  |  |  |  |
Schedule of Incentive Plans and Deferred Compensation [Line Items] |  |  |  |  |  |  |
Shares purchased for share-based compensation awards (in shares) | shares |  |  | 1,610 |  |  |  |
Equity Compensation Plan | Class A common stock | Performance Period Equity Compensation Plan 2018 |  |  |  |  |  |  |
Schedule of Incentive Plans and Deferred Compensation [Line Items] |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award, share-based liabilities paid |  | $ 3.2 |  | $ 0.3 |  |  |
Average price per share for shares purchased (in dollars per share) | $ / shares |  |  |  | $ 190.68 |  |  |
Equity Compensation Plan | Class A common stock | Performance Period Equity Compensation Plan 2018 | Performance Shares |  |  |  |  |  |  |
Schedule of Incentive Plans and Deferred Compensation [Line Items] |  |  |  |  |  |  |
Shares purchased for share-based compensation awards (in shares) | shares |  |  |  | 1,786 |  |  |

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