Debt (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Short-term borrowings
Maximum amount borrowed	$ 601,500	$ 305,000	$ 92,700
Weighted average interest rate	2.73%	3.69%	2.89%

Reflects the calculation as of the balance-sheet date of the average interest rate weighted by the amount of short-term debt outstanding by type or by instrument at that time.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19

-Subparagraph b

-Article 5

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Namespace Prefix:	us-gaap_
Data Type:	us-types:percentItemType
Balance Type:	na
Period Type:	instant