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Marketable Securities (Details) - USD ($)
9 Months Ended 12 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 112,790,000 $ 200,966,000
Gross Unrealized Gains 12,000 56,000
Gross Unrealized Losses (39,000) (138,000)
Fair Value 112,763,000 200,884,000
Other-than-temporary unrealized losses 0 0
U.S. Treasury securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 12,770,000 4,826,000
Gross Unrealized Gains 0 0
Gross Unrealized Losses (11,000) (1,000)
Fair Value 12,759,000 4,825,000
Government-sponsored enterprises    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 22,995,000 29,764,000
Gross Unrealized Gains 0 5,000
Gross Unrealized Losses (12,000) (10,000)
Fair Value 22,983,000 29,759,000
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 77,025,000 166,376,000
Gross Unrealized Gains 12,000 51,000
Gross Unrealized Losses (16,000) (127,000)
Fair Value $ 77,021,000 $ 166,300,000