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Derivative financial instruments - Information About Interest Rate Swap Agreements (Detail) (Fixed rate long-term borrowings [Member], USD $)
In Thousands, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2013
Dec. 31, 2012
Fixed rate long-term borrowings [Member]
   
Derivative [Line Items]    
Notional amount $ 900,000 $ 900,000
Average maturity (in years) 4 years 1 month 6 days 4 years 4 months 24 days
Weighted-average rate, Fixed 6.07% 6.07%
Weighted-average rate, Variable 1.83% 1.85%