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Shareholders' Equity - Summary of Activities in Common Stock (Detail) - USD ($)
shares in Thousands, $ in Thousands
 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Equity [Abstract]    
Purchases of common stock, Shares 0 (42)
Stock option exercises, Shares 722 534
Employee stock plan purchases, Shares 23 27
Purchases of common stock, Amount $ 0 $ (6,171)
Stock option exercises, Amount 44,317 23,360
Employee stock plan purchases, Amount $ 3,760 $ 3,427