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Income Taxes - Additional Information (Detail) - USD ($) | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 30, 2018 | Dec. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Income Tax Disclosure [Line Items] |  |  |  |  |  |
Tax cuts and jobs act, incomplete accounting, estimated income tax expense |  | $ 11,900,000 |  |  |  |
Tax cuts and jobs act, incomplete accounting, estimated write-down of deferred tax assets |  | 9,200,000 |  |  |  |
Tax cuts and jobs act, incomplete accounting, change in tax rate estimate income tax expense related to unrepatriated foreign earnings |  | 2,700,000 |  |  |  |
Tax cuts and jobs act, complete accounting, income tax expense (benefit) | $ (4,000,000.0) |  |  |  |  |
Deferred tax benefit recorded as an adjustment to other comprehensive income |  |  | $ 2,800,000 | $ 1,700,000 |  |
Foreign losses |  | 3,091,000 | 3,097,000 | 3,091,000 |  |
State losses and credit carry-forwards |  | 3,854,000 | 8,164,000 | 3,854,000 |  |
Federal loss and research carry-forwards |  | 3,058,000 | 17,495,000 | 3,058,000 |  |
Foreign and domestic carry-forwards, research and development tax credits, unamortized research and development cost and state credit carry-forwards |  |  | 28,800,000 |  |  |
Net change in valuation allowance |  |  | (200,000) |  |  |
Cash and cash equivalents |  | 86,433,000 | 105,504,000 | 86,433,000 |  |
Short-term investments |  | 16,129,000 | 3,246,000 | 16,129,000 |  |
Short-term liquidity amount |  | 102,600,000 | 108,700,000 | 102,600,000 |  |
Income tax benefit (expense) from stock options exercised adjustment to equity |  |  | 0 | 0 | $ 0 |
Unrecognized tax benefits |  | 2,366,000 | 1,868,000 | 2,366,000 | 2,226,000 | $ 2,537,000
Unrecognized tax benefits, effective tax rate |  | 2,200,000 | 1,700,000 | 2,200,000 | 1,700,000 |
Accrued interest and penalties |  | 800,000 | 700,000 | 800,000 | $ 800,000 |
Foreign Subsidiaries [Member] |  |  |  |  |  |
Income Tax Disclosure [Line Items] |  |  |  |  |  |
Short-term liquidity amount |  | $ 56,800,000 | $ 87,100,000 | $ 56,800,000 |  |
Short-term liquidity, in percentage |  | 55.40% | 80.10% | 55.40% |  |
Minimum [Member] |  |  |  |  |  |
Income Tax Disclosure [Line Items] |  |  |  |  |  |
Operating loss carry forwards expiration year |  |  | 2019 |  |  |
Maximum [Member] |  |  |  |  |  |
Income Tax Disclosure [Line Items] |  |  |  |  |  |
Operating loss carry forwards expiration year |  |  | 2030 |  |  |
Foreign [Member] |  |  |  |  |  |
Income Tax Disclosure [Line Items] |  |  |  |  |  |
Valuation allowance established against loss carryforwards |  |  | $ 2,400,000 |  |  |
State [Member] |  |  |  |  |  |
Income Tax Disclosure [Line Items] |  |  |  |  |  |
Valuation allowance established against loss and credit carryforwards |  |  | $ 3,400,000 |  |  |

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