Investments and Fair Value Measurements Short-Term and Long-Term Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Total available for sale short-term and long-term marketable securities	$ 172,690	$ 311,979
Fair Value, Measurements, Recurring | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	164,890	272,280
Gross Unrealized Gains	4	3
Gross Unrealized Losses	(265)	(252)
Total available for sale short-term and long-term marketable securities	164,629	272,031
Fair Value, Measurements, Recurring | Short-term Investments | Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	32,656	58,503
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(1)
Total available for sale short-term and long-term marketable securities	32,656	58,502
Fair Value, Measurements, Recurring | Short-term Investments | Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	84,246	145,728
Gross Unrealized Gains	1	3
Gross Unrealized Losses	(205)	(174)
Total available for sale short-term and long-term marketable securities	84,042	145,557
Fair Value, Measurements, Recurring | Short-term Investments | U.S. government agency bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	14,013	3,013
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(41)	(7)
Total available for sale short-term and long-term marketable securities	13,972	3,006
Fair Value, Measurements, Recurring | Short-term Investments | U.S. government treasury bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	32,953	60,650
Gross Unrealized Gains	3	0
Gross Unrealized Losses	(19)	(70)
Total available for sale short-term and long-term marketable securities	32,937	60,580
Fair Value, Measurements, Recurring | Short-term Investments | Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,022	4,386
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Total available for sale short-term and long-term marketable securities	1,022	4,386
Fair Value, Measurements, Recurring | Long-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,134	40,090
Gross Unrealized Gains	1	2
Gross Unrealized Losses	(74)	(144)
Total available for sale short-term and long-term marketable securities	8,061	39,948
Fair Value, Measurements, Recurring | Long-term Investments | Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,128	25,067
Gross Unrealized Gains	1	2
Gross Unrealized Losses	0	(76)
Total available for sale short-term and long-term marketable securities	1,129	24,993
Fair Value, Measurements, Recurring | Long-term Investments | U.S. government agency bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,006	15,023
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(74)	(68)
Total available for sale short-term and long-term marketable securities	$ 6,932	$ 14,955