Derivative financial instruments (Details) (Fixed rate long-term borrowings [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 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)	5.4	6.4
Weighted-average rate, fixed	6.07%	6.07%
Weighted-average rate, variable	2.07%	1.84%
Estimated fair value gain	$ 147,302	$ 96,637

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

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

Amount of unrealized gain (loss) related to the fair value of interest rate derivatives designated as fair value hedging instruments. Recognized in earnings and offsets the gain (loss) on the hedged item to the extent that the fair value hedge is determined to be effective.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4A

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6935481&loc=SL5618551-113959

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4C

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6935481&loc=SL5624171-113959

+ 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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