Financial Instruments - Summary of Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Total investments (available for sale), Cost	$ 160,780	$ 123,128
Total investments (available for sale), Gross Unrealized Gain	60	3
Total investments (available for sale), Gross Unrealized (Loss)	(90)	(856)
Total investments (available for sale), Fair value	160,750	122,275
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total investments (available for sale), Cost	151,183	115,278
Total investments (available for sale), Gross Unrealized Gain	60	3
Total investments (available for sale), Gross Unrealized (Loss)	(90)	(380)
Total investments (available for sale), Fair value	151,153	114,901
Short-term Investments [Member] \| Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total investments (available for sale), Cost	2,250	1,500
Total investments (available for sale), Gross Unrealized (Loss)		(11)
Total investments (available for sale), Fair value	2,250	1,489
Short-term Investments [Member] \| Non-governmental Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total investments (available for sale), Cost	108,987	76,999
Total investments (available for sale), Gross Unrealized Gain	37
Total investments (available for sale), Gross Unrealized (Loss)	(67)	(242)
Total investments (available for sale), Fair value	108,957	76,757
Short-term Investments [Member] \| Treasury and Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Total investments (available for sale), Cost	39,946	36,779
Total investments (available for sale), Gross Unrealized Gain	23	3
Total investments (available for sale), Gross Unrealized (Loss)	(23)	(127)
Total investments (available for sale), Fair value	39,946	36,655
Restricted short-term investments [Member] \| Non-governmental Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total investments (available for sale), Cost	9,597
Total investments (available for sale), Fair value	$ 9,597
Long Term Investments [Member] \| Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total investments (available for sale), Cost		7,850
Total investments (available for sale), Gross Unrealized (Loss)		(476)
Total investments (available for sale), Fair value		$ 7,374

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Paragraph 2
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
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