Indemnity Supplemental Information (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
Available-for-sale securities, at fair value:
Fixed maturities	$ 9,980.0		$ 9,571.0
Equity securities	740.0		875.0
Trading securities, at fair value	2,981.0		3,223.0
Limited partnerships	927.0		979.0
Other invested assets	22.0		21.0
Total investments	14,650.0		14,669.0
Cash and cash equivalents	618.0	$ 379.0	514.0	$ 452.0
Premiums receivable from policyholders	1,417.0		1,281.0
Reinsurance recoverable	162.0		161.0
Deferred income tax asset	51.0		37.0
Deferred acquisition costs	656.0		595.0
Other assets	561.0		501.0
Total assets	18,115.0		17,758.0
Liabilities
Losses and loss expense reserves	3,923.0		3,853.0
Life policy and deposit contract reserves	1,848.0		1,812.0
Unearned premiums	3,109.0		2,834.0
Deferred income tax liability	283.0		490.0
Other liabilities, adjusted for consolidation presentation	827.0		786.0
Total liabilities	9,990.0		9,775.0
Shareholders' equity and noncontrolling interest
Total Indemnity shareholders' equity	750.0		703.0
Noncontrolling interest in consolidated entity - Exchange	7,375.0		7,280.0
Total equity	8,125.0		7,983.0	7,550.0
Total liabilities, shareholders' equity, and noncontrolling interest	18,115.0		17,758.0
Indemnity
Available-for-sale securities, at fair value:
Fixed maturities	588.0		564.0
Equity securities	20.0		25.0
Limited partnerships	95.0		113.0
Other invested assets	1.0		1.0
Total investments	704.0		703.0
Cash and cash equivalents	122.0	45.0	92.0	49.0
Deferred income tax asset	51.0		37.0
Other assets	110.0		127.0
Receivables from the Exchange and other affiliates	385.0		335.0
Note receivable from EFL	25.0		25.0
Total assets	1,397.0		1,319.0
Liabilities
Other liabilities, adjusted for consolidation presentation	647.0		616.0
Total liabilities	647.0		616.0
Shareholders' equity and noncontrolling interest
Total Indemnity shareholders' equity	750.0		703.0
Total equity	750.0		703.0	734.0
Total liabilities, shareholders' equity, and noncontrolling interest	1,397.0		1,319.0
Exchange
Available-for-sale securities, at fair value:
Fixed maturities	9,392.0		9,007.0
Equity securities	720.0		850.0
Trading securities, at fair value	2,981.0		3,223.0
Limited partnerships	832.0		866.0
Other invested assets	21.0		20.0
Total investments	13,946.0		13,966.0
Cash and cash equivalents	496.0		422.0
Premiums receivable from policyholders	1,417.0		1,281.0
Reinsurance recoverable	162.0		161.0
Deferred acquisition costs	656.0		595.0
Other assets	451.0		374.0
Total assets	17,128.0		16,799.0
Liabilities
Losses and loss expense reserves	3,923.0		3,853.0
Life policy and deposit contract reserves	1,848.0		1,812.0
Unearned premiums	3,109.0		2,834.0
Deferred income tax liability	283.0		490.0
Other liabilities, adjusted for consolidation presentation	590.0		530.0
Total liabilities	9,753.0		9,519.0
Shareholders' equity and noncontrolling interest
Noncontrolling interest in consolidated entity - Exchange	7,375.0		7,280.0
Total equity	7,375.0		7,280.0	$ 6,816.0
Total liabilities, shareholders' equity, and noncontrolling interest	17,128.0		16,799.0
Payable to Indemnity	381.0		331.0
Erie Family Life Insurance Company (EFL)
Available-for-sale securities, at fair value:
Note receivable from EFL	25.0
Shareholders' equity and noncontrolling interest
Payable to Indemnity	$ 4.0		4.0
Note receivable from EFL, annual interest rate (as a percent)	6.70%
Note receivable from EFL, interest income recognized by Indemnity	$ 1.3	$ 1.3
Reclassifications and eliminations
Available-for-sale securities, at fair value:
Receivables from the Exchange and other affiliates	(385.0)		(335.0)
Note receivable from EFL	(25.0)		(25.0)
Total assets	(410.0)		(360.0)
Liabilities
Other liabilities, adjusted for consolidation presentation	(410.0)		(360.0)
Total liabilities	(410.0)		(360.0)
Shareholders' equity and noncontrolling interest
Total liabilities, shareholders' equity, and noncontrolling interest	$ (410.0)		$ (360.0)