Fair Value Measurements - Fair Values and Unrealized Losses on Investments (Detail) - Corporate Debt [Member] - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities [Line Items]
Less than Twelve Months, Estimated fair value	$ 136,667	$ 70,651
Less than Twelve Months, Gross unrealized losses	(449)	(270)
Twelve Months or More, Estimated fair value	2,000
Twelve Months or More, Gross unrealized losses	(1)
Estimated fair value, Total	138,667	70,651
Gross unrealized losses, Total	$ (450)	$ (270)

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27337-111563

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27290-111563

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This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

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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 7
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27337-111563

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
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-Paragraph 6
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-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27290-111563

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-Publisher FASB
-Name Accounting Standards Codification
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