Short-Term and Long-Term Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Total available for sale short-term and long-term marketable securities	$ 174,952	$ 311,979
Fair Value, Measurements, Recurring | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	165,176	272,280
Gross Unrealized Gains	1	3
Gross Unrealized Losses	(437)	(252)
Total available for sale short-term and long-term marketable securities	164,740	272,031
Fair Value, Measurements, Recurring | Short-term Investments | Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	20,447	58,503
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	(1)
Total available for sale short-term and long-term marketable securities	20,446	58,502
Fair Value, Measurements, Recurring | Short-term Investments | Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	106,583	145,728
Gross Unrealized Gains	1	3
Gross Unrealized Losses	(333)	(174)
Total available for sale short-term and long-term marketable securities	106,251	145,557
Fair Value, Measurements, Recurring | Short-term Investments | U.S. government agency bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,999	3,013
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(52)	(7)
Total available for sale short-term and long-term marketable securities	8,947	3,006
Fair Value, Measurements, Recurring | Short-term Investments | U.S. government treasury bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	28,146	60,650
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(51)	(70)
Total available for sale short-term and long-term marketable securities	28,095	60,580
Fair Value, Measurements, Recurring | Short-term Investments | Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,001	4,386
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Total available for sale short-term and long-term marketable securities	1,001	4,386
Fair Value, Measurements, Recurring | Long-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,297	40,090
Gross Unrealized Gains	1	2
Gross Unrealized Losses	(86)	(144)
Total available for sale short-term and long-term marketable securities	10,212	39,948
Fair Value, Measurements, Recurring | Long-term Investments | Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,265	25,067
Gross Unrealized Gains	1	2
Gross Unrealized Losses	(20)	(76)
Total available for sale short-term and long-term marketable securities	3,246	24,993
Fair Value, Measurements, Recurring | Long-term Investments | U.S. government agency bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,032	15,023
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(66)	(68)
Total available for sale short-term and long-term marketable securities	$ 6,966	$ 14,955