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

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

-Publisher AICPA

-Name Accounting Research Bulletin (ARB)

-Number 43

-Chapter 1

-Section B

-Paragraph 7

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

-Topic 505

-SubTopic 30

-Section 45

-Paragraph 1

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

Name:	us-gaap_TreasuryStockAcquiredAverageCostPerShare
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

Stock Repurchase Program (Details) (USD $) In Millions, except Share data, unless otherwise specified	0 Months Ended	6 Months Ended	0 Months Ended		12 Months Ended
	Oct. 17, 2007	Jun. 30, 2011	Dec. 14, 2011 Stock Repurchase Program Additional Authorization [Member]	Feb. 22, 2011 Stock Repurchase Program Additional Authorization [Member]	Dec. 31, 2011 Stock Repurchase Program Additional Authorization [Member]	Dec. 31, 2011 Treasury Stock [Member]
Stock Repurchase Program [Line Items]
Common stock repurchase authorized amount	$ 150.0		$ 150.0	$ 100.0
Total stock repurchased, shares		863,466			3,960,555	2,355
Total stock repurchased, value					99
Common stock repurchase price per share		$ 27.94			$ 24.99
Treasury stock repurchase authorization amount available					$ 1