Investments and Fair Value Measurements Short-Term and Long-Term Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Total available for sale short-term and long-term marketable securities	$ 292,011	$ 107,572
Fair Value, Measurements, Recurring | Short-term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	291,799	98,586
Gross Unrealized Gains	227	1
Gross Unrealized Losses	(15)	(127)
Total available for sale short-term and long-term marketable securities	292,011	98,460
Fair Value, Measurements, Recurring | Short-term Investments | Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	33,259	17,793
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Total available for sale short-term and long-term marketable securities	33,259	17,793
Fair Value, Measurements, Recurring | Short-term Investments | Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	152,452	45,100
Gross Unrealized Gains	169	0
Gross Unrealized Losses	(10)	(48)
Total available for sale short-term and long-term marketable securities	152,611	45,052
Fair Value, Measurements, Recurring | Short-term Investments | U.S. government agency bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,233	19,981
Gross Unrealized Gains	6	0
Gross Unrealized Losses	0	(77)
Total available for sale short-term and long-term marketable securities	10,239	19,904
Fair Value, Measurements, Recurring | Short-term Investments | U.S. government treasury bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	91,832	15,292
Gross Unrealized Gains	48	0
Gross Unrealized Losses	(5)	(1)
Total available for sale short-term and long-term marketable securities	91,875	15,291
Fair Value, Measurements, Recurring | Short-term Investments | Foreign bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,004
Gross Unrealized Gains	4
Gross Unrealized Losses	0
Total available for sale short-term and long-term marketable securities	4,008
Fair Value, Measurements, Recurring | Short-term Investments | Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	19	420
Gross Unrealized Gains	0	1
Gross Unrealized Losses	0	(1)
Total available for sale short-term and long-term marketable securities	$ 19	420
Fair Value, Measurements, Recurring | Long-term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		9,091
Gross Unrealized Gains		23
Gross Unrealized Losses		(2)
Total available for sale short-term and long-term marketable securities		9,112
Fair Value, Measurements, Recurring | Long-term Investments | Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		4,957
Gross Unrealized Gains		5
Gross Unrealized Losses		(2)
Total available for sale short-term and long-term marketable securities		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		2,235
Gross Unrealized Gains		9
Gross Unrealized Losses		0
Total available for sale short-term and long-term marketable securities		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