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Marketable Securities - Available-for-sale Securities by Maturities (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Summary of maturities, at par, of marketable securities    
2018 $ 10,710 $ 20,227
2019 38,717 30,297
2020 20,123 20,423
2021 400 0
Total 69,950 70,947
Certificates of deposit    
Summary of maturities, at par, of marketable securities    
2018 2,211 4,306
2019 2,311 2,311
2020 1,799 1,799
2021 0 0
Total 6,321 8,416
U.S. Treasury and agency notes    
Summary of maturities, at par, of marketable securities    
2018 3,351 6,399
2019 17,574 14,599
2020 9,174 9,171
2021 0 0
Total 30,099 30,169
Corporate notes    
Summary of maturities, at par, of marketable securities    
2018 4,325 7,954
2019 13,721 6,430
2020 7,150 6,450
2021 400 0
Total 25,596 20,834
Municipal notes    
Summary of maturities, at par, of marketable securities    
2018 823 1,568
2019 5,111 6,957
2020 2,000 3,003
2021 0 0
Total $ 7,934 $ 11,528