Marketable Securities - Summary of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Summary of available-for-sale securities
Available-for-sale securities, Cost	$ 32,975	$ 42,112
Available-for-sale securities, Estimated Fair Value	32,815	42,140
Level 1 | Certificates of deposit
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	4,810	2,522
Unrecognized losses for less than 12 months, Estimated Fair value	4,797	2,492
Unrecognized losses for more than 12 months, Cost	239	837
Unrecognized losses for more than 12 months, Estimated Fair value	238	832
Unrecognized gains, Cost	2,800	5,379
Unrecognized gains, Estimated Fair value	2,805	5,395
Available-for-sale securities, Cost	7,849	8,738
Available-for-sale securities, Estimated Fair Value	7,840	8,719
Level 2 | U.S. Treasury and agency notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	860	1,919
Unrecognized losses for less than 12 months, Estimated Fair value	857	1,910
Unrecognized losses for more than 12 months, Cost	0	702
Unrecognized losses for more than 12 months, Estimated Fair value	0	700
Unrecognized gains, Cost	736	1,182
Unrecognized gains, Estimated Fair value	738	1,207
Available-for-sale securities, Cost	1,596	3,803
Available-for-sale securities, Estimated Fair Value	1,595	3,817
Level 2 | Corporate notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	14,638	3,872
Unrecognized losses for less than 12 months, Estimated Fair value	14,516	3,841
Unrecognized losses for more than 12 months, Cost	2,080	4,423
Unrecognized losses for more than 12 months, Estimated Fair value	2,061	4,405
Unrecognized gains, Cost	3,334	16,897
Unrecognized gains, Estimated Fair value	3,339	16,963
Available-for-sale securities, Cost	20,052	25,192
Available-for-sale securities, Estimated Fair Value	19,916	25,209
Level 2 | Municipal notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	1,742	739
Unrecognized losses for less than 12 months, Estimated Fair value	1,725	733
Unrecognized losses for more than 12 months, Cost	301	457
Unrecognized losses for more than 12 months, Estimated Fair value	298	456
Unrecognized gains, Cost	1,435	3,183
Unrecognized gains, Estimated Fair value	1,441	3,206
Available-for-sale securities, Cost	3,478	4,379
Available-for-sale securities, Estimated Fair Value	$ 3,464	$ 4,395