Fair Value Measurements - Additional Information (Detail)	12 Months Ended
Dec. 31, 2018 USD ($) MarketableSecurity	Dec. 31, 2017 USD ($) MarketableSecurity
Debt Instrument Fair Value Carrying Value [Line Items]
Transfers among the Level 1, Level 2 and Level 3 categories \| $	$ 0	$ 0
Number of marketable securities loss position due to fluctuations in interest rates \| MarketableSecurity	145	11
Impairment of assets \| $	$ 0	$ 0
U.S. Corporate Bonds [Member]
Debt Instrument Fair Value Carrying Value [Line Items]
Number of marketable securities not part of remaining contractual maturities of one year or less \| MarketableSecurity	2
Maximum [Member]
Debt Instrument Fair Value Carrying Value [Line Items]
Marketable securities, remaining contractual maturities	1 year

Debt instrument fair value carrying value.

No definition available.

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- Definition

Amount of transfers of assets measured on a recurring basis among Level 1, Level 2 and Level 3.

+ References

No definition available.

+ Details

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- Definition

The remaining contractual period for marketable securities.

+ References

No definition available.

+ Details

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- Definition

Number of marketable securities not part of remaining contractual maturities of one year or less.

+ References

No definition available.

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- Definition

Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=109226348&loc=d3e2420-110228

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- Definition

Number of investment positions in available-for-sale investments in a continuous unrealized loss position for less than one year or normal operating cycle, if longer, for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

No definition available.

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