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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
6 Months Ended 12 Months Ended
Jun. 30, 2013
Dec. 31, 2012
Fixed Rate Long-Term Borrowings [Member]
   
Derivative [Line Items]    
Notional amount $ 1,400,000 $ 900,000
Average maturity (in years) 4 years 2 months 12 days 4 years 4 months 24 days
Weighted-average rate, Fixed 4.42% 6.07%
Weighted-average rate, Variable 1.27% 1.85%