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Marketable Securities (Details) - USD ($)
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 200,966,000 $ 142,882,000
Gross Unrealized Gains 56,000 44,000
Gross Unrealized Losses (138,000) (49,000)
Fair Value 200,884,000 142,877,000
Available-for-sale securities, mature in one year or less, amortized cost basis 192,985,000  
Available-for-sale securities, mature in one year or less, fair value 192,898,000  
Available-for-sale securities, maturing between one and five years, amortized cost basis 7,981,000  
Available-for-sale securities, maturing between one and five years, fair value 7,986,000  
Available-for-sale securities, other than temporary impairment losses 0 0
U.S. Treasury securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 4,826,000 9,354,000
Gross Unrealized Gains 0 1,000
Gross Unrealized Losses (1,000) (6,000)
Fair Value 4,825,000 9,349,000
Government-sponsored enterprises    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 29,764,000 22,055,000
Gross Unrealized Gains 5,000 1,000
Gross Unrealized Losses (10,000) (9,000)
Fair Value 29,759,000 22,047,000
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 166,376,000 111,473,000
Gross Unrealized Gains 51,000 42,000
Gross Unrealized Losses (127,000) (34,000)
Fair Value $ 166,300,000 $ 111,481,000