Marketable Securities - Schedule of Available-for-sale Marketable Securities and Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Cash and cash equivalents:
Cash and cash equivalents, Amortized Cost Value	$ 56,088	$ 118,564
Cash and cash equivalents, Gross Unrealized Gains	0	0
Cash and cash equivalents, Gross Unrealized Losses	0	0
Cash and cash equivalents, Fair Value	56,088	118,564
Marketable securities:
Marketable securities, Amortized Cost Value	312,686	312,006
Marketable securities, Gross Unrealized Gains	0	1
Marketable securities, Gross Unrealized Losses	(1,483)	(315)
Marketable securities, Fair Value	311,203	311,692
Domestic corporate debt securities
Marketable securities:
Marketable securities, Amortized Cost Value	207,827	207,320
Marketable securities, Gross Unrealized Gains	0	1
Marketable securities, Gross Unrealized Losses	(1,185)	(235)
Marketable securities, Fair Value	206,642	207,086
Domestic corporate commercial paper
Marketable securities:
Marketable securities, Amortized Cost Value	29,963	29,844
Marketable securities, Gross Unrealized Gains	0	0
Marketable securities, Gross Unrealized Losses	(12)	(7)
Marketable securities, Fair Value	29,951	29,837
Agency bonds
Marketable securities:
Marketable securities, Amortized Cost Value	74,896	74,842
Marketable securities, Gross Unrealized Gains	0	0
Marketable securities, Gross Unrealized Losses	(286)	(73)
Marketable securities, Fair Value	74,610	74,769
Cash
Cash and cash equivalents:
Cash and cash equivalents, Amortized Cost Value	9,981	73,302
Cash and cash equivalents, Fair Value	9,981	73,302
Money market funds
Cash and cash equivalents:
Cash and cash equivalents, Amortized Cost Value	31,110	325
Cash and cash equivalents, Fair Value	31,110	325
Domestic corporate commercial paper
Cash and cash equivalents:
Cash and cash equivalents, Amortized Cost Value	14,997	44,937
Cash and cash equivalents, Gross Unrealized Gains	0	0
Cash and cash equivalents, Gross Unrealized Losses	0	0
Cash and cash equivalents, Fair Value	$ 14,997	$ 44,937

Cash And Cash Equivalents, Accumulated Gross Unrealized Gain, Before Tax

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Cash And Cash Equivalents, Accumulated Gross Unrealized Loss, Before Tax

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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Amount of debt securities categorized neither as held-to-maturity nor trading.

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Amount 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. Excludes cash and cash equivalents within disposal group and discontinued operation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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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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