Marketable Securities - Summary of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Summary of available-for-sale securities
Available-for-sale securities, Cost	$ 26,773	$ 32,975
Available-for-sale securities, Estimated Fair Value	26,675	32,815
Level 1 | Certificates of deposit
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	1,868	4,810
Unrecognized losses for less than 12 months, Estimated Fair value	1,863	4,797
Unrecognized losses for more than 12 months, Cost	0	239
Unrecognized losses for more than 12 months, Estimated Fair value	0	238
Unrecognized gains, Cost	3,320	2,800
Unrecognized gains, Estimated Fair value	3,329	2,805
Available-for-sale securities, Cost	5,188	7,849
Available-for-sale securities, Estimated Fair Value	5,192	7,840
Level 2 | U.S. Treasury and agency notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	947	860
Unrecognized losses for less than 12 months, Estimated Fair value	946	857
Unrecognized losses for more than 12 months, Cost	0	0
Unrecognized losses for more than 12 months, Estimated Fair value	0	0
Unrecognized gains, Cost	857	736
Unrecognized gains, Estimated Fair value	859	738
Available-for-sale securities, Cost	1,804	1,596
Available-for-sale securities, Estimated Fair Value	1,805	1,595
Level 2 | Corporate notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	11,658	14,638
Unrecognized losses for less than 12 months, Estimated Fair value	11,592	14,516
Unrecognized losses for more than 12 months, Cost	1,053	2,080
Unrecognized losses for more than 12 months, Estimated Fair value	1,042	2,061
Unrecognized gains, Cost	3,431	3,334
Unrecognized gains, Estimated Fair value	3,435	3,339
Available-for-sale securities, Cost	16,142	20,052
Available-for-sale securities, Estimated Fair Value	16,069	19,916
Level 2 | Municipal notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	2,556	1,742
Unrecognized losses for less than 12 months, Estimated Fair value	2,526	1,725
Unrecognized losses for more than 12 months, Cost	271	301
Unrecognized losses for more than 12 months, Estimated Fair value	269	298
Unrecognized gains, Cost	812	1,435
Unrecognized gains, Estimated Fair value	814	1,441
Available-for-sale securities, Cost	3,639	3,478
Available-for-sale securities, Estimated Fair Value	$ 3,609	$ 3,464