Summary of Significant Accounting Policies - Summary of Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Summary Of Significant Policies [Line Items]
Cash equivalents	$ 63,133	$ 42,509
Marketable securities	136,547	57,359
Total cash equivalents and marketable securities	199,680	99,868
Money Market Funds [Member]
Summary Of Significant Policies [Line Items]
Cash equivalents	61,133	14,742
U.S. Government Securities [Member]
Summary Of Significant Policies [Line Items]
Cash equivalents		27,767
Marketable securities	49,239	34,191
Corporate Bonds [Member]
Summary Of Significant Policies [Line Items]
Marketable securities	60,831	22,168
Commercial Paper [Member]
Summary Of Significant Policies [Line Items]
Cash equivalents	2,000
Marketable securities	26,477	1,000
Quoted Prices in Active Markets, (Level 1) [Member]
Summary Of Significant Policies [Line Items]
Cash equivalents	61,133	14,742
Total cash equivalents and marketable securities	61,133	14,742
Quoted Prices in Active Markets, (Level 1) [Member] \| Money Market Funds [Member]
Summary Of Significant Policies [Line Items]
Cash equivalents	61,133	14,742
Significant Other Observable Inputs (Level 2) [Member]
Summary Of Significant Policies [Line Items]
Cash equivalents	2,000	27,767
Marketable securities	136,547	57,359
Total cash equivalents and marketable securities	138,547	85,126
Significant Other Observable Inputs (Level 2) [Member] \| U.S. Government Securities [Member]
Summary Of Significant Policies [Line Items]
Cash equivalents		27,767
Marketable securities	49,239	34,191
Significant Other Observable Inputs (Level 2) [Member] \| Corporate Bonds [Member]
Summary Of Significant Policies [Line Items]
Marketable securities	60,831	22,168
Significant Other Observable Inputs (Level 2) [Member] \| Commercial Paper [Member]
Summary Of Significant Policies [Line Items]
Cash equivalents	2,000
Marketable securities	$ 26,477	$ 1,000

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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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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