Fair Market Value of Investments with Unrealized Losses Not Deemed to be Other-Than Temporarily Impaired (Detail) $ in Thousands	Dec. 31, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Unrealized Loss Position, Less than 12 Months, Fair Value	$ 15,000
Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	23
Unrealized Loss Position, Greater than 12 Months, Fair Value	0
Unrealized Loss Position, Greater than 12 Months, Gross Unrealized Losses	0
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Unrealized Loss Position, Less than 12 Months, Fair Value	1,494
Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	1
Unrealized Loss Position, Greater than 12 Months, Fair Value	0
Unrealized Loss Position, Greater than 12 Months, Gross Unrealized Losses	0
Corporate bonds and medium-term notes
Schedule of Available-for-sale Securities [Line Items]
Unrealized Loss Position, Less than 12 Months, Fair Value	9,478
Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	17
Unrealized Loss Position, Greater than 12 Months, Fair Value	0
Unrealized Loss Position, Greater than 12 Months, Gross Unrealized Losses	0
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Unrealized Loss Position, Less than 12 Months, Fair Value	1,096
Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	4
Unrealized Loss Position, Greater than 12 Months, Fair Value	0
Unrealized Loss Position, Greater than 12 Months, Gross Unrealized Losses	0
U.S. treasury and agency securities
Schedule of Available-for-sale Securities [Line Items]
Unrealized Loss Position, Less than 12 Months, Fair Value	2,932
Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	1
Unrealized Loss Position, Greater than 12 Months, Fair Value	0
Unrealized Loss Position, Greater than 12 Months, Gross Unrealized Losses	$ 0

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
-SubTopic 10
-Section 50
-Paragraph 7
-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 that have been in a continuous loss position for less than twelve months.

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This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

+ 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
-Name Accounting Standards Codification
-Topic 320
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-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27290-111563

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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)(1)
-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
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27337-111563

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