Marketable Securities - Available-for-sale Securities by Maturities (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Summary of maturities, at par, of marketable securities
2017	$ 4,314	$ 6,921
2018	13,511	13,571
2019	6,012	5,773
2020	0	0
Total	23,837	26,265
Certificates of deposit
Summary of maturities, at par, of marketable securities
2017	294	531
2018	4,306	4,306
2019	324	324
2020	0	0
Total	4,924	5,161
U.S. Treasury and agency notes
Summary of maturities, at par, of marketable securities
2017	1,234	1,234
2018	444	444
2019	142	142
2020	0	0
Total	1,820	1,820
Corporate notes
Summary of maturities, at par, of marketable securities
2017	2,051	4,316
2018	7,073	7,133
2019	4,421	4,232
2020	0	0
Total	13,545	15,681
Municipal notes
Summary of maturities, at par, of marketable securities
2017	735	840
2018	1,688	1,688
2019	1,125	1,075
2020	0	0
Total	$ 3,548	$ 3,603