Marketable Securities - Summary of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Cost
Summary of available-for-sale securities
Total Available-for-sale Securities	$ 70,520	$ 71,066
Cost | Level 1 | Certificates of deposit
Summary of available-for-sale securities
Marketable Securities with unrecognized losses for less than 12 months	6,625	6,238
Marketable Securities with unrecognized losses for more than 12 months	987	102
Marketable Securities with unrecognized gains	0	2,088
Available-for-sale securities, Cost	7,612	8,428
Cost | Level 2 | U.S. Treasury and agency notes
Summary of available-for-sale securities
Marketable Securities with unrecognized losses for less than 12 months	30,010	29,741
Marketable Securities with unrecognized losses for more than 12 months	137	137
Marketable Securities with unrecognized gains	3	152
Available-for-sale securities, Cost	30,150	30,030
Cost | Level 2 | Corporate notes
Summary of available-for-sale securities
Marketable Securities with unrecognized losses for less than 12 months	21,856	18,230
Marketable Securities with unrecognized losses for more than 12 months	2,499	2,804
Marketable Securities with unrecognized gains	0	0
Available-for-sale securities, Cost	24,355	21,034
Cost | Level 2 | Municipal notes
Summary of available-for-sale securities
Marketable Securities with unrecognized losses for less than 12 months	6,995	10,298
Marketable Securities with unrecognized losses for more than 12 months	955	999
Marketable Securities with unrecognized gains	453	277
Available-for-sale securities, Cost	8,403	11,574
Fair Value
Summary of available-for-sale securities
Total Available-for-sale Securities	69,952	70,868
Fair Value | Level 1 | Certificates of deposit
Summary of available-for-sale securities
Marketable Securities with unrecognized losses for less than 12 months	6,563	6,222
Marketable Securities with unrecognized losses for more than 12 months	984	100
Marketable Securities with unrecognized gains	0	2,089
Available-for-sale securities, Cost	7,547	8,411
Fair Value | Level 2 | U.S. Treasury and agency notes
Summary of available-for-sale securities
Marketable Securities with unrecognized losses for less than 12 months	29,780	29,669
Marketable Securities with unrecognized losses for more than 12 months	136	135
Marketable Securities with unrecognized gains	4	153
Available-for-sale securities, Cost	29,920	29,957
Fair Value | Level 2 | Corporate notes
Summary of available-for-sale securities
Marketable Securities with unrecognized losses for less than 12 months	21,662	18,159
Marketable Securities with unrecognized losses for more than 12 months	2,479	2,788
Marketable Securities with unrecognized gains	0	0
Available-for-sale securities, Cost	24,141	20,947
Fair Value | Level 2 | Municipal notes
Summary of available-for-sale securities
Marketable Securities with unrecognized losses for less than 12 months	6,948	10,288
Marketable Securities with unrecognized losses for more than 12 months	943	987
Marketable Securities with unrecognized gains	453	278
Available-for-sale securities, Cost	$ 8,344	$ 11,553