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