Stockholder's Equity (Details) (USD $)	3 Months Ended	6 Months Ended
Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Summary of shares repurchased under stock purchase program
Number of shares repurchased	6,787	117,176	6,787	117,176
Average price of shares repurchased	$ 37.70	$ 37.75	$ 37.70	$ 37.75
Shares remaining to be repurchased	234,964	426,724	234,964	426,724

Total cost of shares repurchased divided by the total number of shares repurchased.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 30

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6405813&loc=d3e23239-112655

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

-Publisher AICPA

-Name Accounting Research Bulletin (ARB)

-Number 43

-Section B

-Paragraph 7

-Subparagraph b

-Chapter 1

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

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