Investments and Fair Value Measurements Short-Term and Long-Term Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Total available for sale short-term and long-term marketable securities	$ 342,391	$ 107,572
Fair Value, Measurements, Recurring | Short-term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	297,303	98,586
Gross Unrealized Gains	146	1
Gross Unrealized Losses	(27)	(127)
Total available for sale short-term and long-term marketable securities	297,422	98,460
Fair Value, Measurements, Recurring | Short-term Investments | Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	29,556	17,793
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Total available for sale short-term and long-term marketable securities	29,556	17,793
Fair Value, Measurements, Recurring | Short-term Investments | Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	125,299	45,100
Gross Unrealized Gains	74	0
Gross Unrealized Losses	(8)	(48)
Total available for sale short-term and long-term marketable securities	125,365	45,052
Fair Value, Measurements, Recurring | Short-term Investments | U.S. government agency bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	20,507	19,981
Gross Unrealized Gains	2	0
Gross Unrealized Losses	(13)	(77)
Total available for sale short-term and long-term marketable securities	20,496	19,904
Fair Value, Measurements, Recurring | Short-term Investments | U.S. government treasury bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	108,945	15,292
Gross Unrealized Gains	56	0
Gross Unrealized Losses	(6)	(1)
Total available for sale short-term and long-term marketable securities	108,995	15,291
Fair Value, Measurements, Recurring | Short-term Investments | Foreign bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	12,976
Gross Unrealized Gains	14
Gross Unrealized Losses	0
Total available for sale short-term and long-term marketable securities	12,990
Fair Value, Measurements, Recurring | Short-term Investments | Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	20	420
Gross Unrealized Gains	0	1
Gross Unrealized Losses	0	(1)
Total available for sale short-term and long-term marketable securities	20	420
Fair Value, Measurements, Recurring | Long-term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	44,917	9,091
Gross Unrealized Gains	56	23
Gross Unrealized Losses	(4)	(2)
Total available for sale short-term and long-term marketable securities	44,969	9,112
Fair Value, Measurements, Recurring | Long-term Investments | Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	23,879	4,957
Gross Unrealized Gains	40	5
Gross Unrealized Losses	(1)	(2)
Total available for sale short-term and long-term marketable securities	23,918	4,960
Fair Value, Measurements, Recurring | Long-term Investments | U.S. government agency bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		1,399
Gross Unrealized Gains		8
Gross Unrealized Losses		0
Total available for sale short-term and long-term marketable securities		1,407
Fair Value, Measurements, Recurring | Long-term Investments | U.S. government treasury bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	21,038	2,235
Gross Unrealized Gains	16	9
Gross Unrealized Losses	(3)	0
Total available for sale short-term and long-term marketable securities	$ 21,051	2,244
Fair Value, Measurements, Recurring | Long-term Investments | Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		500
Gross Unrealized Gains		1
Gross Unrealized Losses		0
Total available for sale short-term and long-term marketable securities		$ 501