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Income Taxes - Reconciliation of Beginning and Ending Amounts of Unrecognized Tax Benefits (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]      
Beginning Balance $ 36,291 $ 35,803 $ 35,643
Increases related to current and prior year tax positions 291 488 160
Ending Balance $ 36,582 $ 36,291 $ 35,803