Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
Dec. 31, 2016 USD ($) Securities Segment shares	Dec. 31, 2015 USD ($) Securities
Significant Of Accounting Policies [Line Items]
Unrecognized tax benefits	$ 0	$ 0
Realized gains or losses on marketable securities	$ 0
Minimum period to liquidate	12 months	12 months
Debt securities in an unrealized loss position \| Securities	158	74
Aggregate fair value of debt securities in an unrealized loss position	$ 335,400,000	$ 207,400,000
Concentration of credit risk	The Company has not experienced any credit losses in such accounts and does not believe it is exposed to any significant credit risk on these funds.
Off- balance sheet concentrations of credit risk	The Company has no off-balance sheet concentrations of credit risk, such as foreign currency exchange contracts, option contracts or other hedging arrangements.
Financial assets or liabilities that were classified as Level 3	$ 0	0
Transfers of assets from Level 1 to Level 2		0.0
Transfers of assets from Level 2 to Level 1		0
Transfers of liabilities from Level 1 to Level 2		0
Transfers of liabilities from Level 2 to Level 1		0
Number of operating segments \| Segment	1
Stock options granted \| shares	1,556,240
Government Securities [Member]
Significant Of Accounting Policies [Line Items]
Transfers of assets from Level 1 to Level 2	$ 11,000,000
Performance-Based Stock Option Grants [Member]
Significant Of Accounting Policies [Line Items]
Stock options granted \| shares	323,339
Level 3 [Member]
Significant Of Accounting Policies [Line Items]
Financial assets classified as Level 3	$ 0	0
Financial liabilities classified as Level 3	$ 0	$ 0
Maximum [Member]
Significant Of Accounting Policies [Line Items]
Investment maturity period, non-current	2 years	2 years
Investment maturity period, current	1 year	1 year
Minimum [Member]
Significant Of Accounting Policies [Line Items]
Investment maturity period, non-current	1 year	1 year

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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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Number of investment positions in available-for-sale investments that have been in a continuous unrealized loss position for greater than or equal to a year for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

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Description of financial instruments, excluding certain derivatives, with off-balance sheet credit risk (for example, standby letters of credit or financial guarantees), including (1) information about the (shared) activity, region or economic characteristic that identifies the concentration, (2) the face or contract amount of the instrument, (3) the nature and terms of the instrument, including the market risk, cash requirements and accounting policies for it, (4) the maximum amount of loss due to credit risk that would be incurred if the counterparties failed completely to perform according to the terms of the contracts, and any security or collateral that proved to be of no value, (5) the entity's policy of requiring collateral or other security to support financial instruments subject to credit risk, information about access to collateral or other security, and a brief description of the security supporting those financial instruments, and (6) the policy of entering into and a brief description of master netting arrangements to mitigate the credit risk, and the extent to which such arrangements would reduce the maximum amount of loss due to credit risk.

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Description of credit risk that arises from having significant balances in excess of the Federal Deposit Insurance Corporation or other insurance limits on deposits in financial institutions.

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Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Fair value of financial obligations, including, but not limited to, debt instruments, derivative liabilities, federal funds purchased and sold under agreements to repurchase, securities loaned or sold under agreements to repurchase, financial instruments sold not yet purchased, guarantees, line of credit, loans and notes payable, servicing liability, and trading liabilities.

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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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Gross number of share options (or share units) granted during the period.

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Amount of unrecognized tax benefits.

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