Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended	12 Months Ended
Mar. 31, 2017 USD ($) Segment	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)
Significant Accounting Policies [Line Items]
Number of operating segments \| Segment	1
Maturity of highly liquid investments	Three months or less
Maturity of certificates of deposit, commercial paper, corporate notes and corporate bonds	More than three months
Investment securities, available-for-sale	$ 344,183,051		$ 335,458,459
Investment securities, available for sale noncurrent	21,600,000		25,500,000
Investment securities, unrealized loss	19,000		25,000
Carrying value of cash held in money market funds	12,700,000		10,700,000
Allowance for accounts receivable	0		0
Impairment losses recognized	$ 0
Assumed expected dividend rate	0.00%
Service period for granted RSUs, vest in annual installment	3 years	3 years
Percentage of amortization expenses		100.00%
Excess tax benefits for tax deductions	$ 0	$ 0
Significant Unobservable Inputs (Level 3) [Member]
Significant Accounting Policies [Line Items]
Assets measured using quoted prices	0		0
Liabilities measured using quoted prices	0		0
Adjustments for New Accounting Pronouncement [Member]
Significant Accounting Policies [Line Items]
Deferred tax assets	7,000,000
Contractual Maturity Dates More Than One Year and Less Than Two Years [Member]
Significant Accounting Policies [Line Items]
Investment securities, available-for-sale	$ 43,800,000		$ 47,900,000
Minimum [Member]
Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life	3 years
Maximum [Member]
Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life	5 years

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Maturity period of liquid investments to be considered as cash equivalents.

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Percentage of amortization expenses.

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Significant accounting policies.

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A valuation allowance for trade and other receivables due to an Entity within one year (or the normal operating cycle, whichever is longer) that are expected to be uncollectible.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of investment in debt and equity securities categorized neither as trading securities nor held-to-maturity securities and intended be sold or mature one year or operating cycle, if longer.

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Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.

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Investments in debt and equity securities which are categorized neither as held-to-maturity nor trading and which are intended to be sold or mature more than one year from the balance sheet date or operating cycle, if longer. Such securities are reported at fair value; unrealized gains (losses) related to Available-for-sale Securities are excluded from earnings and reported in a separate component of shareholders' equity (other comprehensive income), unless the Available-for-sale security is designated as a hedge or is determined to have had an other than temporary decline in fair value below its amortized cost basis. All or a portion of the unrealized holding gain (loss) of an Available-for-sale security that is designated as being hedged in a fair value hedge is recognized in earnings during the period of the hedge, as are other than temporary declines in fair value below the cost basis for investments in equity securities and debt securities that an entity intends to sell or it is more likely than not that it will be required to sell before the recovery of its amortized cost basis. Other than temporary declines in fair value below the cost basis for debt securities categorized as Available-for-sale that an entity does not intend to sell and for which it is not more likely than not that the entity will be required to sell before the recovery of its amortized cost basis are bifurcated into credit losses and losses related to all other factors. Other than temporary declines in fair value below cost basis related to credit losses are recognized in earnings, and losses related to all other factors are recognized in other comprehensive income.

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Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards.

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Amount of cash outflow for realized tax benefit related to deductible compensation cost reported on the entity's tax return for equity instruments in excess of the compensation cost for those instruments recognized for financial reporting purposes.

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- Definition

The aggregate amount of write-downs for impairments recognized during the period for long lived assets held for use (including those held for disposal by means other than sale).

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Fair value of financial and nonfinancial obligations.

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Investment in short-term money-market instruments (such as commercial paper, banker's acceptances, repurchase agreements, government securities, certificates of deposit, and so forth) which are highly liquid (that is, readily convertible to known amounts of cash) and so near their maturity that they present an insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify as cash equivalents by definition. Original maturity means an original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

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Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

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Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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