Short-Term and Long-Term Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Total available for sale short-term and long-term marketable securities	$ 375,296	$ 310,764
Fair Value, Measurements, Recurring [Member] | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	316,523	251,086
Gross Unrealized Gains	33	38
Gross Unrealized Losses	(102)	(143)
Total available for sale short-term and long-term marketable securities	316,454	250,981
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	62,348	42,397
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(6)
Total available for sale short-term and long-term marketable securities	62,348	42,391
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	164,661	122,788
Gross Unrealized Gains	31	22
Gross Unrealized Losses	(61)	(121)
Total available for sale short-term and long-term marketable securities	164,631	122,689
Fair Value, Measurements, Recurring [Member] | Short-term Investments | U.S. government agency bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,769	28,903
Gross Unrealized Gains	0	9
Gross Unrealized Losses	(8)	(4)
Total available for sale short-term and long-term marketable securities	7,761	28,908
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Municipal securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		5,852
Gross Unrealized Gains		0
Gross Unrealized Losses		(5)
Total available for sale short-term and long-term marketable securities		5,847
Fair Value, Measurements, Recurring [Member] | Short-term Investments | U.S. government treasury bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	78,370	45,146
Gross Unrealized Gains	2	7
Gross Unrealized Losses	(33)	(7)
Total available for sale short-term and long-term marketable securities	78,339	45,146
Fair Value, Measurements, Recurring [Member] | Short-term Investments | Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,375	6,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Total available for sale short-term and long-term marketable securities	3,375	6,000
Fair Value, Measurements, Recurring [Member] | Long-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	58,830	59,887
Gross Unrealized Gains	43	13
Gross Unrealized Losses	(31)	(117)
Total available for sale short-term and long-term marketable securities	58,842	59,783
Fair Value, Measurements, Recurring [Member] | Long-term Investments | Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	45,815	40,889
Gross Unrealized Gains	40	8
Gross Unrealized Losses	(18)	(85)
Total available for sale short-term and long-term marketable securities	45,837	40,812
Fair Value, Measurements, Recurring [Member] | Long-term Investments | U.S. government agency bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,990	6,805
Gross Unrealized Gains	3	0
Gross Unrealized Losses	0	(16)
Total available for sale short-term and long-term marketable securities	7,993	6,789
Fair Value, Measurements, Recurring [Member] | Long-term Investments | U.S. government treasury bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,025	12,016
Gross Unrealized Gains	0	5
Gross Unrealized Losses	(13)	(16)
Total available for sale short-term and long-term marketable securities	$ 5,012	12,005
Fair Value, Measurements, Recurring [Member] | Long-term Investments | Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		177
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Total available for sale short-term and long-term marketable securities		$ 177