Incentive and Deferred Compensation Plans - Narrative (Details)	1 Months Ended			9 Months Ended	12 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) plan $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares
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					$ 5,000,000
Compensation cost related to share-based compensation plan awards					12,000,000.0	$ 7,300,000	$ 6,300,000
Tax benefits recognized in income related to award expense					2,500,000	$ 1,500,000	$ 1,300,000
Total unrecognized compensation cost related to share-based compensation plan awards pertaining to open performance periods					$ 6,800,000
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 2018-2020
Incentive and deferred compensation plans
Compensation cost related to share-based compensation plan awards				$ 11,200,000
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	$ 7,400,000
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		$ 8,200,000
Long-term incentive plan awards | Performance Period Long Term Incentive Plan 2015-2017
Incentive and deferred compensation plans
Compensation cost related to share-based compensation plan awards			$ 8,300,000
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					$ 155.92	$ 176.34	$ 119.28
Total cost of shares purchased for share-based compensation awards					$ 600,000	$ 800,000	$ 1,400,000
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					3,934	4,387	12,005
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					$ 3,400,000
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					$ 900,000	$ 1,100,000	$ 800,000
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					$ 201.78	$ 194.62	$ 122.19
Total cost of shares purchased for share-based compensation awards					$ 1,500,000	$ 1,400,000	$ 1,100,000
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					7,401	7,370	9,285
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					$ 700,000	$ 500,000	$ 400,000
Total unrecognized compensation cost related to share-based compensation plan awards pertaining to open performance periods					$ 600,000
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 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					$ 300,000
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						$ 400,000
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
Equity compensation plan | Performance Period Equity Compensation Plan 2015 | Class A common stock
Incentive and deferred compensation plans
Average price per share for shares purchased (in dollars per share) | $ / shares							$ 117.39
Total cost of shares purchased for share-based compensation awards							$ 700,000
Equity compensation plan | Performance Period Equity Compensation Plan 2015 | Class A common stock | Performance shares
Incentive and deferred compensation plans
Shares purchased for share-based compensation awards (in shares) | shares							5,830
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					48.00%
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					59.00%	49.00%	68.00%