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Marketable Securities (Details) - USD ($)
 3 Months Ended 12 Months Ended
Mar. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 207,671,000 $ 200,966,000
Gross Unrealized Gains 23,000 56,000
Gross Unrealized Losses (131,000) (138,000)
Fair Value 207,563,000 200,884,000
Mature in one year or less, Amortized Cost Basis 202,686,000  
Mature in one year or less, Fair Value 202,585,000  
Mature between one and five years, Amortized Cost Basis 4,985,000  
Mature between one and five years, Fair Value 4,978,000  
Other-than-temporary unrealized losses 0 0
U.S. Treasury securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 13,835,000 4,826,000
Gross Unrealized Gains 0 0
Gross Unrealized Losses (14,000) (1,000)
Fair Value 13,821,000 4,825,000
Government-sponsored enterprises    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 37,942,000 29,764,000
Gross Unrealized Gains 0 5,000
Gross Unrealized Losses (27,000) (10,000)
Fair Value 37,915,000 29,759,000
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 155,894,000 166,376,000
Gross Unrealized Gains 23,000 51,000
Gross Unrealized Losses (90,000) (127,000)
Fair Value $ 155,827,000 $ 166,300,000