Investments and Fair Value Measurements Short-Term and Long-Term Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Total available for sale short-term and long-term marketable securities	$ 192,388	$ 311,979
Fair Value, Measurements, Recurring | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	184,443	272,280
Gross Unrealized Gains	1	3
Gross Unrealized Losses	(147)	(252)
Total available for sale short-term and long-term marketable securities	184,297	272,031
Fair Value, Measurements, Recurring | Short-term Investments | Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	24,675	58,503
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(1)
Total available for sale short-term and long-term marketable securities	24,675	58,502
Fair Value, Measurements, Recurring | Short-term Investments | Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	75,461	145,728
Gross Unrealized Gains	1	3
Gross Unrealized Losses	(94)	(174)
Total available for sale short-term and long-term marketable securities	75,368	145,557
Fair Value, Measurements, Recurring | Short-term Investments | U.S. government agency bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11,665	3,013
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(31)	(7)
Total available for sale short-term and long-term marketable securities	11,634	3,006
Fair Value, Measurements, Recurring | Short-term Investments | U.S. government treasury bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	72,622	60,650
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(22)	(70)
Total available for sale short-term and long-term marketable securities	72,600	60,580
Fair Value, Measurements, Recurring | Short-term Investments | Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	20	4,386
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Total available for sale short-term and long-term marketable securities	20	4,386
Fair Value, Measurements, Recurring | Long-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,162	40,090
Gross Unrealized Gains	3	2
Gross Unrealized Losses	(74)	(144)
Total available for sale short-term and long-term marketable securities	8,091	39,948
Fair Value, Measurements, Recurring | Long-term Investments | Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,127	25,067
Gross Unrealized Gains	3	2
Gross Unrealized Losses	0	(76)
Total available for sale short-term and long-term marketable securities	1,130	24,993
Fair Value, Measurements, Recurring | Long-term Investments | U.S. government agency bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,035	15,023
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(74)	(68)
Total available for sale short-term and long-term marketable securities	$ 6,961	$ 14,955