Summary of Significant Accounting Policies (Direct Premium Attributable By Segment) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Annual direct premiums written	$ 2,655,720	$ 2,579,683	$ 2,558,825
Private Passenger Automobile [Member]
Segment Reporting Information [Line Items]
Annual direct premiums written	2,140,531	2,105,602	2,115,763
Homeowners [Member]
Segment Reporting Information [Line Items]
Annual direct premiums written	318,295	285,188	261,560
Commercial Automobile [Member]
Segment Reporting Information [Line Items]
Annual direct premiums written	74,655	75,642	84,503
Other Lines [Member]
Segment Reporting Information [Line Items]
Annual direct premiums written	$ 122,239	$ 113,251	$ 96,999

Premiums written directly by insurer for all property and casualty insurance before adding contracts assumed from other insurers or subtracting any amounts assumed by other insurers.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 17

-Article 12

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 113

-Paragraph 27

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 605

-Section 50

-Paragraph 1

-Subparagraph (b),(c)(1)

-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration