Marketable Securities - Summary of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Summary of available-for-sale securities
Available-for-sale securities, Cost	$ 24,272	$ 26,773
Available-for-sale securities, Estimated Fair Value	24,212	26,675
Level 1 | Certificates of deposit
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	1,726	1,868
Unrecognized losses for more than 12 months, Cost	0	0
Unrecognized gains, Cost	3,221	3,320
Available-for-sale securities, Cost	4,947	5,188
Unrecognized losses for less than 12 months, Estimated Fair Value	1,722	1,863
Unrecognized losses for more than 12 months, Estimated Fair Value	0	0
Unrecognized gains, Estimated Fair value	3,229	3,329
Available-for-sale securities, Estimated Fair Value	4,951	5,192
Level 2 | U.S. Treasury and agency notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	1,047	947
Unrecognized losses for more than 12 months, Cost	0	0
Unrecognized gains, Cost	761	857
Available-for-sale securities, Cost	1,808	1,804
Unrecognized losses for less than 12 months, Estimated Fair Value	1,045	946
Unrecognized losses for more than 12 months, Estimated Fair Value	0	0
Unrecognized gains, Estimated Fair value	762	859
Available-for-sale securities, Estimated Fair Value	1,807	1,805
Level 2 | Corporate notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	9,994	11,658
Unrecognized losses for more than 12 months, Cost	977	1,053
Unrecognized gains, Cost	2,964	3,431
Available-for-sale securities, Cost	13,935	16,142
Unrecognized losses for less than 12 months, Estimated Fair Value	9,954	11,592
Unrecognized losses for more than 12 months, Estimated Fair Value	972	1,042
Unrecognized gains, Estimated Fair value	2,969	3,435
Available-for-sale securities, Estimated Fair Value	13,895	16,069
Level 2 | Municipal notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	2,016	2,556
Unrecognized losses for more than 12 months, Cost	270	271
Unrecognized gains, Cost	1,296	812
Available-for-sale securities, Cost	3,582	3,639
Unrecognized losses for less than 12 months, Estimated Fair Value	1,992	2,526
Unrecognized losses for more than 12 months, Estimated Fair Value	268	269
Unrecognized gains, Estimated Fair value	1,299	814
Available-for-sale securities, Estimated Fair Value	$ 3,559	$ 3,609