Marketable Securities - Schedule of Available-for-sale Marketable Securities and Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Cash and cash equivalents:
Cash and cash equivalents, Amortized Cost Value	$ 62,014	$ 118,564	$ 446,938
Cash and cash equivalents, Gross Unrealized Gains	0	0
Cash and cash equivalents, Gross Unrealized Losses	0	0
Cash and cash equivalents, Fair Value	62,014	118,564
Marketable securities:
Marketable securities, Amortized Cost Value	215,327	312,006
Marketable securities, Gross Unrealized Gains	0	1
Marketable securities, Gross Unrealized Losses	(972)	(315)
Marketable securities, Fair Value	214,355	311,692
Domestic corporate debt securities
Marketable securities:
Marketable securities, Amortized Cost Value	155,346	207,320
Marketable securities, Gross Unrealized Gains	0	1
Marketable securities, Gross Unrealized Losses	(643)	(235)
Marketable securities, Fair Value	154,703	207,086
Domestic corporate commercial paper
Marketable securities:
Marketable securities, Amortized Cost Value		29,844
Marketable securities, Gross Unrealized Gains		0
Marketable securities, Gross Unrealized Losses		(7)
Marketable securities, Fair Value		29,837
Agency bonds
Marketable securities:
Marketable securities, Amortized Cost Value	59,981	74,842
Marketable securities, Gross Unrealized Gains	0	0
Marketable securities, Gross Unrealized Losses	(329)	(73)
Marketable securities, Fair Value	59,652	74,769
Cash
Cash and cash equivalents:
Cash and cash equivalents, Amortized Cost Value	15,236	73,302
Cash and cash equivalents, Gross Unrealized Gains	0	0
Cash and cash equivalents, Gross Unrealized Losses	0	0
Cash and cash equivalents, Fair Value	15,236	73,302
Money market funds
Cash and cash equivalents:
Cash and cash equivalents, Amortized Cost Value	46,778	325
Cash and cash equivalents, Gross Unrealized Gains	0	0
Cash and cash equivalents, Gross Unrealized Losses	0	0
Cash and cash equivalents, Fair Value	$ 46,778	325
Domestic corporate commercial paper
Cash and cash equivalents:
Cash and cash equivalents, Amortized Cost Value		44,937
Cash and cash equivalents, Gross Unrealized Gains		0
Cash and cash equivalents, Gross Unrealized Losses		0
Cash and cash equivalents, Fair Value		$ 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 investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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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.

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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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