Income Taxes (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
Sep. 30, 2012	Sep. 30, 2011
Income Taxes
Estimated annual effective tax rates (as a percent)	31.00%	28.00%
Business tax credit carryforwards	$ 30.7

Amount before allocation of valuation allowances of deferred tax asset attributable to deductible general business tax credit carryforwards.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32621-109319

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32632-109319

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 3

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32559-109319

Name:	us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsGeneralBusiness
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant