Acquisition (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	12 Months Ended
Jul. 31, 2010 National Interstate (NATL) [Member]	Dec. 31, 2010 Vanliner Group, Inc. Moving and Storage Insurance Business [Member]
Acquisition (Textuals) [Abstract]
Percentage ownership of National Interstate	52.00%
Cash paid for acquisition	$ 114
Gross written premiums		$ 90
Percentage of total gross written premiums of Vanliner Group, Inc.		78.00%

The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 95
-Paragraph 15, 17
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Investing Activities
-URI http://asc.fasb.org/extlink&oid=6516133

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3213-108585

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration