Investments and Fair Value Measurements - Available for sale securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Available for sale securities:
Amortized Cost	$ 501,837	$ 370,961
Gross Unrealized Gains	8	2
Gross Unrealized Losses	(174)	(735)
Estimated Fair Value	501,671	370,228
Marketable securities
Available for sale securities:
Amortized Cost	207,086	301,906
Gross Unrealized Gains	8	2
Gross Unrealized Losses	(174)	(735)
Estimated Fair Value	206,920	301,173
U.S. government securities \| Level 1
Available for sale securities:
Amortized Cost	48,807	89,896
Gross Unrealized Losses	(86)	(342)
Estimated Fair Value	48,721	89,554
U.S. government agency securities \| Level 2
Available for sale securities:
Amortized Cost	9,852	50,891
Gross Unrealized Gains	2
Gross Unrealized Losses		(113)
Estimated Fair Value	9,854	50,778
Corporate notes \| Level 2
Available for sale securities:
Amortized Cost	57,508	141,226
Gross Unrealized Gains	6	2
Gross Unrealized Losses	(88)	(280)
Estimated Fair Value	57,426	140,948
Commercial paper \| Level 2
Available for sale securities:
Amortized Cost	90,919	19,893
Estimated Fair Value	90,919	19,893
Money market funds \| Level 1
Available for sale securities:
Amortized Cost	294,751	69,055
Estimated Fair Value	$ 294,751	$ 69,055

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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