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Income Taxes - Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]    
Balance at beginning of period $ 7,851 $ 7,753
Tax positions related to the current period    
Gross additions 720 516
Gross reductions 0 0
Tax positions related to prior years    
Gross additions 92 31
Gross reductions (223) (70)
Settlements (411) (379)
Lapses on statutes of limitations 0 0
Balance at end of period $ 8,029 $ 7,851