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Cash Equivalents, Marketable Securities and Equity Securities (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($) item	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Cash Equivalents, Marketable Securities and Equity Securities
Amortized Cost	$ 341,000		$ 329,215
Gross Unrealized Losses	(82)		(76)
Fair Value	$ 340,918		329,139
Equity securities included in other non-current assets:
Number of securities in an unrealized loss position | item	41
Securities held by the Company in an unrealized loss position for less than 12 months	$ 230,300
Number of securities in an unrealized loss position for more than 12 months | item	0
Realized gains (losses) on available-for-sale securities	$ 0	$ 0
Government agency securities
Cash Equivalents, Marketable Securities and Equity Securities
Amortized Cost	29,922		58,979
Gross Unrealized Losses	(15)		(29)
Fair Value	29,907		58,950
Corporate equity securities
Equity securities included in other non-current assets:
Amortized Cost	3,667
Fair Value	3,667
Money market funds
Cash Equivalents, Marketable Securities and Equity Securities
Amortized Cost	106,974		134,635
Fair Value	106,974		134,635
U.S. Treasuries
Cash Equivalents, Marketable Securities and Equity Securities
Amortized Cost	200,437		135,601
Gross Unrealized Losses	(67)		(47)
Fair Value	$ 200,370		$ 135,554

X

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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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