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Income Taxes - Expiry Dates of Federal and State Income Tax Credit Carryforwards (Detail)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
USD ($)
U.S. federal  
Tax Credit Carryforward [Line Items]  
Tax credit carryforwards $ 56,285
U.S. federal | U.S. Foreign Tax Credit  
Tax Credit Carryforward [Line Items]  
Tax credit carryforwards $ 46,639
U.S. federal | U.S. Foreign Tax Credit | Minimum  
Tax Credit Carryforward [Line Items]  
Tax credit carryforwards, expiration year 2021
U.S. federal | U.S. Foreign Tax Credit | Maximum  
Tax Credit Carryforward [Line Items]  
Tax credit carryforwards, expiration year 2026
U.S. federal | U.S. research tax credit  
Tax Credit Carryforward [Line Items]  
Tax credit carryforwards $ 9,623
U.S. federal | U.S. research tax credit | Minimum  
Tax Credit Carryforward [Line Items]  
Tax credit carryforwards, expiration year 2031
U.S. federal | U.S. research tax credit | Maximum  
Tax Credit Carryforward [Line Items]  
Tax credit carryforwards, expiration year 2036
U.S. federal | U.S. business tax credits  
Tax Credit Carryforward [Line Items]  
Tax credit carryforwards $ 23
U.S. federal | U.S. business tax credits | Minimum  
Tax Credit Carryforward [Line Items]  
Tax credit carryforwards, expiration year 2031
U.S. federal | U.S. business tax credits | Maximum  
Tax Credit Carryforward [Line Items]  
Tax credit carryforwards, expiration year 2036
California (U.S.)  
Tax Credit Carryforward [Line Items]  
Tax credit carryforwards $ 15,499
California (U.S.) | State investment tax credits  
Tax Credit Carryforward [Line Items]  
Tax credit carryforwards 858
California (U.S.) | State research tax credits  
Tax Credit Carryforward [Line Items]  
Tax credit carryforwards $ 14,641