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Schedule of Unrecognized Tax Benefits (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Tax Disclosure [Line Items]      
Beginning balance, as of January 1 $ 38,958 $ 36,792 $ 30,418
Tax positions for current year, Additions 8,208 9,766 6,626
Tax positions for prior years, Additions 199 187 792
Tax positions for prior years, Reductions (10,573) (1,960) (708)
Expiration of statutes, Reductions (325) (3,532)  
Settlements with tax authorities, Reductions (204) (2,295) (336)
Ending balance as of December 31 $ 36,263 $ 38,958 $ 36,792