Marketable Securities - Summary of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized gains	$ 1
Cost
Debt Securities, Available-for-sale [Line Items]
Total Available-for-sale Securities	64,138	$ 71,066
Cost | Certificates of deposit | Level 1
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	250	6,238
Marketable Securities with unrecognized losses for more than 12 months	3,861	102
Marketable Securities with unrecognized gains	0	2,088
Total Available-for-sale Securities	4,111	8,428
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	3,112	29,741
Marketable Securities with unrecognized losses for more than 12 months	23,564	137
Marketable Securities with unrecognized gains	3	152
Total Available-for-sale Securities	26,679	30,030
Cost | Corporate notes | Level 2
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	13,696	18,230
Marketable Securities with unrecognized losses for more than 12 months	12,542	2,804
Total Available-for-sale Securities	26,238	21,034
Cost | Municipal notes | Level 2
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	2,994	10,298
Marketable Securities with unrecognized losses for more than 12 months	4,116	999
Marketable Securities with unrecognized gains	0	277
Total Available-for-sale Securities	7,110	11,574
Estimated Fair Value
Debt Securities, Available-for-sale [Line Items]
Total Available-for-sale Securities	63,749	70,868
Estimated Fair Value | Certificates of deposit | Level 1
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	248	6,222
Marketable Securities with unrecognized losses for more than 12 months	3,812	100
Marketable Securities with unrecognized gains	0	2,089
Total Available-for-sale Securities	4,060	8,411
Estimated 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	3,105	29,669
Marketable Securities with unrecognized losses for more than 12 months	23,415	135
Marketable Securities with unrecognized gains	4	153
Total Available-for-sale Securities	26,524	29,957
Estimated Fair Value | Corporate notes | Level 2
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	13,665	18,159
Marketable Securities with unrecognized losses for more than 12 months	12,431	2,788
Total Available-for-sale Securities	26,096	20,947
Estimated Fair Value | Municipal notes | Level 2
Debt Securities, Available-for-sale [Line Items]
Marketable Securities with unrecognized losses for less than 12 months	2,982	10,288
Marketable Securities with unrecognized losses for more than 12 months	4,087	987
Marketable Securities with unrecognized gains	0	278
Total Available-for-sale Securities	$ 7,069	$ 11,553