Derivative financial instruments - Information About Interest Rate Swap Agreements (Detail) (Fixed rate long-term borrowings [Member], USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
Sep. 30, 2012	Dec. 31, 2011
Fixed rate long-term borrowings [Member]
Derivative [Line Items]
Notional amount	$ 900,000	$ 900,000
Average maturity (in years)	4 years 7 months 6 days	5 years 4 months 24 days
Weighted-average rate, fixed	6.07%	6.07%
Weighted-average rate, variable	1.96%	2.07%

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

No definition available.

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- Definition

Average remaining period until maturity of the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ 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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- 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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