Financial Instruments - Summary of Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Total investments (available for sale), Cost	$ 152,051	$ 156,432
Total investments (available for sale), Gross Unrealized Gain	25	32
Total investments (available for sale), Gross Unrealized (Loss)	(156)	(186)
Total investments (available for sale), Fair value	151,920	156,278
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total investments (available for sale), Cost	143,454	147,835
Total investments (available for sale), Gross Unrealized Gain	25	32
Total investments (available for sale), Gross Unrealized (Loss)	(156)	(186)
Total investments (available for sale), Fair value	143,323	147,681
Short-term Investments [Member] \| Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total investments (available for sale), Cost	1,000	4,050
Total investments (available for sale), Fair value	1,000	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	107,023	107,305
Total investments (available for sale), Gross Unrealized Gain	18	22
Total investments (available for sale), Gross Unrealized (Loss)	(80)	(113)
Total investments (available for sale), Fair value	106,961	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	35,431	36,480
Total investments (available for sale), Gross Unrealized Gain	7	10
Total investments (available for sale), Gross Unrealized (Loss)	(76)	(73)
Total investments (available for sale), Fair value	35,362	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	8,597	8,597
Total investments (available for sale), Fair value	$ 8,597	$ 8,597

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
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563

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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
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563

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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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