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Fair Value of Senior Notes (Detail)
$ in Thousands
Dec. 31, 2017
USD ($)
Series 2007A Tranche D [Member]  
Fair Value Of Financial Instruments [Line Items]  
Fair Value of Senior Notes $ 38,126
4.5% Senior Unsecured Notes Due 2026 [Member]  
Fair Value Of Financial Instruments [Line Items]  
Fair Value of Senior Notes $ 365,190