Investments and Fair Value Measurements - Available for sale securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Available for sale securities:
Amortized Cost	$ 334,673	$ 501,837
Gross Unrealized Gains	119	8
Gross Unrealized Losses	(23)	(174)
Estimated Fair Value	334,769	501,671
Marketable securities
Available for sale securities:
Amortized Cost	287,047	207,086
Gross Unrealized Gains	119	8
Gross Unrealized Losses	(23)	(174)
Estimated Fair Value	287,143	206,920
US government securities \| Level 1
Available for sale securities:
Amortized Cost	132,492	48,807
Gross Unrealized Gains	61
Gross Unrealized Losses	(15)	(86)
Estimated Fair Value	132,538	48,721
US government agency securities \| Level 2
Available for sale securities:
Amortized Cost		9,852
Gross Unrealized Gains		2
Estimated Fair Value		9,854
Corporate notes \| Level 2
Available for sale securities:
Amortized Cost	32,227	57,508
Gross Unrealized Gains	41	6
Gross Unrealized Losses	(1)	(88)
Estimated Fair Value	32,267	57,426
Commercial paper \| Level 2
Available for sale securities:
Amortized Cost	122,328	90,919
Gross Unrealized Gains	17
Gross Unrealized Losses	(7)
Estimated Fair Value	122,338	90,919
Money market funds \| Level 1
Available for sale securities:
Amortized Cost	47,626	294,751
Estimated Fair Value	$ 47,626	$ 294,751

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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