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Financial Instruments - Summary of Investments (Detail) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Total short-term investments (available for sale), Cost   $ 161,596
Total short-term investments (available for sale), Gross Unrealized Gain   7
Total short-term investments (available for sale), Gross Unrealized (Loss)   (355)
Total short-term investments (available for sale), Fair value   161,248
Short-term, Municipal Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total short-term investments (available for sale), Cost   830
Total short-term investments (available for sale), Fair value   830
Short-term, Non-governmental Debt Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total short-term investments (available for sale), Cost $ 179,393 125,485
Total short-term investments (available for sale), Gross Unrealized Gain 35 7
Total short-term investments (available for sale), Gross Unrealized (Loss) (337) (222)
Total short-term investments (available for sale), Fair value 179,091 125,270
Short-term, Treasury and Federal Agencies [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total short-term investments (available for sale), Cost 17,417 30,211
Total short-term investments (available for sale), Gross Unrealized Gain 5  
Total short-term investments (available for sale), Gross Unrealized (Loss) (85) (133)
Total short-term investments (available for sale), Fair value 17,337 30,078
Short-term Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total short-term investments (available for sale), Cost 196,810 156,526
Total short-term investments (available for sale), Gross Unrealized Gain 40 7
Total short-term investments (available for sale), Gross Unrealized (Loss) (422) (355)
Total short-term investments (available for sale), Fair value $ 196,428 156,178
Restricted short-term, Non-governmental Debt Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total short-term investments (available for sale), Cost   5,070
Total short-term investments (available for sale), Fair value   $ 5,070