Postretirement Benefits - Cost of Pension Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cost of pension plans:
Service cost for benefits earned	$ 43,492	$ 33,854	$ 38,052
Interest cost on benefit obligation	37,578	39,306	35,382
Expected return on plan assets	(49,411)	(47,484)	(51,260)
Prior service cost amortization	1,343	1,394	1,353
Net actuarial loss amortization	12,125	5,113	12,809
Pension plan cost	$ 45,127	$ 32,183	$ 36,336
Erie Insurance Exchange (EIE)
Postretirement benefits
Postretirement annual benefit expense reimbursed to Indemnity from the exchange and its subsidiaries	59.00%