Postretirement Benefits (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Cost of pension plans:
Service cost for benefits earned	$ 31,106	$ 28,201	$ 30,433
Interest cost on benefit obligation	34,275	33,125	30,755
Expected return on plan assets	(41,267)	(39,520)	(35,921)
Prior service cost amortization	871	696	670
Net actuarial loss amortization	9,301	8,111	14,031
Settlement cost	302
Pension plan cost	$ 34,588	$ 30,613	$ 39,968
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 | Erie Family Life Insurance Company (EFL)
Postretirement benefits
Contingent liability for amount of nonparticipating annuity contracts the employee pension plan purchased from EFL	$ 19,500
Supplemental employee retirement plan (SERP) | Nonqualified plan | Unfunded plan
Cost of pension plans:
Number of former plan participants resulting in charges recognized as settlements	2