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Financial Instruments - Summary of Investments (Detail) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Total investments (available for sale), Cost $ 158,993 $ 156,432
Total investments (available for sale), Gross Unrealized Gain 15 32
Total investments (available for sale), Gross Unrealized (Loss) (135) (186)
Total investments (available for sale), Fair value 158,873 156,278
Short-term Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total investments (available for sale), Cost 151,923 147,835
Total investments (available for sale), Gross Unrealized Gain 15 32
Total investments (available for sale), Gross Unrealized (Loss) (135) (186)
Total investments (available for sale), Fair value 151,803 147,681
Short-term Investments [Member] | Municipal Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total investments (available for sale), Cost 1,350 4,050
Total investments (available for sale), Fair value 1,350 4,050
Short-term Investments [Member] | Non-governmental Debt Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total investments (available for sale), Cost 118,111 107,305
Total investments (available for sale), Gross Unrealized Gain 11 22
Total investments (available for sale), Gross Unrealized (Loss) (74) (113)
Total investments (available for sale), Fair value 118,048 107,214
Short-term Investments [Member] | Treasury and Federal Agencies [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total investments (available for sale), Cost 32,462 36,480
Total investments (available for sale), Gross Unrealized Gain 4 10
Total investments (available for sale), Gross Unrealized (Loss) (61) (73)
Total investments (available for sale), Fair value 32,405 36,417
Restricted short-term investments [Member] | Non-governmental Debt Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total investments (available for sale), Cost 7,070 8,597
Total investments (available for sale), Fair value $ 7,070 $ 8,597