Marketable Securities - Summary of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized gains	$ 63
Cost
Debt Securities, Available-for-sale [Line Items]
Total Available-for-sale Securities	47,369	$ 64,138
Cost | Certificates of deposit | Level 1
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	748	250
Marketable Securities with unrecognized losses for more than 12 months	805	3,861
Marketable Securities with unrecognized gains	2,299	0
Total Available-for-sale Securities	3,852	4,111
Cost | U.S. Treasury and agency notes | Level 2
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	6,615	3,112
Marketable Securities with unrecognized losses for more than 12 months	2,659	23,564
Marketable Securities with unrecognized gains	14,058	3
Total Available-for-sale Securities	23,332	26,679
Cost | Corporate notes | Level 2
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	2,003	13,696
Marketable Securities with unrecognized losses for more than 12 months	1,670	12,542
Marketable Securities with unrecognized gains	13,466	0
Total Available-for-sale Securities	17,139	26,238
Cost | Municipal notes | Level 2
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	0	2,994
Marketable Securities with unrecognized losses for more than 12 months	0	4,116
Marketable Securities with unrecognized gains	3,046	0
Total Available-for-sale Securities	3,046	7,110
Fair Value
Debt Securities, Available-for-sale [Line Items]
Total Available-for-sale Securities	47,423	63,749
Fair Value | Certificates of deposit | Level 1
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	747	248
Marketable Securities with unrecognized losses for more than 12 months	804	3,812
Marketable Securities with unrecognized gains	2,305	0
Total Available-for-sale Securities	3,856	4,060
Fair Value | U.S. Treasury and agency notes | Level 2
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	6,611	3,105
Marketable Securities with unrecognized losses for more than 12 months	2,658	23,415
Marketable Securities with unrecognized gains	14,076	4
Total Available-for-sale Securities	23,345	26,524
Fair Value | Corporate notes | Level 2
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	2,002	13,665
Marketable Securities with unrecognized losses for more than 12 months	1,669	12,431
Marketable Securities with unrecognized gains	13,493	0
Total Available-for-sale Securities	17,164	26,096
Fair Value | Municipal notes | Level 2
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	0	2,982
Marketable Securities with unrecognized losses for more than 12 months	0	4,087
Marketable Securities with unrecognized gains	3,058	0
Total Available-for-sale Securities	$ 3,058	$ 7,069