Related Party (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Related Party Transactions
Rent expense on office space leased from Exchange			$ 14.3
Reimbursements from the Exchange related to rent expense			$ 4.9
Scenario, Forecast [Member]
Related Party Transactions
Rent expense on office space leased from Exchange		$ 17.4
Erie Insurance Exchange (EIE)
Related Party Transactions
Number of field office facilities leased from the Exchange	3