Financial Instruments - Summary of Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Total investments (available for sale), Cost	$ 161,007	$ 161,596
Total investments (available for sale), Gross Unrealized Gain	17	7
Total investments (available for sale), Gross Unrealized (Loss)	(368)	(355)
Total investments (available for sale), Fair value	160,656	161,248
Short-term, Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total investments (available for sale), Cost		830
Total investments (available for sale), Fair value		830
Short-term, Non-governmental Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total investments (available for sale), Cost	139,700	125,485
Total investments (available for sale), Gross Unrealized Gain	17	7
Total investments (available for sale), Gross Unrealized (Loss)	(241)	(222)
Total investments (available for sale), Fair value	139,476	125,270
Short-term, Treasury and Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Total investments (available for sale), Cost	16,987	30,211
Total investments (available for sale), Gross Unrealized (Loss)	(127)	(133)
Total investments (available for sale), Fair value	16,860	30,078
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total investments (available for sale), Cost	156,687	156,526
Total investments (available for sale), Gross Unrealized Gain	17	7
Total investments (available for sale), Gross Unrealized (Loss)	(368)	(355)
Total investments (available for sale), Fair value	156,336	156,178
Restricted short-term, Non-governmental Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total investments (available for sale), Cost	4,320	5,070
Total investments (available for sale), Fair value	$ 4,320	$ 5,070