Marketable Securities - Summary of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized gains	$ 36
Cost
Debt Securities, Available-for-sale [Line Items]
Total Available-for-sale Securities	56,506	$ 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	498	250
Marketable Securities with unrecognized losses for more than 12 months	4,012	3,861
Marketable Securities with unrecognized gains	0	0
Total Available-for-sale Securities	4,510	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	3,888	3,112
Marketable Securities with unrecognized losses for more than 12 months	10,861	23,564
Marketable Securities with unrecognized gains	11,574	3
Total Available-for-sale Securities	26,323	26,679
Cost | Corporate notes | Level 2
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	1,898	13,696
Marketable Securities with unrecognized losses for more than 12 months	6,000	12,542
Marketable Securities with unrecognized gains	11,098	0
Total Available-for-sale Securities	18,996	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	1,417	4,116
Marketable Securities with unrecognized gains	5,260	0
Total Available-for-sale Securities	6,677	7,110
Fair Value
Debt Securities, Available-for-sale [Line Items]
Total Available-for-sale Securities	56,501	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	497	248
Marketable Securities with unrecognized losses for more than 12 months	4,005	3,812
Marketable Securities with unrecognized gains	0	0
Total Available-for-sale Securities	4,502	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	3,883	3,105
Marketable Securities with unrecognized losses for more than 12 months	10,845	23,415
Marketable Securities with unrecognized gains	11,587	4
Total Available-for-sale Securities	26,315	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	1,897	13,665
Marketable Securities with unrecognized losses for more than 12 months	5,990	12,431
Marketable Securities with unrecognized gains	11,114	0
Total Available-for-sale Securities	19,001	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	1,416	4,087
Marketable Securities with unrecognized gains	5,267	0
Total Available-for-sale Securities	$ 6,683	$ 7,069