Fair Value Measurements - Summary of Assets Measured at Fair Value on a Recurring Basis (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Document Period End Date	Mar. 31, 2015
Marketable securities:
Marketable securities	$ 171,929us-gaap_AvailableForSaleSecurities	$ 184,757us-gaap_AvailableForSaleSecurities
Money market mutual funds [Member]
Marketable securities:
Marketable securities	13,199	1,919
U.S. agency obligations [Member]
Marketable securities:
Marketable securities	153,203	177,257
U.S. treasury securities [Member]
Marketable securities:
Marketable securities	5,527	5,581
Fair Value, Measurements, Recurring
Marketable securities:
Assets, Fair Value Disclosure	171,929	184,757
Fair Value, Measurements, Recurring \| Money market mutual funds [Member]
Cash equivalents:
Cash equivalents	13,199	1,919
Fair Value, Measurements, Recurring \| U.S. agency obligations [Member]
Cash equivalents:
Cash equivalents		1,561
Marketable securities:
Marketable securities	153,203	175,696
Fair Value, Measurements, Recurring \| U.S. treasury securities [Member]
Marketable securities:
Marketable securities	5,527	5,581
Fair Value, Measurements, Recurring \| Level 1 [Member]
Marketable securities:
Assets, Fair Value Disclosure	13,199	1,919
Fair Value, Measurements, Recurring \| Level 1 [Member] \| Money market mutual funds [Member]
Cash equivalents:
Cash equivalents		1,919
Fair Value, Measurements, Recurring \| Level 1 [Member] \| U.S. agency obligations [Member]
Cash equivalents:
Cash equivalents		0
Marketable securities:
Marketable securities	0	0
Fair Value, Measurements, Recurring \| Level 1 [Member] \| U.S. treasury securities [Member]
Marketable securities:
Marketable securities	0	0
Fair Value, Measurements, Recurring \| Level 2 [Member]
Marketable securities:
Assets, Fair Value Disclosure	158,730	182,838
Fair Value, Measurements, Recurring \| Level 2 [Member] \| Money market mutual funds [Member]
Cash equivalents:
Cash equivalents	0	0
Fair Value, Measurements, Recurring \| Level 2 [Member] \| U.S. agency obligations [Member]
Cash equivalents:
Cash equivalents		1,561
Marketable securities:
Marketable securities		175,696
Fair Value, Measurements, Recurring \| Level 2 [Member] \| U.S. treasury securities [Member]
Marketable securities:
Marketable securities		$ 5,581

The end date of the period reflected on the cover page if a periodic report. For all other reports and registration statements containing historical data, it is the date up through which that historical data is presented. If there is no historical data in the report, use the filing date. The format of the date is CCYY-MM-DD.

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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.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

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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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-Name Accounting Standards Codification

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