Incentive and Deferred Compensation Plans - Narrative (Details) $ / shares in Units, $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($)	Dec. 31, 2021 USD ($) plan $ / shares shares	Dec. 31, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($)
Incentive and deferred compensation plans
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 awards
Incentive and deferred compensation plans
Maximum percentage of employee compensation that can be deferred		100.00%
Long-term incentive plan awards
Incentive and deferred compensation plans
Maximum percentage of employee compensation that can be deferred		100.00%
Vesting period of performance and/or service condition(s)		3 years
Maximum dollar amount which may be received by any single participant during any specified period		$ 5.0
Compensation cost related to share-based compensation plan awards		3.0	$ 12.0	$ 7.3
Tax benefits recognized in income related to award expense		0.6	2.5	1.5
Total unrecognized compensation cost related to share-based compensation plan awards pertaining to open performance periods		$ 3.8
Expected period of recognition related to unrecognized compensation cost		2 years
Long-term incentive plan awards | Class A common stock
Incentive and deferred compensation plans
Maximum number of shares authorized for grant (in shares) | shares		1,500,000
Maximum number of shares which may be received by any single participant during any specified period (in shares) | shares		250,000
Long-term incentive plan awards | Performance Period Long Term Incentive Plan 2019-2021
Incentive and deferred compensation plans
Compensation cost related to share-based compensation plan awards	$ 5.4
Long-term incentive plan awards | Performance Period Long-Term Incentive Plan 2018-2020
Incentive and deferred compensation plans
Compensation cost related to share-based compensation plan awards			$ 10.9
Long-term incentive plan awards | Performance Period Long Term Incentive Plan 2017-2019
Incentive and deferred compensation plans
Compensation cost related to share-based compensation plan awards				$ 7.4
Long-term incentive plan awards | Performance Period Long-Term Incentive Plan 2016-2018
Incentive and deferred compensation plans
Compensation cost related to share-based compensation plan awards					$ 8.2
Incentive compensation deferral plan | Class A common stock
Incentive and deferred compensation plans
Vesting percent of the grant in Class A common stock		100.00%
Payment period for vested share credits		3 years
Average price per share for shares purchased (in dollars per share) | $ / shares		$ 232.81	$ 155.92	$ 176.34
Total cost of shares purchased for share-based compensation awards		$ 0.6	$ 0.6	$ 0.8
Incentive compensation deferral plan | Class A common stock | Performance shares
Incentive and deferred compensation plans
Shares purchased for share-based compensation awards (in shares) | shares		2,570	3,934	4,387
Incentive compensation deferral plan | Performance Period Long-Term Incentive Plan 2018-2020 | Class A common stock
Incentive and deferred compensation plans
Average price per share for shares purchased (in dollars per share) | $ / shares		$ 191.25
Total cost of shares purchased for share-based compensation awards		$ 3.4
Incentive compensation deferral plan | Performance Period Long-Term Incentive Plan 2018-2020 | Class A common stock | Performance shares
Incentive and deferred compensation plans
Shares purchased for share-based compensation awards (in shares) | shares		17,881
Incentive compensation deferral plan | Performance Period Long Term Incentive Plan 2017-2019 | Class A common stock
Incentive and deferred compensation plans
Average price per share for shares purchased (in dollars per share) | $ / shares			$ 185.31
Total cost of shares purchased for share-based compensation awards			$ 3.4
Incentive compensation deferral plan | Performance Period Long Term Incentive Plan 2017-2019 | Class A common stock | Performance shares
Incentive and deferred compensation plans
Shares purchased for share-based compensation awards (in shares) | shares			18,126
Stock compensation plan for outside directors
Incentive and deferred compensation plans
Compensation cost related to share-based compensation plan awards		$ 0.8	$ 0.9	$ 1.1
Stock compensation plan for outside directors | Class A common stock
Incentive and deferred compensation plans
Average price per share for shares purchased (in dollars per share) | $ / shares		$ 212.41	$ 201.78	$ 194.62
Total cost of shares purchased for share-based compensation awards		$ 1.1	$ 1.5	$ 1.4
Stock compensation plan for outside directors | Class A common stock | Performance shares
Incentive and deferred compensation plans
Shares purchased for share-based compensation awards (in shares) | shares		5,238	7,401	7,370
Stock compensation plan for outside directors | Three Month Vesting Period | Class A common stock
Incentive and deferred compensation plans
Vesting period of performance and/or service condition(s)		3 months
Vesting percent of the grant in Class A common stock		25.00%
Equity compensation plan
Incentive and deferred compensation plans
Compensation cost related to share-based compensation plan awards		$ 0.2	$ 0.7	$ 0.5
Total unrecognized compensation cost related to share-based compensation plan awards pertaining to open performance periods		$ 0.2
Expected period of recognition related to unrecognized compensation cost		3 years
Equity compensation plan | Class A common stock
Incentive and deferred compensation plans
Maximum number of shares authorized for grant (in shares) | shares		100,000
Maximum number of shares which may be received by any single participant during any specified period (in shares) | shares		5,000
Equity compensation plan | Performance Period Equity Compensation Plan 2018 | Class A common stock
Incentive and deferred compensation plans
Average price per share for shares purchased (in dollars per share) | $ / shares		$ 242.01
Total cost of shares purchased for share-based compensation awards		$ 0.2
Equity compensation plan | Performance Period Equity Compensation Plan 2018 | Class A common stock | Performance shares
Incentive and deferred compensation plans
Shares purchased for share-based compensation awards (in shares) | shares		978
Equity compensation plan | Performance Period Equity Compensation Plan 2017 | Class A common stock
Incentive and deferred compensation plans
Average price per share for shares purchased (in dollars per share) | $ / shares			$ 165.82
Total cost of shares purchased for share-based compensation awards			$ 0.3
Equity compensation plan | Performance Period Equity Compensation Plan 2017 | Class A common stock | Performance shares
Incentive and deferred compensation plans
Shares purchased for share-based compensation awards (in shares) | shares			1,787
Equity compensation plan | Performance Period Equity Compensation Plan 2016 | Class A common stock
Incentive and deferred compensation plans
Average price per share for shares purchased (in dollars per share) | $ / shares				$ 132.35
Total cost of shares purchased for share-based compensation awards				$ 0.4
Equity compensation plan | Performance Period Equity Compensation Plan 2016 | Class A common stock | Performance shares
Incentive and deferred compensation plans
Shares purchased for share-based compensation awards (in shares) | shares				3,246
Erie Insurance Exchange (EIE) | Long-term incentive plan awards
Incentive and deferred compensation plans
Incentive plan compensation cost reimbursed to Indemnity from the exchange and its subsidiaries		46.00%
Erie Insurance Exchange (EIE) | Equity compensation plan
Incentive and deferred compensation plans
Incentive plan compensation cost reimbursed to Indemnity from the exchange and its subsidiaries		33.00%	59.00%	49.00%