Postretirement Benefits (Details) - USD ($) $ in Thousands			3 Months Ended		6 Months Ended
	Apr. 24, 2018	Jan. 19, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Cost of pension plans:
Service cost for benefits earned			$ 9,513	$ 7,776	$ 19,026	$ 15,553
Interest cost on benefits obligation			8,845	8,568	17,691	17,137
Expected return on plan assets			(12,814)	(10,316)	(25,629)	(20,633)
Prior service cost amortization			338	218	676	436
Net actuarial loss amortization			3,202	2,325	6,404	4,650
Pension plan cost			9,084	$ 8,571	$ 18,168	$ 17,143
Erie Insurance Exchange (EIE)
Postretirement Benefits
Postretirement annual benefit expense reimbursed to Indemnity from the Exchange and its subsidiaries (as a percent)					59.00%
Employee pension plan | Qualified plan | Funded plan
Postretirement Benefits
Employer contribution	$ 40,000	$ 40,000
Employee pension plan | Qualified plan | Funded plan | Erie Family Life Insurance Company (EFL)
Postretirement Benefits
Contingent liability for amount of nonparticipating annuity contracts the employee pension plan purchased from EFL			$ 18,300		$ 18,300