Fair Value Measurements (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2016	Sep. 30, 2016	Dec. 31, 2015
Assets:
Total, Estimated Fair Value	$ 255,097	$ 255,097	$ 247,001
Transfer of assets between levels	0	0
Recurring fair value measurement \| Level 1
Assets:
Money market fund	42,490	42,490	31,761
Total, Estimated Fair Value	132,955	132,955	98,818
Recurring fair value measurement \| Level 1 \| U.S. treasury notes
Assets:
Available for sale securities	57,085	57,085	35,054
Recurring fair value measurement \| Level 1 \| Certificates of deposit
Assets:
Available for sale securities	33,380	33,380	32,003
Recurring fair value measurement \| Level 2
Assets:
Total, Estimated Fair Value	122,142	122,142	148,183
Recurring fair value measurement \| Level 2 \| U.S. government agency securities
Assets:
Available for sale securities	122,142	122,142	148,183
Total \| Recurring fair value measurement
Assets:
Money market fund	42,490	42,490	31,761
Total, Estimated Fair Value	255,097	255,097	247,001
Total \| Recurring fair value measurement \| U.S. treasury notes
Assets:
Available for sale securities	57,085	57,085	35,054
Total \| Recurring fair value measurement \| U.S. government agency securities
Assets:
Available for sale securities	122,142	122,142	148,183
Total \| Recurring fair value measurement \| Certificates of deposit
Assets:
Available for sale securities	$ 33,380	$ 33,380	$ 32,003

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- Definition

Sum of investments and unrestricted cash measured at fair value as of the balance sheet date.

+ References

No definition available.

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- Definition

Total of debt securities categorized neither as held-to-maturity nor trading which are intended be sold or mature more than one year from the balance sheet date or operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26626-111562

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Cash Equivalents
-URI http://asc.fasb.org/extlink&oid=6507016

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