Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Dec. 31, 2010
Revenues
Premiums earned/life policy revenue	$ 1,137		$ 1,061	$ 3,333	$ 3,138
Net investment income	105		108	326	326
Net realized investment gains (losses)	169		(422)	358	(234)
Net impairment losses recognized in earnings	0		0	0	0
Equity in earnings of limited partnerships	41		40	99	150
Service agreement and other revenue	8		9	24	26
Total revenues	1,460		796	4,140	3,406
Benefits and expenses
Insurance losses and loss expenses	912		848	2,571	2,724
Policy acquisition and underwriting expenses	278		253	835	749
Total benefits and expenses	1,190		1,101	3,406	3,473
Income (loss) before income taxes	270		(305)	734	(67)
Provision for income taxes	86		(125)	234	(54)
Net income (loss)	184		(180)	500	(13)	268
Management operations
Segment information
Indemnity's maximum management fee rate as a percent of premiums written and assumed by the Exchange				25.00%
Reduction in commission expense due to adjustment		6.0		6.0		0.5	0.7
Indemnity commission expense adjustment		This amount represents the reimbursement by the North Carolina Reinsurance Facility (NCRF) for commissions Indemnity paid to agents on the surcharges collected on behalf of the NCRF in prior periods. This amount was incorrectly recorded as a benefit to the Exchange in prior periods.		This amount represents the reimbursement by the North Carolina Reinsurance Facility (NCRF) for commissions Indemnity paid to agents on the surcharges collected on behalf of the NCRF in prior periods. This amount was incorrectly recorded as a benefit to the Exchange in prior periods.
Revenues
Management fee revenue	305		280	882	816
Service agreement and other revenue	8		8	23	25
Total revenues	313		288	905	841
Benefits and expenses
Cost of management operations	247		226	734	667
Total benefits and expenses	247		226	734	667
Income (loss) before income taxes	66		62	171	174
Provision for income taxes	23		22	60	61
Net income (loss)	43		40	111	113
Property and casualty insurance operations
Revenues
Premiums earned/life policy revenue	1,118		1,045	3,279	3,089
Total revenues	1,118		1,045	3,279	3,089
Benefits and expenses
Insurance losses and loss expenses	890		823	2,501	2,653
Policy acquisition and underwriting expenses	329		302	963	882
Total benefits and expenses	1,219		1,125	3,464	3,535
Income (loss) before income taxes	(101)		(80)	(185)	(446)
Provision for income taxes	(36)		(28)	(65)	(156)
Net income (loss)	(65)		(52)	(120)	(290)
Life insurance operations
Revenues
Premiums earned/life policy revenue	19		16	55	50
Service agreement and other revenue	0		1	1	1
Total revenues	19		17	56	51
Benefits and expenses
Insurance losses and loss expenses	24		26	74	75
Policy acquisition and underwriting expenses	8		7	25	21
Total benefits and expenses	32		33	99	96
Income (loss) before income taxes	(13)		(16)	(43)	(45)
Provision for income taxes	(4)		(6)	(15)	(16)
Net income (loss)	(9)		(10)	(28)	(29)
Investment operations
Revenues
Net investment income	108		111	334	334
Net realized investment gains (losses)	169		(422)	358	(234)
Net impairment losses recognized in earnings	0		0	0	0
Equity in earnings of limited partnerships	41		40	99	150
Total revenues	318		(271)	791	250
Benefits and expenses
Income (loss) before income taxes	318		(271)	791	250
Provision for income taxes	103		(113)	254	57
Net income (loss)	215		(158)	537	193
Eliminations
Revenues
Premiums earned/life policy revenue				(1)	(1)
Net investment income	(3)		(3)	(8)	(8)
Management fee revenue	(305)		(280)	(882)	(816)
Total revenues	(308)		(283)	(891)	(825)
Benefits and expenses
Cost of management operations	(247)		(226)	(734)	(667)
Insurance losses and loss expenses	(2)		(1)	(4)	(4)
Policy acquisition and underwriting expenses	(59)		(56)	(153)	(154)
Total benefits and expenses	(308)		(283)	(891)	(825)
Erie Family Life Insurance Company ("EFL") | Life insurance operations
Segment information
Total investment revenues	25		29	77	83
Benefits and expenses
Income (loss) before income taxes	$ 12		$ 13	$ 34	$ 38