Derivative financial instruments (Details) (Fixed rate long-term borrowings [Member], USD $) In Thousands, unless otherwise specified	Sep. 30, 2011	Dec. 31, 2010
Fixed rate long-term borrowings [Member]
Information about interest rate swap agreements
Notional amount	$ 900,000	$ 900,000
Average maturity (in years)	5.6	6.4
Weighted-average rate, fixed	6.07%	6.07%
Weighted-average rate, variable	1.87%	1.84%

Information about interest rate swap agreements.

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- Definition

Average fixed interest rate related to the group of interest rate derivatives.

+ Details

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- Definition

The average time remaining until maturity on the group of derivatives identified.

+ Details

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- Definition

Average variable interest rate related to the group of interest rate derivatives.

+ Details

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- Definition

Aggregate notional amount of all derivatives designated as a fair value hedging instrument. The notional amount relates to a number of currency units, shares, bushels, pounds, or other units specified in a derivative instrument.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Notional Amount

-URI http://asc.fasb.org/extlink&oid=6519104

+ Details

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