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Fair Value of Long-Term Debt (Detail) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, Recorded Amount $ 695,820 $ 699,288
Long-term debt, Fair Value $ 754,802 $ 764,784