Acquisitions (Details 3) (Customer relationship assets)	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Customer relationship assets
Acquired Finite-Lived Intangible Assets
Weighted average life of customer relationship assets associated with acquisitions	17 years	20 years	10 years

Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 30

-Section 50

-Paragraph 1

-Subparagraph (a)(3)

-URI http://asc.fasb.org/extlink&oid=7658586&loc=d3e16265-109275

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration