Marketable Securities - Additional Information (Detail)	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2018 USD ($) Security	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) Security	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities remaining maturities greater than three years	$ 0		$ 0		$ 0
Unrealized gains (losses) on available-for-sale securities, before tax	100,000	$ (100,000)	(100,000)	$ (100,000)
Unrealized gains (losses) on available-for-sale securities, income tax expense	$ 0	0	$ 0	0
Number of investment grade fixed income security \| Security	62		62
Other-than-temporary impaired			$ 0		$ 0
Debt Securities [Member] \| Investment Income [Member]
Schedule Of Available For Sale Securities [Line Items]
Net realized gains or losses available for sale securities	$ 0	$ 0	$ 0	$ 500,000

Available for sale debt securities remaining maturities greater than three years.

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- Definition

Amount of realized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position.

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Amount before tax and reclassification adjustments of unrealized holding gain (loss) on available-for-sale securities.

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Amount of tax expense (benefit) before reclassification adjustments of unrealized holding gain (loss) on available-for-sale securities.

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Amount of other-than-temporary impairment (OTTI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), recognized in earnings and other comprehensive loss (OCI).

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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